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Yes. New workers are not the only ones who might be unacclimatized. Workers can lose their heat tolerance during an extended absence (e.g., vacation or sick leave). They can also lose heat acclimatization during the winter, when temperatures are cooler. Existing workers are at increased risk of heat-related illness in these situations:


In the above situations, employers should allow workers to gain heat tolerance gradually. Use the same protection strategies that are used for new workers. Maintain the additional heat protections for at least one week. Unacclimatized workers who feel fine on their first day in warm conditions might develop heat-related illness on a subsequent day.

New workers need time to acclimatize unless they have previously worked in hot environments. To prevent heat-related illnesses, they should work shorter workdays in the heat during their first 1-2 weeks. OSHA and NIOSH recommend the "Rule of 20 percent" for building heat tolerance:

To become acclimatized to heat, workers should perform job tasks that are similar in intensity to their expected work. For example, if a new worker has been hired to lay bricks outdoors in hot weather, then he should lay bricks during his first week. Doing light work may not acclimatize a worker to the demands of their job.

The effect of heat acclimatization on aerobic exercise performance can be quite dramatic, such that acclimatized subjects can easily complete tasks in the heat that earlier were difficult or impossible (Sawka et al., 1996, 2003). For example, the decrement in perform-ance during a self-paced time-trial in the heat is also partly recovered after 1 wk of acclimatization and almost fully restored after 2 wk of acclimatization (Racinais et al., 2015). The improved exercise capability and improved cardiovascular stability likely change in parallel. Figure 3 demonstrates improved aerobic exercise capability and cardiovascular stability with heat acclimatization. When 45 subjects attempted a 20-km march in a desert climate, 20 subjects suffered from syncope during the initial day, while by day 5 of the acclimatization program, no cases of syncope occurred (Bean & Eichna, 1943).

Fluid balance improvements from heat acclimatization include better matching of thirst to body water needs (Bean & Eichna, 1943; Eichna et al., 1945; Périard et al., 2015), increased total body water and increased blood volume (Mack & Nadel, 1996; Sawka & Coyle, 1999). An unacclimatized person may secrete sweat with a sodium concentration of 60 mmol.L-1 or higher and, if sweating profusely, can lose large amounts of sodium (Sawka et al., 1996). With heat acclimatization, the sweat glands conserve sodium by secreting sweat with a sodium concentration as low as 10 mmol.L-1. The retention of sodium is likely an important contributor to the increased total body water (Mack & Nadel, 1996). Athletes need to ensure that they consume adequate amounts of sodium (via food and beverage), particularly early in the acclimatization process, as salt deficits can lead to dehydration despite consuming plenty of fluids (Mack & Nadel, 1996).

Heat acclimatization (or acclimation) is a biological adaptation that reduces physiological strain (e.g., heart rate and body temperatures), improves comfort, improves exercise capacities and reduces the risks of serious heat illness during exposure to heat stress. The biological adaptations include integrated thermoregulatory, cardiovascular, fluid-electrolyte, metabolic and molecular responses. Heat acclimatization occurs when repeated exercise-heat exposures are sufficiently stressful to invoke profuse sweating and elevate body temperatures. Generally about 1-2-wk of daily exposures of 90 min are required; but highly aerobic fit athletes can heat acclimatize in half that time. Heat acclimatization is specific to the climatic heat stress (desert or tropic) and physical exercise intensities the athletes are exposed to, which should simulate the expected competitive environment. There are strategies and suggestions that trainers, coaches and athletes can follow to optimally induce heat acclimatization prior to sports competition.

Sea-level (SL) natives acclimatizing to high altitude (HA) increase their acute ventilatory response to hypoxia (AHVR), but HA natives have values for AHVR below those for SL natives at SL (blunting). HA natives who live at SL retain some blunting of AHVR and have more marked blunting to sustained (20-min) hypoxia. This study addressed the question of what happens when HA natives resident at SL return to HA: do they acclimatize like SL natives or revert to the characteristics of HA natives? Fifteen HA natives resident at SL were studied, together with 15 SL natives as controls. Air-breathing end-tidal Pco(2) and AHVR were determined at SL. Subjects were then transported to 4,300 m, where these measurements were repeated on each of the following 5 days. There were no significant differences in the magnitude or time course of the changes in end-tidal Pco(2) and AHVR between the two groups. We conclude that HA natives normally resident at SL undergo ventilatory acclimatization to HA in the same manner as SL natives.

Altitude is defined on the following scale High (8,000 - 12,000 feet [2,438 - 3,658 meters]), Very High (12,000 - 18,000 feet [3,658 - 5,487 meters]), and Extremely High (18,000+ feet [5,500+ meters]). Since few people have been to such altitudes, it is hard to know who may be affected. There are no specific factors such as age, sex, or physical condition that correlate with susceptibility to altitude sickness. Some people get it and some people don't, and some people are more susceptible than others. Most people can go up to 8,000 feet (2,438 meters) with minimal effect. If you haven't been to high altitude before, it's important to be cautious. If you have been at that altitude before with no problem, you can probably return to that altitude without problems as long as you are properly acclimatized.

The major cause of altitude illnesses is going too high too fast. Given time, your body can adapt to the decrease in oxygen molecules at a specific altitude. This process is known as acclimatization and generally takes 1-3 days at that altitude. For example, if you hike to 10,000 feet (3,048 meters), and spend several days at that altitude, your body acclimatizes to 10,000 feet (3,048 meters). If you climb to 12,000 feet (3,658 meters), your body has to acclimatize once again. A number of changes take place in the body to allow it to operate with decreased oxygen.

AMS is common at high altitudes. At elevations over 10,000 feet (3,048 meters), 75% of people will have mild symptoms. The occurrence of AMS is dependent upon the elevation, the rate of ascent, and individual susceptibility. Many people will experience mild AMS during the acclimatization process. Symptoms usually start 12-24 hours after arrival at altitude and begin to decrease in severity about the third day. The symptoms of Mild AMS are headache, dizziness, fatigue, shortness of breath, loss of appetite, nausea, disturbed sleep, and a general feeling of malaise. Symptoms tend to be worse at night and when respiratory drive is decreased. Mild AMS does not interfere with normal activity and symptoms generally subside within 2-4 days as the body acclimatizes. As long as symptoms are mild, and only a nuisance, ascent can continue at a moderate rate. When hiking, it is essential that you communicate any symptoms of illness immediately to others on your trip. AMS is considered to be a neurological problem caused by changes in the central nervous system. It is basically a mild form of High Altitude Cerebral Edema (see below).

Moderate AMS includes severe headache that is not relieved by medication, nausea and vomiting, increasing weakness and fatigue, shortness of breath, and decreased coordination (ataxia). Normal activity is difficult, although the person may still be able to walk on their own. At this stage, only advanced medications or descent can reverse the problem. Descending even a few hundred feet (70-100 meters) may help and definite improvement will be seen in descents of 1,000-2,000 feet (305-610 meters). Twenty-four hours at the lower altitude will result in significant improvements. The person should remain at lower altitude until symptoms have subsided (up to 3 days). At this point, the person has become acclimatized to that altitude and can begin ascending again. The best test for moderate AMS is to have the person "walk a straight line" heel to toe. Just like a sobriety test, a person with ataxia will be unable to walk a straight line. This is a clear indication that immediate descent is required. It is important to get the person to descend before the ataxia reaches the point where they cannot walk on their own (which would necessitate a litter evacuation).

There are two other severe forms of altitude illness, High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE). Both of these happen less frequently, especially to those who are properly acclimatized. When they do occur, it is usually with people going too high too fast or going very high and staying there. The lack of oxygen results in leakage of fluid through the capillary walls into either the lungs or the brain.

1792, "habituate (something) to a new climate," from French acclimater, verb formed from à "to" (see ad-) + climat (see climate). Intransitive sense "adapt to a new climate" is from 1861. Related: Acclimated; acclimating. The extended form acclimatize is now more common in the older sense of this word (generally in reference to plants or animals), leaving to this word the intransitive sense, which more often refers to humans.

Acclimate, acclimatise, and acclimatize share one of their definitions: to accustom or become accustomed to a new environment or situation. In British and Australian English, acclimatise appears most often and is generally preferred over the alternatives. In American English, acclimatize is recommended by many dictionaries and usage guides, yet acclimate, which is actually the older word, is far more common. Canadian writers tend to use acclimatize. 041b061a72


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