Countering Heat Stress

Fluid Replacement Reccomendations

       Top on lists of medical and governmental prescriptions to combat risks of heat stress is to drink plenty of water to replace fluid lost through sweating.  The amount of water needed is a function of weather, workload and personal physical attributes.  The slower the wind and the greater the temperature, humidity, and direct sunlight, the more water a body needs.  While wind reduces the risk of heat illness by increasing the rate of sweat evaporation, the effect of full sun can be equivalent to a 13 degree increase in air temperature (EPA, A Guide to Heat Stress in Agriculture).  Since the body generates more heat during heavy work than during light or moderate work, the amount of water needed also rises with the strenuousness  of the task performed.  Finally, personal attributes, such as the degree of acclimatization, age, gender, weight and fitness also affect necessary water intake.  Workers who are younger, better rested, and more physically fit are less likely to suffer heat illness.  Also less likely to be ill are workers that have gradually adjusted to working in a warm or hot environment.

       Mainly because thirst does not provide a timely or precise signal of the body's need for water, most workers exposed to hot conditions do not sufficiently match their fluid consumption to sweat loss.  Thirst is not usually perceived until an individual reaches a water deficit of about 2 percent of body weight.  "Numerous investigators report that ad libitum water intake results in incomplete water replacement or voluntary dehydration during exercise and/or heat exposure."  (Sawka, Montain, and Latzka, 1996)  "A worker . . . should not depend on thirst to signal when and how much to drink. Instead, the worker should drink 5 to 7 ounces of fluids every 15 to 20 minutes to replenish the necessary fluids in the body. ["Working in Hot Environments," NIOSH pamphlet , 4/92, p. 4]
 

Additional Readings:

Preventing Heat Stress in Agriculture
Wei Zhao and Ann L. Kersting