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Decay of acclimation and time for re-acclimation.

Ashley CD; Ferron JM
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R03-OH-007836, 2006 Dec; :1-28
Work in a warm or hot environment coupled with high metabolic loads and/or work in protective clothing can bring about considerable heat strain (e.g. increased heart rate and core body temperature). Acclimation by increasing exposure to work in a hot environment is one potent control used to reduce heat stress and decrease signs of heat strain. Documented physiological responses of acclimation include improved circulatory efficiency and enhanced sweating. Once workers are acclimated, physiological adaptations will persist as long as workers remain in the hot environment. Workers may be absent from the hot environment due to training, vacation, illness or normal shift change, and some adaptations may be lost requiring some degree of re-acclimation. OSHA recommends 5 days of re-acclimation for an absence from the heat of two weeks or more. These guidelines seem arbitrary as the decay of acclimation has not been fully explored. Also, re-acclimation has not been thoroughly evaluated. It seems logical that time for re-acclimation will depend on the degree of decay, and the time of complete decay of acclimation has not been established. The basic philosophy of the experimental design was to determine the rate of decay of physiological adaptations of acclimation and the time to re-establish acclimation after complete loss of physiological adaptations. The first goal was to establish acclimation and then to determine the loss pattern of acclimation in 7-day increments over a period of approximately 6 weeks. Using a 3-day plateau in core body temperature as indication of full acclimation, acclimation was complete in approximately 8 days, suggesting that one can become acclimated in approximately 6 days, days 7 and 8 being used to confirm acclimation. To estimate the complete decay of acclimation, first the time for acclimation was considered. Based on the loss of physiological adaptation, it was estimated that on average, physiological adaptations for acclimation are completely lost in 42.5 days, or approximately 6 weeks. The second goal of the study was to identify the time for re-acclimation based on the previous data collected. Based on this pattern, the loss pattern for 1/3 and 2/3 total loss of acclimation (2 weeks and 4 weeks, respectively) was determined. In our group of participants, there was no statistical difference in the time for re-acclimation with an average time for re-acclimation of 4.5 days. Adequate acclimation can have a positive effect on heat tolerance by inducing physiological changes which help to reduce body temperature for a given work rate. Time away from the heat may result in loss of acclimation; increasing the risk of heat illness, injury and death. Our data suggest that complete loss of acclimation occurs with 6 weeks away from the heat. Further, our data suggest that re-acclimation may not be dependent on time away from the heat.
Hot-environments; Heat-acclimatization; Heat-exposure; Heat-stress; Humans; Women; Men; Physiological-stress; Physiological-effects
Candi D. Ashley; University of South Florida, School of Physical Education and Exercise Science, 4202 E. Fowler Ave. - PED214, Tampa, FL 33620-8600
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National Institute for Occupational Safety and Health
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University of South Florida