With such a comprehensive and high quality The Effective Temperature index Houghton and Yaglou was originally established to provide a method for determining the relative effects of air temperature and humidity on comfort. The accumulation of moisture is mostly a drawback, because of discomfort and risk of after-chill. The SPX product line includes: Exposed skin transfers more heat than covered skin and thus has a profound influence on the intrinsic insulation. The move is presumably on the back of the resurgence in popularity of the ketogenic and associate. A Revolutionary New Thermogenic!
Although not demonstrated in all human studies 68,69 , these results indicate that alternate-day fasting might mimic the insulin sensitizing effects observed in rodents on alternate-day fasting diet, and that the effect might be due to increased adiponectin levels.
Sex differences were also observed in another study where healthy men and women were fasted on alternate days. In this study, HDL levels were increased in women only, and triglycerides were decreased in men but not women Increased insulin sensitivity was suggested by decreased insulin levels with unaltered glucose levels.
In this study, blood pressure was unaltered, but the study duration was merely 22 days. In contrast, one trial showed decreased blood pressure and resting heart rates in subjects on modified alternate-day fasting regimens for 10 weeks, suggesting that longer intervention periods might be needed for this effect to occur There is, however, conflicting data from another study that utilized a two week crossover study design and randomized eight healthy men to a modified alternate-day fasting diet or a standard diet.
No differences were observed in body weight, blood lipids, glucose metabolism or hormone levels, and there was a decrease in energy expenditure after the 2 week period in the alternate-day fasting group More controlled studies, with larger sample sizes and longer study durations are thus needed to bring clarification in this matter.
No human trial has directly examined intermittent fasting and tumor physiology. A single two day fast increases endogenous GH-production fivefold, reflecting the metabolic adaptation to fasting, including increased hepatic glucose production, lipolysis and nitrogen conservation However no significant changes in IGF-1 are seen after a single fast period in human subjects, suggesting that repeated fasts and longer intervention periods might be necessary to mimic the changes in IGF-1 and altered cancer growth observed in some rat studies.
Whether a prolonged alternate-day fasting regimen can alter IGF-1 levels in humans remains an area for future research. Furthermore, no human trials to date have examined the effects of intermittent fasting on neuronal health or life span.
The exact mechanism by which calorie restriction and intermittent fasting exhibits its effects on various organ systems remains unknown. Alternating periods of anabolism and catabolism during intermittent fasting might further increase the cellular stress resistance. Other displayed effects are increased production of neutrophilic factors and antioxidant enzymes, ketone body formation and altered metabolism enzyme production 5.
A variety of questions often arises when discussing intermittent fasting and human health. It is often believed that blood sugar levels will fall to pathological levels if prolonged fasts are implemented. A characteristic decline in mood and energy levels before lunch among humans is often attributed to a drop in blood sugar. However when actually testing blood sugar levels in healthy subjects prone to this phenomena, no actual decline in blood sugar to pathologic levels was seen during a 24 hour fast Furthermore, a double-blind, placebo-controlled study of two days of calorie deprivation showed no adverse effect on cognitive performance, activity, sleep, and mood, when the subjects were unaware of the calorie content of the treatments The homeostasis of body weight regulation and hunger signaling is composed of complex circuits of both central signals including orexin, neuropeptide Y, melanin concentrating hormone and alpha-melanocyte, and peripheral signals from the gut and adipose tissue, such as ghrelin, peptide YY and leptin The interplay between these and other endocrine signaling systems and its effect on body weight regulation and subjective feelings of hunger and satiety remains largely unknown.
The hunger response however seems to be highly adaptive in different meal patterns. Ghrelin, a gut derived hormone, is considered a meal-initiation signal. It increases during fasting and usually peaks in concentration before an anticipated meal, paired with increased feelings of hunger, and decreases after feeding.
Increases in subjective feelings of hunger might be the single most important factor to consider when discussing the applicability of intermittent fasting as a therapeutic or preventive intervention in human subjects. In obese patients, a 14 day total fast lead to strikingly decreased body weights and decreased blood pressure, without causing increased hunger sensations.
Thus a hunger suppressing effect of prolonged fasting was demonstrated This anorexic effect might be attributed to the evolutionary purpose of seeking for nutrients in absence of food.
The experiment, dating back to , was effective and well tolerated. Only one study has directly examined the feelings of hunger and fullness in non-obese subjects on an intermittent fasting diet, by using a mm visual analog scale The subjects were fasted on alternate days and reported an increased feeling of hunger from 37 to 56 mm and decrease in feeling of fullness from 43 to 23 mm when the dietary intervention was initiated. The magnitude of hunger did however not change during the intervention period as repeated measurements were taken, and feelings of fullness actually increased some over time.
The duration of this study was only 22 days and it is still purely speculative whether and adaptation to the new meal pattern would occur in a longer time span. In contrast, modified alternate-day fasting in obese asthmatic patients did not significantly increase the subjective perception of hunger from baseline during the eight week long intervention period Whether repeated bouts of short term fasting can alter hunger hormone signaling or demonstrate the same anorexic effect as the long term fast described above is highly speculative and an interesting area for future research.
It is commonly believed that multiple small meals increase metabolism and lead to increased overall energy expenditure. Following every meal there is an increase in expenditure due to the processing of the nutrients, commonly referred to Thermic Effect of Food TEF A common belief therefore is that increased meal frequency leads to increased TEF and increased overall energy expenditure with multiple meals, and that intermittent fasting accordingly would decrease metabolic rate and lead to increased fat accumulation and possibly obesity.
According to current research though, TEF is proportional to the calorie content and vary with macronutrient composition with the highest increase in energy expenditure observed with a high protein diet and not meal frequency per se , as demonstrated by the equal TEF in different meal patterns under iso-caloric conditions 79, Furthermore, one study examined alterations in resting metabolic rate in human subjects on alternate-day fasting diets, and found no changes after a 22 day period According to these findings, any potential decreases in metabolic rate would be due to decreased total calorie intake and not fasting per se.
Increased levels of both ACTH and corticosteroids can be noted in rodents maintained on alternate-day fasting diets compared with rats fed ad libitum 28,29, Apart from the obvious notion that cortisol is one of the major hormones responsible for glucose utilization during fasting, the question arises whether the increased stress in any way could be harmful to the human organism.
The molecular stress response in intermittently fasted subjects seems markedly different from the one associated with uncontrolled stress. In contrast, in uncontrolled stress, down regulation of the mineral corticoid receptor has been noted.
Furthermore, deleterious stress responses are associated with a decrease in the expression of brain-derived neurotrophic factor BDNF , a response quite the opposite of calorie restriction and intermittent fasting, where increased concentrations of BDNF have been observed in numerous studies 4.
In conclusion, the controlled stress response from intermittent fasting seems fundamentally different from the one by uncontrolled physiological and psychological stress. Conversely, In line with the mechanisms described above, the increased stress might be one of the necessary factors for initiating molecular resistance for larger stressors, and thus promote some of the beneficial effects of intermittent fasting.
One potential serious side effect of intermittent fasting would be loss of muscle mass. Theoretically, food deprivation would result in depleted hepatic glycogen stores, leading to increased proteolysis and flux of amino acids from skeletal muscle for hepatic de novo gluconeogenesi s, to maintain healthy blood glucose concentrations. As discussed previously though, a 24 hour short term fast is insufficient in duration to deplete liver glycogen stores in healthy subjects Up to 40 hours of total fasting does not stimulate catabolic processes and lead to skeletal muscle atrophy Modified alternate-day fasting and loss of lean body mass was investigated in only one study in the systematic search.
No loss of fat free mass in the absence of weight loss was observed compared to a control group fed a standardized diet Furthermore, an increase in ketone body concentrations has been observed in subjects on alternate-day fasting diets in both human and animal studies 17, Ketone bodies spare skeletal muscle from breakdown by providing non-glucose energy substrate for various tissues, of which the brain is the most important, and thus decrease the need for protein-derived substrates for gluconeogenetic conversion to maintain glucose homeostasis Available data thus suggests that short term fasting does not deplete hepatic glycogen stores to the extent that markedly increased proteolysis and gluconeogenesis becomes necessary to maintain healthy glucose concentrations.
Still this notion needs to be clarified in future research of longer duration. Intermittent fasting in the form of alternate day fasting in many instances reduces overall energy intake, with no obvious adverse effects, and thus becomes a model of calorie restriction in both human and animal subjects.
Secondary to reduced energy intake and weight loss, effects such as reduced risk factors for cardiovascular disease, and improved glucose metabolism have been demonstrated in both animal and human subjects on true and modified alternate-day fasting diets. In rats, protection against ischemic injury and improved survival has been demonstrated in both myocardial and cerebral ischemic events. Other beneficial effects, such as slowing the neuronal aging process and increasing cognitive functions and memory, have been observed.
Additionally, calorie restriction can reduce cancer risk and increase life span in rodent models on alternate-day fasting diets. Some effects occur even if the subject maintains body weight, suggesting that the reduced meal frequency or prolonged time in the fasted state might have some additional effects regardless of overall calorie restriction and weight loss.
In humans, modified alternate-day fasting diets might be easier to adhere to and they seemingly lead to less pronounced weight loss than true alternate-day fasting. Without causing weight loss, effects such as improved fasting insulin have been demonstrated in both animals and humans. In line with these findings, adiponectin increases in rats and humans on both true and modified alternate-day fasting diets in the absence of calorie restriction. Additionally, in mice, fat redistribution from visceral to subcutaneous stores has been observed despite unaltered overall body weight.
If this effect proves to be true in human subjects it could propose reduced disease risk despite unaltered body weight. Animal data further indicate some beneficial effects of intermittent fasting diets even without calorie restriction. Neuronal health improvements such as resistance to excitotoxic injury have been observed. Alternate-day fasting in animals also leads to improved recovery after induced spinal cord injuries and progressive demyelinating disease of the peripheral nervous system, in the absence of calorie restriction.
Furthermore, in animal studies, changes associated with retareded tumorgenesis, such as decreased cell proliferation rates in various cell lines and decreased incidence of lymphoma, have been observed. Whether these observations are valid in human subjects as well remains an interesting area for future research. Future research is warranted to test whether the health promoting effects described in animal studies have some validity in humans.
We are in the very infancy of research on intermittent fasting in human subjects and future studies with larger sample sizes, longer durations and of better study design must be completed before any definite conclusions can be made regarding intermittent fasting and human health and the applicability to modern lifestyle. My sincere gratitude to Staffan Lindeberg and Bengt Zöller for helping me set up the systematic search, for all the intellectually stimulating discussions and for the guidance in writing this review.
The complete search term used was: Alternate day fasting and body weight, glucose metabolism and cardiovascular health in humans.
Very interesting stuff, I love that even though some of the references were a bit over my head the latin names for some illnessess and the implications of these, etc most of it was completely comprehensible eventhough it was made for medical purposes. Great summary of some of the work that has been done to date. Some more recent studies showing timing i. Keep up the nice work. Appreciate you and Berkhan sharing your work with us!
Thanks for taking the time to write this up, Bojan. It was a very nice, in-depth look at the studies surrounding IF and written in a way that can appeal to the medical-savvy people, and the general public interested in health like myself. There used to be a time when people were trained to write and communicate, and from the above article this time has clearly passed. I can see that you are using this article to try to bring people in and eventually sell them something.
The way you are approaching it wont work-unless you really think people are going to read 10, words of science crap instead of just closing your site and going elsewhere. While it may seem like jargon to you, it is the preferred lexicon for scholarly publications.
Do NOT presume their level of education. I once listened to an Immunology professor at a University complain that the only articles he could understand in Science the journal were the ones on Immunology. Trust me; if you were to pick up and try to read a scholarly academic article outside of your field, you would react the way Jeff is reacting.
Oh dear Jeff, you have clearly shown that this level of academia is lost on you. This is an excellent review of the available literature, written by a highly educated person.
This is an extremely disrespectful comment that shows your lack of intelligence. Think things through before you comment in future. It is Your comment that is extremely disrespectful comment and shows a lack of intelligence and knowledge of how many different styles there are in the world. I have an MS in Microbiology and have been employed as a programmer and technical writer for over 25 years.
Both Jeff and Notjeff seriously? Notjeff is sadly also correct. However, Audience is Everything. If you want this information to be read and understood by people who are not your thesis committee or people with an MD or PhD in your field, you need to re-write it.
This study confirms much of the anecdotal evidence that I had already heard about the possible long-term effects of intermittent fasting. Interesting timing of this post as I was just talking with a co-worker about the benefits of intermittent fasting and the general practice of fasting. Although it has been a while many years since I have practiced fasting, it is a ritual that I have studied and used its healthy benefits in the past. I have used short fasts of less than 36 hours and longer fasts of 7 days for personal research,purposes and practice.
After having lived several years in an unhealthy way, I have re-tooled my lifestyle into a sustainable healthy way of living and have lost 55 pounds in the past year. The subject of intermittent fasting came up as I believe that I will begin a regular practice of IF as a part of this lifestyle. I am looking forward the time when I can read your complete work as I have only scanned it this morning.
Thanks for sharing your work with me. Thank you for sharing your research. The popular models of IF being 5: Do you have any results on their benefits from your research?
I need to leave a quick comment just as I started reading The best and biggest recource base for the IF is by far Martin Berkhan, and his website leangains. Now back to the reading…. It is a great summary and I do appreciate your work on this.
Thanks for sharing this. It seems like most of the research in this area has been done on type two Diabetics. Ori may in fact be the first well-known proponent of IF and Warrior Diet should be recognized as one of original mainstream publications advocating IF for general health and physique improvements….
I spend much of my time researching information on fasting from peer reviewed journals through my university. I find the health benefits of IF, especially autophagy, are profound but tend to be poorly understood. I have the article you posted. What do you know about this topic? Lots of writing but I was conflicted.. You talked in the beginning about this being your baby.. You made it sound like you had stumbled on some gold nuggets of previously unknown information.
Nice resource of information overall but I still fail to understand why so many people regard IF as some unique idea or think of it as a new way to solve an old problem. I tried it for a full year before realizing that my results were identical to when I ate 6 meals a day for a decade. Is it staving off disease in me a little longer and giving me a future benefit in that respect? Possibly but not conclusively. It matters little and this just seems to justify that.
Eat 6 meals if you like or eat 1 or 2. I can see great benefit in people linking to the article as a place to find many important studies on fasting. Most appreciate the hard work, and you choosing to publish this free for all to read and benefit after the hours of work. Will put several links up to it. It is true that junk food can totally screw up any health benefits of IF and the people that bash it did just that, or did unhealthy low carb or too low calorie or had lots of stress in other areas of life.
IF can also backfire if done to extreme. I mean come on. Thank you for all the work of putting it together! From perspective of 1. Mood and energy 2. Long-term health and longevity 3. Physical and mental performance. All other things being equal, according to current data, should someone training two or three times a week fast once a week for the whole day or daily for about hours? Also, assuming the three days workout regimen Mon, Wed, Fri or two days workout regimen Mon and Fri what would be the best day to fast to get most of the benefits and sacrifice as little anabolism as possible?
Thank you for sharing it! Is there any scientific evidence to support the answer to this question? What of the following types of fasting should one do if he or she is not interested in weight loss but rather muscle gain , but is very interested in all other health benefits fasting might provide and specifically in the benefit below related to glomerulosclerosis as this one can be life-saving!
Intermittent fasting daily 1. Alternate day fasting 2. Rats fed on alternate days showed preserved kidney function as demonstrated by preserved glomerular filtration rate and renal plasma flow, compared to rats fed ad libitum.
Effect of long-term, alternate day feeding on renal function in aging conscious rats. Although scientific in its approach, quite accessible to the rest of us to understand and assimilate.
I can usually go to somewhere between 2 -4 pm. But should I push it too long — I often do! What is causing it?!
The symptoms you describe are associated with detoxing. You say the better your diet the less you experience them. The quality of your food matters, as the more toxins your body has to eliminate the more symptoms you will feel during your food-free periods. You should be able to push through these symptoms and stretch the length of time without food.
Disregard this comment, all. It seems like the carb reduction in paleo is helping. Maybe try being more strict with carb intake, 20g daily.
I certainly appreciate a scientific paper on weight loss versus the advertisement-laced, fad based articles. I have read articles that state while research shows benefits for male subjects rodent and human alike , that longer periods of fasting were detrimental for females.
I came across this thesis while researching for an article I am writing about IF for women specifically. I have been practicing IF for a few years and despite many mishaps have finally hit a sweet spot with it. However, as you said have struggled to find reliable research, or in fact anything written on the subject at all.
However, this is what I have found so far:. Already lost 28 lbs and broke all my personal best in the major lifts. Thank you for sharing your hard work. Great read Bojan, thank you for posting it online. I especially liked the section regarding the Ramadan fast. And congratulations on your MD! Oh, my blood work looks incredible as well, with my total cholesterol in the s despite very high HDL and my triglycerides in the mid 50s. Your research changes lives. This was incredibly well written, fascinating and informative.
That day has finally come, it seems my fear of disappointment was just stopping me from looking a little harder. S Ramachandran, Dawkins and the Lancet Journal. I met a trainer who has a giant of a sculpted body. He told me something about slingshot training which was based on eating and fasting cycles.
He said the body releases growth hormones when we fast. Now it makes heck of a sense. Also, while fasting during Ramadan is mandatory, it is encouraged among Muslims to fast every Monday and Thursday. Very few people do it however. Like most good things in life. Fasting is prescribed to you as it was prescribed to those before you, that ye may learn self-restraint. Thank you so much for sharing. I have been studying fasting for years on my own body as well as teaching it to others.
I hope you write a book some day. I really appreciate not having to immerse myself in a stack of papers to understand some of the real science beyond IF. Just studies in rodents by now. With increased ketones due to short period fasting and Preventing the loss of non fat mass. Can you advise a optimal period in hours of fasting followed by hours not fasting?
Also on none fasting days. Would a ketogenic diet play some role in pre and post fast to minimise shock to the body and bring ketone levels up before a total nutritional cut off? And any thoughts on carb loading or cheat days relating to IF?
Why have you not had your work published in a peer reviewed journal? I find that highly suspicious. All that sounds great… but for a sec. I am from Greece and from family of building constructors.
Those people were strong, lean, with stamina, poor, happy and optimistic. Now we are sick all the time, unsatisfied and with all my respect to the author work, with all that scientific data, — we forgetting that not only the quran reinforce fasting but the christianity too, vigorously i must say, — actually we exploring the proven results of fasting mechanism, without the way to reinforce the fundamental mentality of how to decide to live frugal. Just a thought for further research: If so, why not settle on one unified term?
If not, why are they used interchangeably throughout the article? Intermittent Fasting is the general classification covering all forms of short regular fasting, Alternate Day Fasting is a specific form of Intermittent Fasting where food is only eaten every second day. Thank you for sharing your work. Intermittent fasting seems to be a generally safe and effective regime; your year-long efforts help reveal the scientific underpinnings.
I was a half-assed pescatarian for many years. I went full vegan in March Going vegan was hard, especially the beginning. In March I was around My partner gets home late, loves to cook and thinks nothing of eating a gigantic steak and full meal at 11pm.
It was a tough change for me. Now, I limit my food to window of time. Mid-to-late morning thru till around 6pm at the very latest. It seems to me like I almost need those occasional times to re-jumpstart the downward trend. I swim for 20 minutes a day. I think dropping your body temp for a while also is good for kick-starting your body energy. I love to dance and dance around our condo all the time. I work from home.
I realized that I was doing some kind of intermittent fasting and it sure seems to line up with my experience. I feel euphoric often. I feel so dang healthy I want to shout it from the rooftops! Now, I eat for nutrition. I listen to my body more. I feel like a different person. I feel clean and wonderful inside! Sorry, I meant so say I went Vegan in March My weight loss has been about a month journey.
Is there a spot where I can get more basic how to details on the types of IF…and also figure our which is best more weight loss. I am not obese…just modern day over weight. Mind blown, thank you so incredibly much. I only take about 3 grams. And one small lime, which has about 5mL of juice. Congratulations on acquiring your MD. I will hopefully be following in your footsteps when I graduate in Thank you for the Great work you have done!
Its a appreciated as much as I appreciat my kids! Thank you so much for sharing your research and information. It is refreshing to know that someone is giving the information to anyone that wants to access it for free. Superb review, thank you so much for sharing. I am an undergraduate studying nutrition and I am currently writing my dissertation on time to exhaustion in male subjects during intermittent fasting. The collation of literature here has helped me no end. Thank you so much again for sharing and congratulations on achieving your MD!
I added this to my website, which is all about intermittent fasting information, to share with others. Very nice review, Sir; very useful to me. The only result of your paper is the last sentenace: Very little in the morning, a glass of fresh juice; very little in the evening, a few nuts or some cheese. My proper meal consists of good protein and fat, a decent serving of veg and some fruit — and never forget your dark chocolate!
No wheat, no added sugar — but also no actual rules. So does that count as IF? Is there any science on that model? I stopped weighing myself many years ago. I also need less sleep. If the condition is adequately treated, victims generally feel well within a few hours and recover without sequelae. If not, it may readily proceed to heat stroke. Heat stroke is a serious medical emergency which may result in death. It is a complex clinical condition in which uncontrollable hyperthermia causes tissue damage.
Such an elevation of body temperature is caused initially by severe heat congestion due to excessive heat load, and the resultant hyperthermia induces dysfunction of the central nervous system, including failure of the normal thermoregulatory mechanism, thus accelerating elevation of the body temperature. Heat stroke occurs basically in two forms: The former develops in very young, elderly, obese or unfit individuals undertaking normal activities during prolonged exposure to high environmental temperatures, whereas the latter occurs particularly in young, active adults during physical exertion.
In addition, there is a mixed form of heat stoke presenting features consistent with both of the above forms. Elderly individuals, particularly those who have underlying chronic illness, such as cardiovascular diseases, diabetes mellitus and alcoholism, and those taking certain medications, especially psychotropic drugs, are at a high risk of classical heat stroke.
During sustained heat waves, for example, the mortality rate for the population older than 60 years has been recorded as more than ten times greater than that for the population aged 60 and under.
A similarly high mortality in the elderly population has also been reported among Muslims during the Mecca pilgrimage, where the mixed form of heat stroke has been found to be prevalent. Factors predisposing the elderly to heat stroke, other than chronic diseases as mentioned above, include reduced thermal perception, sluggish vasomotor and sudomotor sweating reflex responses to changes in thermal load, and reduced capacity for acclimatization to heat.
Individuals who work or exercise vigorously in hot, humid environments are at a high risk of exertion-induced heat illness, whether heat exhaustion or heat stroke.
Athletes undergoing high physical stress can fall victim to hyperthermia by producing metabolic heat at a high rate, even when the environment is not very hot, and have often suffered heat stress illness as a result. Relatively unfit non-athletes are at a lesser risk in this regard as long as they realize their own capacity and limit their exertions accordingly. However, when they play sports for fun and are highly motivated and enthusiastic, they often try to exert themselves at an intensity beyond that for which they have been trained, and may succumb to heat illness usually heat exhaustion.
Poor acclimatization, inadequate hydration, unsuitable dress, alcohol consumption and skin illness causing anhidrosis reduction in or lack of sweating , notably prickly heat see below , all aggravate the symptoms. Children are more susceptible to heat exhaustion or heat stroke than adults. They produce more metabolic heat per unit mass, and are less able to dissipate heat because of a relatively low capacity to produce sweat.
The diagnosis of heat stroke is easy to establish when this triad of criteria is met. However, it may be missed when one of those criteria is absent, obscure or overlooked. For example, unless core temperature is measured properly and without delay, severe hyperthermia may not be recognized; or, in a very early stage of exertion-induced heat stroke, sweating may still persist or may even be profuse and the skin may be wet.
The onset of heat stroke is usually abrupt and without precursory symptoms, but some patients with impending heat stroke may have symptoms and signs of disturbances of the central nervous system. They include headache, nausea, dizziness, weakness, drowsiness, confusion, anxiety, disorientation, apathy, aggressiveness and irrational behaviour, tremor, twitching and convulsion.
Once heat stroke occurs, disturbances of the central nervous system are present in all cases. The level of consciousness is often depressed, deep coma being most common. Seizures occur in the majority of cases, especially in physically fit individuals.
Signs of cerebellar dysfunction are prominent and may persist. Pin-pointed pupils are frequently seen. Cerebellar ataxia lack of muscular coordination , hemiplegia paralysis of one side of the body , aphasia and emotional instability may persist in some of survivors. Vomiting and diarrhoea often occur. Tachypnoea rapid breathing is usually present initially and the pulse may be weak and rapid.
Hypotension, one of the most common complications, results from marked dehydration, extensive peripheral vasodilatation and eventual depression of cardiac muscle. Acute renal failure may be seen in severe cases, especially in exertion-induced heat stroke. Haemorrhages occur in all parenchymal organs, in the skin where they are called petechiae and in the gastro-intestinal tract in severe cases. Clinical haemorrhagic manifestations include melaena dark-coloured, tarry faeces , haematemesis blood vomiting , haematuria bloody urine , haemoptysis spitting blood , epistaxis nosebleed , purpura purple spots , ecchymosis black and blue marks and conjunctival haemorrhage.
Intravascular coagulation occurs commonly. Haemorrhagic diathesis bleeding tendency is usually associated with disseminated intra-vascular coagulation DIC. DIC occurs predominantly in exertion-induced heat stroke, where the fibrinolytic clot-dissolving activity of plasma is increased. On the other hand, a decrease in platelet count, prolongation of prothrombin time, depletion of coagulation factors and increased level of fibrin degradation products FDP are provoked by whole-body hyperthermia.
Patients with evidence of DIC and bleeding have higher core temperature, lower blood pressure, lower arterial blood pH and pO 2 , a higher incidence of oliguria or anuria and of shock, and a higher mortality rate. Shock is also a common complication. It is attributable to peripheral circulatory failure and is aggravated by DIC, which causes dissemination of clots in the microcirculatory system.
Proper measurement of core temperature is mandatory: Measurement of oral and axillary temperatures should be avoided because they can vary significantly from real core temperature. The objective of treatment measures is to lower body temperature by reducing heat load and promoting heat dissipation from the skin. The treatment includes moving the patient to a safe, cool, shady and well-ventilated place, removing unnecessary clothing, and fanning.
Cooling the face and head may promote beneficial brain cooling. The efficiency of some cooling techniques has been questioned. It has been argued that placing cold packs over major blood vessels in the neck, groin and axillae and immersion of the body in cold water or covering it with iced towels may promote shivering and cutaneous vasoconstriction, thus actually impeding cooling efficiency.
Traditionally, immersion in an ice-water bath, combined with vigorous skin massage to minimize cutaneous vasoconstriction, has been recommended as the treatment of choice, once the patient is brought to a medical facility. This method of cooling has several disadvantages: This method of cooling can reduce the core temperature by 0. Measures to prevent convulsions, seizures and shivering should also be initiated at once.
Tracheal intubation to protect the airway, insertion of a cardiac catheter to estimate central venous pressure, placement of a gastric tube and insertion of a urinary catheter may also be included among additional recommended measures. For the prevention of heat stroke, a wide variety of human factors should be taken into account, such as acclimatization, age, build, general health, water and salt intake, clothing, peculiarities of religious devotion and ignorance of, or liability to neglect, regulations intended to promote public health.
Prior to physical exertion in a hot environment, workers, athletes or pilgrims should be informed of the work load and the level of heat stress they may encounter, and of the risks of heat stroke. The level of activity should be matched to the ambient temperature, and physical exertion should be avoided or at least minimized during the hottest hours of the day. During physical exertion, free access to water is mandatory.
Since electrolytes are lost in sweat and the opportunity for voluntary ingestion of water may be limited, thus delaying restitution from thermal dehydration, electrolytes should also be replaced in case of profuse sweating. Proper clothing is also an important measure.
Clothes made of fabrics which are both water-absorbent and permeable to air and water vapour facilitate heat dissipation. Miliaria is the most common skin disorder associated with heat load. It occurs when the delivery of sweat onto the skin surface is prevented due to obstruction of the sweat ducts. Sweat retention syndrome ensues when anhidrosis inability to release sweat is widespread over the body surface and predisposes the patient to heat stroke.
Miliaria is commonly induced by physical exertion in a hot, humid environment; by febrile diseases; by the application of wet compresses, bandages, plaster casts or adhesive plaster; and by wearing poorly permeable clothes. Miliaria can be classified into three types, according to the depth of sweat retention: Miliaria crystallina is caused by retention of sweat within or just beneath the horny layer of the skin, where tiny, clear, non-inflammatory blisters can be seen.
This type of miliaria is otherwise symptomless, the least distressing, and heals spontaneously in a few days, when the blisters break out to leave scales. Miliaria rubra occurs when intense heat load causes prolonged and profuse sweating. It is the most common type of miliaria, in which sweat accumulates in the epidermis. Red papules, vesicles or pustules are formed, accompanied by burning and itching sensations prickly heat.
The sweat duct is plugged at the terminal portion. The production of the plug is attributable to the action of resident aerobic bacteria, notably cocci, which increase in population greatly in the horny layer when it is hydrated with sweat. They secrete a toxin which injures the horny epithelial cells of the sweat duct and provokes an inflammatory reaction, precipitating a cast within the lumen of the sweat duct.
Infiltration by leukocytes creates an impaction which completely obstructs the passage of sweat for several weeks. In miliaria profunda, sweat is retained in the dermis, and produces flat, inflammatory papules, nodules and abscesses, with less itching than in miliaria rubra. The occurrence of this type of miliaria is commonly confined to the tropics.
It may develop in a progressive sequence from miliaria rubra after repeated bouts of profuse sweating, as the inflammatory reaction extends downwards from the upper skin layers.
The term achieved currency during the Second World War, when troops deployed to tropical theatres suffered from heat rash and heat intolerance. It is a modality of sweat retention syndrome encountered in hot, humid tropical environments. It is characterized by anhidrosis and miliaria-like rashes, accompanied by symptoms of heat congestion, such as palpitation, rapid pulsation, hyperthermia, headache, weakness and gradually to rapidly progressing inability to tolerate physical activity in the heat.
It is usually preceded by widespread miliaria rubra. The initial and essential treatment of miliaria and sweat retention syndrome is to transfer the affected person to a cool environment.
Application of chemical bacteriostats is effective in preventing the expansion of microflora, and is preferable to the use of antibiotics, which may lead these micro-organisms to acquire resistance. The impactions in the sweat duct slough off after about 3 weeks as a result of epidermal renewal. In this article, a variety of techniques are described which can be used to minimize the incidence of heat disorders and reduce the severity of cases when they do occur. Interventions fall into five categories: Factors outside the worksite which may affect thermal tolerance should not be ignored in the evaluation of the extent of exposure and consequently in elaborating preventive strategies.
For example, total physiological burden and the potential susceptibility to heat disorders will be much higher if heat stress continues during off-duty hours through work at second jobs, strenuous leisure activities, or living in unremittingly hot quarters.
In addition, nutritional status and hydration may reflect patterns of eating and drinking, which may also change with season or religious observances. Candidates for hot trades should be generally healthy and possess suitable physical attributes for the work to be done. Obesity and cardiovascular disease are conditions that add to the risks, and individuals with a history of previous unexplained or repetitive heat illness should not be assigned to tasks involving severe heat stress.
Various physical and physiological characteristics which may affect heat tolerance are discussed below and fall into two general categories: Workers should be informed of the nature of heat stress and its adverse effects as well as the protective measures provided in the workplace. They should be taught that heat tolerance depends to a large extent upon drinking enough water and eating a balanced diet. In addition, workers should be taught the signs and symptoms of heat disorders, which include dizziness, faintness, breathlessness, palpitations and extreme thirst.
They should also learn the basics of first aid and where to call for help when they recognize these signs in themselves or others. Management should implement a system for reporting heat- related incidents at work. Children and very small adults face two potential disadvantages for work in hot environments. First, externally imposed work represents a greater relative load for a body with a small muscle mass, inducing a greater rise in core body temperature and more rapid onset of fatigue.
In addition, the higher surface-to-mass ratio of small people may be a disadvantage under extremely hot conditions. These factors together may explain why men weighing less than 50 kg were found to be at increased risk for heat illness in deep mining activities.
Early laboratory studies on women seemed to show that they were relatively intolerant to work in heat, compared with men. However, we now recognize that nearly all of the differences can be explained in terms of body size and acquired levels of physical fitness and heat acclimatization.
However, there are minor sex differences in heat dissipation mechanisms: Although the menstrual cycle is associated with a shift in basal body temperature and slightly alters thermoregulatory responses in women, these physiological adjustments are too subtle to influence heat tolerance and thermoregulatory efficiency in real work situations.
When allowance is made for individual physique and fitness, men and women are essentially alike in their responses to heat stress and their ability to acclimatize to work under hot conditions. For this reason, selection of workers for hot jobs should be based on individual health and physical capacity, not gender.
Very small or sedentary individuals of either sex will show poor tolerance for work in heat. Severe maternal hyperthermia over-heating due to illness appears to increase the incidence of foetal malformation, but there is no evidence of a similar effect from occupational heat stress. Although various ethnic groups have originated in differing climates, there is little evidence of inherent or genetic differences in response to heat stress.
All humans appear to function as tropical animals; their ability to live and work in a range of thermal conditions reflects adaptation through complex behaviour and development of technology. Seeming ethnic differences in response to heat stress probably relate to body size, individual life history and nutritional status rather than to inherent traits. Industrial populations generally show a gradual decline in heat tolerance after age There is some evidence of an obligatory, age-associated reduction in cutaneous vasodilatation widening of the cavity of blood vessels of the skin and maximal sweat rate, but most of the change can be attributed to alterations in lifestyle which reduce physical activity and increase the accumulation of body fat.
Age does not appear to impair heat tolerance or ability to acclimatize if the individual maintains a high level of aerobic conditioning. However, ageing populations are subject to increasing incidence of cardiovascular disease or other pathologies which may impair individual heat tolerance. As noted above, early findings of group differences in heat tolerance which were attributed to gender, race or age are now viewed as manifestations of aerobic capacity and heat acclimatization.
In some cases activity on the job may provide the necessary physical training, but most industrial jobs are less strenuous and require supplementation through a regular exercise programme for optimal fitness. Loss of aerobic capacity detraining is relatively slow, so that weekends or vacations of 1 to 2 weeks cause only minimal changes.
Serious declines in aerobic capacity are more likely to occur over weeks to months when injury, chronic illness or other stress causes the individual to change lifestyle. Acclimatization to work in heat can greatly expand human tolerance for such stress, so that a task which is initially beyond the capability of the unacclimatized person may become easier work after a period of gradual adjustment. Individuals with a high level of physical fitness generally display partial heat acclimatization and are able to complete the process more quickly and with less stress than sedentary persons.
Season may also affect the time which must be allowed for acclimatization; workers recruited in summer may already be partly heat acclimatized, while winter hires will require a longer period of adjustment.
In most situations, acclimatization can be induced through gradual introduction of the worker to the hot task. For instance, the new recruit may be assigned to hot work only in the morning or for gradually increasing time periods during the first few days.
Such acclimatization on the job should take place under close supervision by experienced personnel; the new worker should have standing permission to withdraw to cooler conditions any time symptoms of intolerance occur. Extreme conditions may warrant a formal protocol of progressive heat exposure such as that used for workers in the South African gold mines.
Maintenance of full heat acclimatization requires exposure to work in heat three to four times per week; lower frequency or passive exposure to heat have a much weaker effect and may allow gradual decay of heat tolerance. However, weekends off work have no measurable effect on acclimatization.
High body fat content has little direct effect on thermoregulation, as heat dissipation at the skin involves capillaries and sweat glands which lie closer to the skin surface than the subcutaneous fat layer of skin.
However, obese persons are handicapped by their excess body weight because every movement requires greater muscular effort and therefore generates more heat than in a lean person.
In addition, obesity often reflects an inactive lifestyle with resulting lower aerobic capacity and absence of heat acclimatization. Medical conditions and other stresses. Examples include febrile illness higher than normal body temperature , recent immunization, or gastroenteritis with associated disturbance of fluid and electrolyte balance. Skin conditions such as sunburn and rashes may limit ability to secrete sweat.
In addition, susceptibility to heat illness may be increased by prescription medications, including sympathomimetics, anticholinergics, diuretics, phenothiazines, cyclic antidepressants, and monoamine-oxidase inhibitors. Alcohol is a common and serious problem among those who work in heat.
Alcohol not only impairs intake of food and water, but also acts as a diuretic increase in urination as well as disturbing judgement. The adverse effects of alcohol extend many hours beyond the time of intake. Alcoholics who suffer heat stroke have a far higher mortality rate than non-alcoholic patients. Evaporation of sweat is the main path for dissipating body heat and becomes the only possible cooling mechanism when air temperature exceeds body temperature. Water requirements cannot be reduced by training, but only by lowering the heat load on the worker.
Human water loss and rehydration have been extensively studied in recent years, and more information is now available. A human weighing 70 kg can sweat at a rate of 1. Such loss would be incapacitating unless at least part of the water were replaced during the work shift. However, since water absorption from the gut peaks at about 1. Drinking to satisfy thirst is not enough to keep a person well hydrated. Most people do not become aware of thirst until they have lost 1 to 2 l of body water, and persons highly motivated to perform hard work may incur losses of 3 to 4 l before clamorous thirst forces them to stop and drink.
Paradoxically, dehydration reduces the capacity to absorb water from the gut. Therefore, workers in hot trades must be educated regarding the importance of drinking enough water during work and continuing generous rehydration during off-duty hours.
Management must provide ready access to water or other appropriate drinks which encourage rehydration. The following details are a vital part of any programme for hydration maintenance:.
Flavourings may be used to improve the acceptance of water. For this reason it is better to offer water or dilute, flavoured beverages and to avoid carbonation, caffeine and drinks with heavy concentrations of sugar or salt.
Although sweat is hypotonic lower salt content compared to blood serum, high sweat rates involve a continuous loss of sodium chloride and small amounts of potassium, which must be replaced on a daily basis. In addition, work in heat accelerates the turnover of trace elements including magnesium and zinc.
All of these essential elements should normally be obtained from food, so workers in hot trades should be encouraged to eat well-balanced meals and avoid substituting candy bars or snack foods, which lack important nutritional components.
Some diets in industrialized nations include high levels of sodium chloride, and workers on such diets are unlikely to develop salt deficits; but other, more traditional diets may not contain adequate salt. Under some conditions it may be necessary for the employer to provide salty snacks or other supplementary foods during the work shift.
The vital component of any beverage is water, but electrolyte drinks may be useful in persons who have already developed significant dehydration water loss combined with electrolyte depletion salt loss. These drinks are generally high in salt content and should be mixed with equal or greater volumes of water before consumption. A much more economical mixture for oral rehydration can be made according to the following recipe: Workers should not be given salt tablets, as they are easily abused, and overdoses lead to gastro-intestinal problems, increased urine output and greater susceptibility to heat illness.
The common goal of modification to work practices is to lower time-averaged heat stress exposure and to bring it within acceptable limits.
This can be accomplished by reducing the physical workload imposed on an individual worker or by scheduling appropriate breaks for thermal recovery. Individual effort levels can be lowered by reducing external work such as lifting, and by limiting required locomotion and static muscle tension such as that associated with awkward posture.
These goals may be reached by optimizing task design according to ergonomic principles, providing mechanical aids or dividing the physical effort among more workers. The simplest form of schedule modification is to allow individual self-pacing. This ability to voluntarily adjust work rate probably depends on awareness of cardiovascular stress and fatigue.
Human beings cannot consciously detect elevations in core body temperature; rather, they rely on skin temperature and skin wettedness to assess thermal discomfort. An alternative approach to schedule modification is the adoption of prescribed work-rest cycles, where management specifies the duration of each work bout, the length of rest breaks and the number of repetitions expected.
Thermal recovery takes much longer than the period required to lower respiratory rate and work-induced heart rate: Lowering core temperature to resting levels requires 30 to 40 min in a cool, dry environment, and takes longer if the person must rest under hot conditions or while wearing protective clothing. If a constant level of production is required, then alternating teams of workers must be assigned sequentially to hot work followed by recovery, the latter involving either rest or sedentary tasks performed in a cool place.
If cost were no object, all heat stress problems could be solved by application of engineering techniques to convert hostile working environments to hospitable ones. A wide variety of techniques may be used depending on the specific conditions of the workplace and available resources.
Traditionally, hot industries can be divided into two categories: In hot-dry processes, such as metal smelting and glass production, workers are exposed to very hot air combined with strong radiant heat load, but such processes add little humidity to the air. In contrast, warm-moist industries such as textile mills, paper production and mining involve less extreme heating but create very high humidities due to wet processes and escaped steam. The most economical techniques of environmental control usually involve reduction of heat transfer from the source to the environment.
Hot air may be vented outside the work area and replaced with fresh air. Hot surfaces can be covered with insulation or given reflective coatings to reduce heat emissions, simultaneously conserving heat which is needed for the industrial process.
A second line of defence is large-scale ventilation of the work area to provide a strong flow of outside air. The most expensive option is air conditioning to cool and dry the atmosphere in the workplace. Although lowering air temperature does not affect transmission of radiant heat, it does help to reduce the temperature of the walls and other surfaces which may be secondary sources of convective and radiative heating.
When overall environmental control proves impractical or uneconomical, it may be possible to ameliorate thermal conditions in local work areas. Local or even portable reflective shielding may be interposed between the worker and a radiant heat source.
Alternatively, modern engineering techniques may allow construction of remote systems to control hot processes so that workers need not suffer routine exposure to highly stressful heat environments.
For instance, a spring heat wave can precipitate an epidemic of heat illness among workers who are not yet heat acclimatized as they would be in summer.
Management should therefore implement a system for predicting weather-related changes in heat stress so that timely precautions can be taken. Work in extreme thermal conditions may require personal thermal protection in the form of specialized clothing.
Passive protection is provided by insulative and reflective garments; insulation alone can buffer the skin from thermal transients. Reflective aprons may be used to protect personnel who work facing a limited radiant source. Another form of passive protection is the ice vest, which is loaded with slush or frozen packets of ice or dry ice and is worn over an undershirt to prevent uncomfortable chilling of the skin. The phase change of the melting ice absorbs part of the metabolic and environmental heat load from the covered area, but the ice must be replaced at regular intervals; the greater the heat load, the more frequently the ice must be replaced.
Ice vests have proven most useful in deep mines, ship engine rooms, and other very hot, humid environments where access to freezers can be arranged.
Active thermal protection is provided by air- or liquid-cooled garments which cover the entire body or some portion of it, usually the torso and sometimes the head. The simplest systems are ventilated with the surrounding, ambient air or with compressed air cooled by expansion or passage through a vortex device. Air cooling can theoretically take place through convection temperature change or evaporation of sweat phase change. However, the effectiveness of convection is limited by the low specific heat of air and the difficulty in delivering it at low temperatures in hot surroundings.
Most air-cooled garments therefore operate through evaporative cooling. The worker experiences moderate heat stress and attendant dehydration, but is able to thermoregulate through natural control of the sweat rate. Air cooling also enhances comfort through its tendency to dry the underclothing. Disadvantages include 1 the need to connect the subject to the air source, 2 the bulk of air distribution garments and 3 the difficulty of delivering air to the limbs.
These systems circulate a water-antifreeze mixture through a network of channels or small tubes and then return the warmed liquid to a heat sink which removes the heat added during passage over the body. The heat sink may dissipate thermal energy to the environment through evaporation, melting, refrigeration or thermoelectric processes.
Liquid-cooled garments offer far greater cooling potential than air systems. A full-coverage suit linked to an adequate heat sink can remove all metabolic heat and maintain thermal comfort without the need to sweat; such a system is used by astronauts working outside their spacecraft.
However, such a powerful cooling mechanism requires some type of comfort control system which usually involves manual setting of a valve which shunts part of the circulating liquid past the heat sink. Liquid-cooled systems can be configured as a back pack to provide continuous cooling during work. Any cooling device which adds weight and bulk to the human body, of course, may interfere with the work at hand. For instance, the weight of an ice vest significantly increases the metabolic cost of locomotion, and is therefore most useful for light physical work such as watch-standing in hot compartments.
Systems which tether the worker to a heat sink are impractical for many types of work. Intermittent cooling may be useful where workers must wear heavy protective clothing such as chemical protective suits and cannot carry a heat sink or be tethered while they work.
Removing the suit for each rest break is time consuming and involves possible toxic exposure; under these conditions, it is simpler to have the workers wear a cooling garment which is attached to a heat sink only during rest, allowing thermal recovery under otherwise unacceptable conditions. The human body exchanges heat with its environment by various pathways: Conduction is the transmission of heat between two solids in contact. Such exchanges are observed between the skin and clothing, footwear, pressure points seat, handles , tools and so on.
In practice, in the mathematical calculation of thermal balance, this heat flow by conduction is approximated indirectly as a quantity equal to the heat flow by convection and radiation which would take place if these surfaces were not in contact with other materials. Convection is the transfer of heat between the skin and the air surrounding it. Air circulation, known as natural convection, is thus established at the surface of the body.
This exchange becomes greater if the ambient air passes over the skin at a certain speed: Moreover, it receives the radiation emitted by neighbouring surfaces. F clR is the factor by which clothing reduces radiation heat exchange. This expression may be replaced by a simplified equation of the same type as that for exchanges by convection:.
Every wet surface has on it a layer of air saturated with water vapour. If the atmosphere itself is not saturated, the vapour diffuses from this layer towards the atmosphere. The layer then tends to be regenerated by drawing on the heat of evaporation 0. P sk,s is the saturated pressure of water vapour at the temperature of the skin expressed in kPa.
P a is the ambient partial pressure of water vapour expressed in kPa. F pcl is the factor of reduction of exchanges by evaporation due to clothing. A correction factor operates in the calculation of heat flow by convection, radiation and evaporation so as to take account of clothing.
In the case of cotton clothing, the two reduction factors F clC and F clR may be determined by:. As regards the reduction of heat transfer by evaporation, the correction factor F pcl is given by the following expression:.
An insulation of 1 clo corresponds to 0. ISO standard gives the thermal insulation provided by different combinations of clothing. In the case of special protective clothing that reflects heat or limits permeability to vapour under conditions of heat exposure, or absorbs and insulates under conditions of cold stress, individual correction factors must be used.
To date, however, the problem remains poorly understood and the mathematical predictions remain very approximate. The appliances and methods for measuring these physical parameters of the environment are the subject of ISO standard , which describes the different types of sensor to use, specifies their range of measurement and their accuracy, and recommends certain measurement procedures.
This section summarizes part of the data of that standard, with particular reference to the conditions of use of the most common appliances and apparatus. There are numerous types of thermometers on the market.
Mercury thermometers are the most common. Their advantage is accuracy, provided that they have been correctly calibrated originally. Their main disadvantages are their lengthy response time and lack of automatic recording ability. Electronic thermometers, on the other hand, generally have a very short response time 5 s to 1 min but may have calibration problems. Whatever the type of thermometer, the sensor must be protected against radiation. This is generally ensured by a hollow cylinder of shiny aluminium surrounding the sensor.
Such protection is ensured by the psychrometer, which will be mentioned in the next section. The saturated water vapour pressure P S,t at any temperature t is given by:. The psychrometric diagram figure The parameters of humidity most often used in practice are:.
The range of measurement and the accuracy recommended are 0. The mean radiant temperature t r has been defined previously; it can be determined in three different ways:. The black sphere thermometer consists of a thermal probe, the sensitive element of which is placed at the centre of a completely closed sphere, made of a metal that is a good conductor of heat copper and painted matt black so as to have a coefficient of absorption in the infrared zone close to 1.
The sphere is positioned in the workplace and subjected to exchanges by convection and radiation. The temperature of the globe t g then depends on the mean radiant temperature, the air temperature and the air velocity. For a standard black globe 15 cm in diameter, the mean temperature of radiation can be calculated from the temperature of the globe on the basis of the following expression:.
In practice, the need must be stressed to maintain the emissivity of the globe close to 1. The main limitation of this type of globe is its long response time of the order of 20 to 30 min, depending on the type of globe used and the ambient conditions. The measurement is valid only if the conditions of radiation are constant during this period of time, and this is not always the case in an industrial setting; the measurement is then inaccurate. These response times apply to globes 15 cm in diameter, using ordinary mercury thermometers.
They are shorter if sensors of smaller thermal capacity are used or if the diameter of the globe is reduced. The equation above must therefore be modified to take account of this difference in diameter. The WBGT index makes direct use of the temperature of the black globe. It is then essential to use a globe 15 cm in diameter.
On the other hand, other indices make use of the mean radiant temperature. A smaller globe can then be selected to reduce the response time, provided that the equation above is modified to take account of it. The air velocity must be measured disregarding the direction of air flow. Otherwise, the measurement must be undertaken in three perpendicular axes x, y and z and the global velocity calculated by vectorial summation:. The range of measurements recommended by ISO standard extends from 0.
It should be measured as a 1- or 3-min average value. There are two categories of appliances for measuring air velocity: The measurement is carried out by counting the number of turns made by the vanes during a certain period of time.
In this way the mean velocity during that period of time is obtained in a discontinuous manner. These anemometers have two main disadvantages:. They are very directional and have to be oriented strictly in the direction of the air flow. When this is vague or unknown, measurements have to be taken in three directions at right angles. The range of measurement extends from about 0.
This limitation to low velocities is important when, for instance, it is a question of analysing a thermal comfort situation where it is generally recommended that a velocity of 0. They are appliances giving an instantaneous estimate of speed at one point of space: These appliances are also often very directional, and the remarks above also apply. Finally, the measurement is correct only from the moment when the temperature of the appliance has reached that of the environment to be evaluated.
This response can be powerful and effective, but it can also produce a strain on the body which leads to discomfort and eventually to heat illness and even death. It is important therefore to assess hot environments to ensure the health and safety of workers. Heat stress indices provide tools for assessing hot environments and predicting likely thermal strain on the body. Limit values based upon heat stress indices will indicate when that strain is likely to become unacceptable.
The mechanisms of heat stress are generally understood, and work practices for hot environments are well established. These include knowledge of the warning signs of heat stress, acclimatization programmes and water replacement.
There are still many casualties, however, and these lessons seem to have to be relearned. In , Leithead and Lind described an extensive survey and concluded that heat disorders occur for one or more of the following three reasons:. They concluded that many deaths can be attributed to neglect and lack of consideration and that even when disorders do occur, much can be done if all the requirements for the correct and prompt remedial treatment are available.
A heat stress index is a single number which integrates the effects of the six basic parameters in any human thermal environment such that its value will vary with the thermal strain experienced by the person exposed to a hot environment. The index value measured or calculated can be used in design or in work practice to establish safe limits. Much research has gone into determining the definitive heat stress index, and there is discussion about which is best.
For example, Goldman presents 32 heat stress indices, and there are probably at least double that number used throughout the world. Many indices do not consider all six basic parameters, although all have to take them into conside ration in application. The use of indices will depend upon individual contexts, hence the production of so many.
Some indices are inadequate theoretically but can be justified for specific applications based on experience in a particular industry. The recent surge in standardization e. It will be necessary, however, to gain experience with the use of any new index.
Most heat stress indices consider, directly or indirectly, that the main strain on the body is due to sweating. For example, the more sweating required to maintain heat balance and internal body temperature, the greater the strain on the body. For an index of heat stress to represent the human thermal environment and predict heat strain, a mechanism is required to estimate the capacity of a sweating person to lose heat in the hot environment.
An index related to evaporation of sweat to the environment is useful where persons maintain internal body temperature essentially by sweating.
These conditions are generally said to be in the prescriptive zone WHO Hence deep body temperature remains relatively constant while heart rate and sweat rate rise with heat stress. At the upper limit of the prescriptive zone ULPZ , thermoregulation is insufficient to maintain heat balance, and body temperature rises.
This is termed the environmentally driven zone WHO In this zone heat storage is related to internal body temperature rise and can be used as an index to determine allowable exposure times e.
Heat stress indices can be conveniently categorized as rational, empirical or direct. Rational indices are based upon calculations involving the heat balance equation; empirical indices are based on establishing equations from the physiological responses of human subjects e.
The most influential and widely used heat stress indices are described below. The Heat Stress Index is the ratio of evaporation required to maintain heat balance Ereq to the maximum evaporation that could be achieved in the environment Emax , expressed as a percentage Belding and Hatch Equations are provided in table The HSI as an index therefore is related to strain, essentially in terms of body sweating, for values between 0 and Mild to moderate heat strain.
Little effect on physical work but possible effect on skilled work. Severe heat strain, involving threat to health unless physically fit. Very severe heat strain. Personnel should be selected by medical examination. Ensure adequate water and salt intake. An important improvement is the recognition that not all sweat evaporates. An important improvement is the recognition that not all sweat evaporates e. For outdoor conditions with solar load, T g is replaced with T a and allowance made for solar load R s by:.
Similar to the other rational indices, SW req is derived from the six basic parameters air temperature T a , radiant temperature T r , relative humidity air velocity v , clothing insulation I cl , metabolic rate M and external work W. From this the evaporation required is calculated from:. Equations are provided for each component see table From the required evaporation E req and maximum evaporation E max and sweating efficiency r , the following are calculated:.
If the men are clothed, increase the wet bulb temperature by 1. The P4SR is determined from figure The P4SR is then:. Givoni and Goldman provide equations for predicting heart rate of persons soldiers in hot environments. They define an index for heart rate IHR from a modification of predicted equilibrium rectal temperature,. The equilibrium heart rate HR f in beats per minute is then given by:. This index calculated sweating required for heat balance from an improved heat balance equation but, most importantly, also provided a practical method of interpretation of calculations by comparing what is required with what is physiologically possible and acceptable in humans.
Extensive discussions and laboratory and industrial evaluations CEC of this index led to it being accepted as International Standard ISO b. Differences between observed and predicted responses of workers led to the inclusion of cautionary notes concerning methods of assessing dehydration and evaporative heat transfer through clothing in its adoption as a proposed European Standard prEN Essentially, if what is calculated as required can be achieved, then these are predicted values e.
If they cannot be achieved, the maximum values can be taken e. More detail is given in a decision flow chart see figure If required sweat rate can be achieved by persons and it will not cause unacceptable water loss, then there is no limit due to heat exposure over an 8-hour shift.
If not, the duration-limited exposures DLE are calculated from the following:. If the above are not satisfied, then:.
Fuller details are given in ISO b. More developments with this approach can be made; however, an alternative approach is to use a thermal model. Givoni and Goldman , also provide empirical prediction models for the assessment of heat stress. The Effective Temperature index Houghton and Yaglou was originally established to provide a method for determining the relative effects of air temperature and humidity on comfort. Three subjects judged which of two climatic chambers was warmer by walking between the two.
Using different combinations of air temperature and humidity and later other parameters , lines of equal comfort were determined. Immediate impressions were made so the transient response was recorded. This had the effect of over-emphasizing the effect of humidity at low temperatures and underestimating it at high temperatures when compared with steady-state responses.
Although originally a comfort index, the use of the black globe temperature to replace dry bulb temperature in the ET nomograms provided the Corrected Effective Temperature CET Bedford Research reported by Macpherson suggested that the CET predicted physiological effects of increasing mean radiant temperature.
ET and CET are now rarely used as comfort indices but have been used as heat stress indices. It is the amount of sweat secreted by fit, acclimatized young men exposed to the environment for 4 hours while loading guns with ammunition during a naval engagement.
The single number index value which summarizes the effects of the six basic parameters is an amount of sweat from the specific population, but it should be used as an index value and not as an indication of an amount of sweat in an individual group of interest. It was acknowledged that outside of the prescriptive zone e. The P4SR nomograms figure The P4SR appears to have been useful under the conditions for which it was derived; however, the effects of clothing are over-simplified and it is most useful as a heat storage index.
Heart rate prediction as an index Fuller and Brouha proposed a simple index based on the prediction of heart rate HR in beats per minute. Body temperature and pulse rates are measured during recovery following a work cycle or at specified times during the working day.
At the end of a work cycle the worker sits on a stool, oral temperature is taken and the following three pulse rates are recorded:. The ultimate criterion in terms of heat strain is an oral temperature of The component of thermal strain possible heat stress index can be calculated from:.
It was developed in a US Navy investigation into heat casualties during training Yaglou and Minard as an approximation to the more cumbersome Corrected Effective Temperature CET , modified to account for the solar absorptivity of green military clothing. WBGT limit values were used to indicate when military recruits could train. It was found that heat casualties and time lost due to cessation of training in the heat were both reduced by using the WBGT index instead of air temperature alone.
The specification of the measuring instruments is provided in the standard, as are WBGT limit values for acclimatized or non- acclimatized persons see table For example, for a resting acclimatized person in 0.
The simplicity of the index and its use by influential bodies has led to its widespread acceptance. Like all direct indices it has limitations when used to simulate human response, and should be used with caution in practical applications. It is possible to buy portable instruments which determine the WBGT index e.
Dasler , provides WBGT limit values based on a prediction of exceeding any two physiological limits from experimental data of impermissible strain. The limits are given by:. This index therefore uses the WBGT direct index in the environmentally driven zone see figure The temperature of a wet black globe of appropriate size can be used as an index of heat stress. The principle is that it is affected by both dry and evaporative heat transfer, as is a sweating man, and the temperature can then be used, with experience, as a heat stress index.
Olesen describes WGT as the temperature of a 2. The temperature is read when equilibrium is reached after about 10 to 15 minutes of exposure. It is a 3-inch The sensing element of a thermometer is located at the centre of the sphere, and the temperature is read on a colour coded dial. Lind proposed a simple, direct index used for storage- limited heat exposure and based on a weighted summation of aspirated wet bulb temperature T wb and dry bulb temperature T db:.
Allowable exposure times for mine rescue teams were based on this index. It is widely applicable but is not appropriate where there is significant thermal radiation. NIOSH provides a comprehensive description of working practices for hot environments, including preventive medical practices. A proposal for medical supervision of individuals exposed to hot or cold environments is provided in ISO CD It should always be remembered that it is a basic human right, which was affirmed by the Declaration of Helsinki, that, when possible, persons can withdraw from any extreme environment without need of explanation.
Where exposure does take place, defined working practices will greatly improve safety. It is a reasonable principle in environmental ergonomics and in industrial hygiene that, where possible, the environmental stressor should be reduced at the source. NIOSH divides control methods into five types. These are presented in table Reduce surface temperatures or place reflective shield between radiant source and workers.
Change emissivity of surface. Use doors which open only when access required. Increase air movement, decrease water vapour pressure. Use fans or air conditioning. Wet clothing and blow air across person. Perform jobs at cooler times of day and year. Provide cool areas for rest and recovery. Extra personnel, worker freedom to interrupt work, increase water intake. Keep workers physically fit. Ensure water loss is replaced and maintain electrolyte balance if necessary.
Supervisors trained in recognizing signs of heat illness and in first aid. Basic instruction to all personnel on personal precautions, use of protective equipment and effects of non-occupational factors e.