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Acclimatize
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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 competiti