Summary

This study is concerned with all types of thermal stress, from the bitter cold winters typical of northern Québec to the sweltering heat in certain industries. Six thermal indices were analyzed in detail, three applicable to cold, one to thermal comfort and two to extremely hot conditions.

Review of the scientific literature on exposure to cold revealed that phenotypic as well as genotypic adaptations have been noted in people exposed to cold environments, though these are of lesser scope than behavioural adaptations. Together these physiological adjustments, called acclimatization, significantly affect physiological responses measured in terms of dexterity, strength, learning capacity, memory span, response time and working memory. Performance and alertness are poorer in workers who are not acclimatized and these workers may be at greater risk when exposed to extreme cold. It is thus important to know and take into consideration the origins and recent history of working in cold environments of those who face such conditions. 

The preceding notwithstanding, behavioural adaptation, acquired in just a few exposures to cold, is more significant. It is thus crucial to insist on training and protective measures, such as heated shelters and suitable clothing, in a cold exposure management program. In any case, all the literature indicates that a surveillance program is fully warranted, especially in case of cold exposure of workers who are not acclimatized and not accustomed to working in the cold.

Three indexes for cold exposure were examined: the Saskatchewan cold condition guidelines, IREQ’s required insulation guidelines and the wind chill index. IREQ’s guidelines were eliminated because clothing insulation values are required for their use, and these are unavailable. Also, application of these guidelines requires that certain parameters be limited to particular ranges that have still not been established. Though the Saskatchewan cold condition guidelines have been included in the publications of key agencies for the last three decades, their scientific basis could not be traced and this is disquieting. The wind chill index, on the other hand, is not only well known to those working in the field but also very well documented. It does not offer much in the way of prevention measures, however, and ways of developing this more precise index to make it a real prevention tool should be considered.

The Predicted Mean Vote/Predicted Percentage Dissatisfied (PMV/PPD) index expresses thermal comfort, bridging cold and hot conditions. It is typically used for indoor spaces and its determination requires specialized equipment, for measuring black globe temperature (Tg). Our paper, however, identifies certain specific conditions when PMV/PPD can be accurately determined without Tg measurement. This index, which indicates the percentage of people dissatisfied with the thermal environment, will be of interest to those responsible for heating/cooling of indoor spaces as well as industrial hygienists.

Last, for excessively hot environments (WBGT > 32 °C), we recommend the predicted heat strain (PHS) model of ISO standard 7933. The scope of application of this standard, in its current version, does not exceed 39.8 °C-WBGT, but it can nonetheless be validly extended to temperatures above 45 °C if two modifications are made: one concerning maximum sweating of unacclimatized subjects and the other dissipation of accumulated heat. These two modifications correct methodological errors in the existing scope of the standard. The Short-Term Exposure Limit suggested in 2009 is not recommended because of obvious inconsistencies and lack of validation in the scientific literature.

Field observations made it possible to identify the instruments and approaches required to use the recommended indices in all types of environments, from the very hot to the very cold. Interpretation and application limits are also given for each index.