Summary

Global warming is undeniable, and its impact on population health is a growing concern among scientists and public policy makers. Specialists are predicting not only that temperatures will rise in Quebec, but also that heat waves and heat emergencies will be longer and more intense. Other environmental parameters may also undergo changes with the increase in outdoor temperatures: for example, ambient concentrations of air pollutants, such as ground-level ozone, could rise.

These changes could have an impact on worker health and safety. The literature reports that exposure to outdoor heat can cause death, disease and possibly an increase in occupational injuries. However, the statistical relationship between outdoor temperature and the incidence of illness or accidents among workers is not well documented and has never been studied in conditions similar to those in Quebec. The same is true regarding the relationship between ozone and acute respiratory disorders in workers, while the effects of simultaneous exposure to heat and ozone have never been studied in this population.

The general objective of our study was to document these associations in a Quebec setting. To do so, we developed statistical models to (a) assess the association between outdoor summer temperature and occupational injuries accepted for compensation by the Commission de la santé et de la sécurité du travail (CSST, now named CNESST) [Quebec workers’ compensation board] in connection with overexposure to heat (sunstroke, fainting, loss of consciousness, etc.) and work-related accidents; (b) explore the association between daily summer levels of tropospheric ozone and occupational injury claims for acute respiratory illnesses accepted for compensation. Last, we also wanted to identify subpopulations, industries and occupations most at risk for occupational injuries related to heat or tropospheric ozone concentrations.

Modelling suggests a log-linear relationship between outdoor summer temperatures in Quebec and daily counts of accepted injury claims for heat-related health problems and work-related accidents. Daily counts of accepted injury claims for heat-related health problems (mean daily number = 0.13, from 0 to 10) and work-related accidents (mean daily number = 306, from 54 to 641) increased an estimated 42% and 0.2%, respectively, for every 1^oC increase in maximum daily temperature (range = −7.8°C to 37.3°C). Analyses also showed that the strength of the associations may vary depending on worker age, industrial sector and occupation’s physical workload (manual labour or other). These results must be interpreted with caution, however, as the study has a number of limitations, including the use of compensation data (which contain little information about the exact circumstances of the occurrence of the injury or health problem) and imprecise workplace temperature and humidity values at the time of injury, which were estimated from regional data.

Given that summer temperatures in Quebec are expected to increase in coming years, it is essential to implement preventive measures specifically targeting those workers most likely to suffer from heat effects and to continue research to advance our knowledge in this field, in particular by using better heat stress indices. The low statistical power of the results regarding the association between acute respiratory illnesses and estimated ozone concentrations precludes drawing any firm conclusions in this respect. As outdoor workers are nonetheless among those most exposed to ground-level ozone and other air pollutants, further study of this subject is clearly warranted.