ONORM EN ISO 7933:2024

Ergonomics of the thermal environment - Analytical determination and interpretation of heat stress using calculation of the predicted heat strain (ISO 7933:2023)

This document describes a predicted heat strain (PHS) model that is used to analytically determine and interpret the heat strain (in terms of water loss and rectal temperature) of an average person in a hot environment and the determination of the maximum permissible exposure period within which the physiological stress is acceptable for 95% of the exposed population (the maximum tolerable rectal temperature and the maximum tolerable water loss are not exceeded in 95% of the affected persons). The different conditions used in this prediction model, and particularly in the heat balance, show the influence of the different physical parameters of the environment on the heat load of the average person. In this way, this document makes it possible to determine which parameters or which groups of parameters can be modified and to what extent this can be done in order to reduce the risk of physiological overstress. In its current form, this assessment method cannot be applied to cases where special protective clothing is worn (e.g. fully reflective clothing, clothing with active cooling and ventilation, impermeable coveralls). This document does not predict the physiological response of individuals, but only considers average individuals who are in good health and able to perform their work. It is therefore intended for use by people trained, for example, in the field of ergonomics or industrial hygiene, as the results may require interpretation by specialists. Recommendations on how and when to apply this model can be found in ISO 8025.