An analytical index is presented to evaluate the thermal stress of cold environments in terms of a minimal required clothing insulation (IREQ) for the maintenance of an adequate heat balance. IREQ is derived from a heat exchange analysis based on heat transfer equations and physiological criteria available in the literature. IREQ provides an integrated measure of the combined effects of climate (air temperature, mean radiant temperature, humidity, and air velocity) and activity and serves as a guideline for the appropriate choice of an adequate insulation value of clothing. If available clothing insulation is less than IREQ, a duration limited exposure based on acceptable levels of body cooling is calculated.