Predicted Heat Strain
Analytical determination and interpretation of heat stress using calculation of the predicted heat strain
The ISO 7933 specifies a method to determine the thermal heat strain experienced by a subject working in a hot environment. It provides a method to calculate the sweat rate and estimate the core temperature. This allows the user to determine how each input parameter affects the above mentioned variable. 
The ISO 7933 International Standard allows for the determination of which parameter or collection of parameters should be modified, and to what extent, to reduce the risk of physiological stresses. The following are the key goals of this International Standard:
- the assessment of thermal stress in situations that are likely to cause an excessive increase in core temperature or water loss in the standard subject;
- determining the maximum exposure time for which the physiological strain is tolerable (no physical damage is to be expected). These exposure times are referred to as "maximum permitted exposure times" in this forecast mode.
The rectal temperature increase must be limited at a maximum value, in the case of non-equilibrium of the thermal balance, such that the likelihood of any harmful impact is extremely low. Finally, regardless of the thermal balance, water loss should be limited to a number, that is compatible with the body's hydromineral equilibrium.
Consequently, our tool reports the maximum allowable exposure times within which the physiological strain is acceptable (no physical damage is to be expected) calculated as a function of:
- rectal temperature;
- water loss of 5% of the body mass for 95% of the population;
- water loss of 7.5% of the body mass for an average person.
The PHS can only be used for:
- dry-bulb temperatures between 15 and 50°C,
- air speeds between 0 and 3 m/s,
- metabolic rates between 1.7 and 6.8 met,
- total clothing insulation between 0.1 and 1.0 clo, and
- when the difference between the mean radiant temperature and the air temperature is between 0 and 60°C.
-  ISO, “ISO 7933 - Ergonomics of the thermal environment — Analytical determination and interpretation of heat stress using calculation of the predicted heat strain.” ISO, Geneva, Switzerland, 2004.