Summary

Exposure to cold or heat triggers a series of compensatory physiological responses enabling the human body to maintain its internal temperature despite thermal stress. These thermoregulation mechanisms are well documented and the resulting physiological changes can modify organ functions related to the absorption and metabolism of chemical substances. Pharmacological and epidemiological studies report an increase in the absorption and effects of certain drugs, and higher human mortality associated with air pollution, on simultaneous exposure to heat and xenobiotics. Extrapolation of these data to occupational health suggests that concurrent exposure to thermal stress and chemical substances is likely to increase the absorption and effects of some xenobiotics.
The primary objective of this research was to survey all data published in recent scientific literature. The second objective was to identify Quebec workers exposed to chemical substances who might be the most affected by thermal stress. Careful attention was given to the presence of chemical substances likely to affect thermoregulation mechanisms.
The literature review was based on a search of Medline, Toxline and Chemical Abstract bibliographic data from January 1990 to June 2012, the purpose being to document physiological changes associated with thermal stress, concurrent exposures to thermal stress and chemical substances and their effects, workers exposed to chemical substances who are mostly likely to be affected by thermal stress, and chemical substances that can affect thermoregulation mechanisms. An approach relying on professional judgment was then used to identify Quebec workplaces where exposure to thermal stress was likely to modify the toxicokinetics of chemical substances. Thirteen experts in thermal stress or industrial hygiene were consulted individually for purposes of classifying the 136 identified jobs in relation to the significance of the issue under study.
The collected data show that there have been few studies on the impact of exposure to cold on the toxicokinetics and effects of chemical substances. The few studies that were surveyed report that exposure to cold thermal stress usually decreases the toxicity of chemical substances. Exposure to heat, on the other hand, is associated with increased pulmonary and cutaneous absorption of xenobiotics, which in turn is often associated with increased toxicity and concentration in body fluids. Biological monitoring of exposure can be used to demonstrate increased pulmonary or cutaneous absorption of contaminants on exposure to heat. The magnitude of the increase depends on the intensity of the thermal stress, exposure levels and the physico-chemical characteristics of the chemical substances.
Of the occupations most affected by this issue in Quebec, 20 are in the non-metallic mineral manufacturing/primary metal processing/metal product manufacturing industry, as well as roofers and firefighters. These work environments should be prioritized in future research intended to further characterize the risk associated with simultaneous exposure to thermal stress and chemical substances. More specifically, exposure to certain contaminants can affect thermoregulation mechanisms and decrease the ability of workers to adapt to heat. This issue primarily affects workers exposed to lead and its inorganic compounds (dust and fumes), certain pesticides (organophosphates and carbamates) and metal oxide fumes (zinc, aluminum, antimony, cadmium, copper, magnesium, manganese, tin).
This survey, by identifying higher-risk occupations, will help guide occupational health professionals in assessing risk in cases of simultaneous exposure to thermal stress and chemical substances. The data compiled in this report may also be used to develop toxicokinetic models for improved risk assessment. Workplace studies could also be done in order to document, in actual conditions, the effect of heat exposure on the absorption and toxicokinetics of chemical substances. Biological monitoring would be a tool of choice in studying this issue.