'fit', healthy adults, dehydration is responsible for almost all of the deleterious effects of working in the heat...

Pokaż mi serce nie opętane zwodniczymi marzeniami, a pokażę ci człowieka szczęśliwego.

Heavy physical exertion in heat will result in sweat rates of about 1 litre per hour.
Sweat rates of up to 2.2 litres per hour are sustainable over periods of one to two hours in fit, healthy individuals with plenty of access to water. However, the limit of the stomach and gut to absorb water is about 1.6 to 1.8 litres per hour on a continuous bases, so sweat rates in the order of 2.2 litres would be extremely dangerous to the individual's health if allowed to continue for the duration of the shift. Dehydration of 3
to 4% of body weight may result in as much as a 50 % reduction in the work rate in hot environments, while dehydration of only 2% in the same environment will cause retardation of mental performance. Thus, working in heat, when accompanied by dehydration affects safety performance either directly or indirectly. Any diuretic, including alcohol and beverages containing caffeine is likely to adversely affect dehydration levels (Brake et al, 1998: 5).
The ability to work in heat may also be affected by drugs that restrict maximum heart rate or cause vaso-constriction.
Prior fitness - While not as effective as exposure to hot environments, intensive training in normal environmental conditions favours acclimatisation.
Natural heat tolerance - The first studies of heat tolerance in mine recruits was conducted by Dreosti in South Africa. Using a particularly severe testing procedure he showed that 15% of the men were 'heat intolerant', 25% were 'heat tolerant' and the remaining 60% were 'normal'. His observations were based on oral temperatures ranging from 950F (350C) to in excess of 1020F ( 390C) (Wyndham, 1965: 167).
Currently, the approach in South African mines is to conduct a short screening test which consists of a 30-minute bench-stepping exercise with an external work rate of 80W in a controlled environment (wet-bulb: 28.00C; dry-bulb: 29.50C; air velocity: 0.4
m/s). An oral temperature of 37.60C, a rectal temperature of 38.90C or a heart rate of 160 beats/min are upper limits for heat tolerance of an individual, and anyone exceeding these limits during the screening test would be deemed heat intolerant. Kielblock and Schutte (1993: 281) have suggested that heat tolerance could be compromised following heat stroke and that the afflicted worker thus rendered permanently unfit for work in hot environments.
Clothing
As previously described, effective thermoregulation of the body is achieved in large part by evaporation. The cooling effect of sweat relies on adequate air circulation close to the surface of the skin. If the worker wears clothing which impedes air flow and limits effective evaporation of sweat, then the thermal load on the body is greatly increased, even though the environment may be within the normal acceptable limits set by heat stress indices (Gillies, 1991: 3).
Loose fitting clothing, which allows a ready movement of the air, should be worn in hot, humid environments. The amount of skin exposed to radiant heat should be minimal, while specially insulated, reflective clothing should be worn in extreme radiant and convective heat environments. Such conditions commonly exist during mines rescue operations and other fire fighting situations.
Mechanically cooled suits operated by forcing cool air through a vortex have been developed for the purpose of microclimate cooling of the individual rather than the environment (Lahey, 1984: 64). These have been met with mixed reception, and while they work effectively to reduce the core temperature of the individual, they also reduce the mobility of the worker.