Comparison of laboratory studies with predictions of the required sweat rate index (ISO 7933) for climates with moderate to high thermal radiation.

For International Standard ISO 7933 (Hot environments-analytical determination and interpretation of thermal stress using calculation of required sweat rate. International Organization for Standardization, Geneva, 1989) it has been questioned whether the heat stress index "required sweat rate" is applicable in environments where mean radiant temperature (t(r)) exceeds dry air temperature (t(a)). Based on a re-analysis of 556 climatic chamber experiments on 16 subjects the observed sweat rates were compared with required sweat rates predicted according to the ISO standard. Under climatic conditions with t(r) > t(a) the predictions overestimate the observed sweat rates on average by 86% in clothed subjects and by 25% in nude subjects. In climates with t(r) = t(a) the predictions highly agreed with the observed sweat rates in nude subjects and were slightly overestimated (13%) in clothed subjects. The misjudgement of the present ISO standard is obviously caused by an inappropriate calculation of radiative heat exchange. Corrections are proposed to improve the validity concerning climates with t(r) > t(a).