This document describes a model (the predicted heat strain (PHS)model) for the analytical determination and interpretation of the thermal stress (in terms of water loss and rectal temperature) experienced by an average person in a hot environment and determines the “maximum allowable exposure times”, with which the physiological strain is acceptable for 95 % of the exposed population (the maximum tolerable rectal temperature and the maximum tolerable water loss are not exceeded by 95 % of the exposed people).
The various terms used in this prediction model, and in particular in the heat balance, show the influence of the different physical parameters of the environment on the thermal stress experienced by the average person. In this way, this document makes it possible to determine which parameter or group of parameters can be changed, and to what extent, in order to reduce the risk of physiological strains.
In its present form, this method of assessment is not applicable to cases where special protective clothing (such as fully reflective clothing, active cooling and ventilation, impermeable coveralls…) is worn.
The model has not been extensively validated for conditions with unsteady environmental parameters, metabolic rate and/or clothing and therefore must be used cautiously in cases where these parameters vary substantially with time. It does not permit to determine validly the duration of time needed for an average person whose rectal temperature has risen to 38 °C or more, to recover a rectal temperature of 36,8 °C.
This document does not predict the physiological response of an individual person, but only considers average persons in good health and fit for the work they perform. It is therefore intended to be used by ergonomists, industrial hygienists, etc. as the outcomes may require expert interpretations. Recommendations about how and when to use this model are given in ISO 16595/WP