Factors affecting heat illness when working in conditions of thermal stress.

In hot working conditions, high sweat rates with excessive loss of body fluids may result in dehydration and electrolyte imbalance. It is well established that dehydration and/or electrolyte disturbances will impair work performance, and, if prolonged or severe, can pose a serious risk to health. The lesser condition of hypohydration is undoubtedly widespread in the workplace, and may be indirectly responsible for less than optimal performance and workplace accidents. With the aid of a new sweat collection method, fluid and electrolyte loss from a population of male workers with varying fitness and body composition has been documented. This has provided the basis for prescribing guidelines of fluid replacement when working in the heat. In addition, the minimum duration of heat exposure required to trigger heat acclimmatization was sought using sweat sodium as an indicator. Rehydration at the rate of 500 ml/h (250 ml every 30 min) is recommended for people working in all but extreme heat (> 45 degrees C). Electrolyte supplements (sodium and potassium) are not generally required in the workplace, but may be warranted in certain circumstances to avoid hyponatremia (> 3 h). The ability to predict the susceptibility of an individual to fluid and electrolyte disturbances cannot be made from age, body composition, ethnicity or VO2max, although a high VO2max appears to enhance heat tolerance. Sodium loss in sweat varies greatly and is not significantly related to sweat rate. Acclimatization results in a significant decrease in sweat sodium and increased sweat rate during summer compared with winter. This advantageous physiological adaptation requires a minimum of 9 h of heat exposure to initiate.