Heat warnings have stretched across Canada this summer, from Atlantic Canada and southern British Columbia to the Prairies, central Canada and the Northwest Territories.

During one mid-July heat wave, humidex readings reached up to 45 C across parts of southern Ontario and western Québec, according to Environment Canada.

For millions of Canadians, those warnings changed day-to-day plans. For many workers, though, the workday continued on construction sites, farms and loading docks, and inside factories, warehouses and restaurant kitchens.

The most visible workplace consequences are heat exhaustion and heat stroke. But heat also creates a less obvious and poorly understood risk: it makes ordinary workplace injuries more likely.

Heat turns routine hazards into injuries

High temperatures cause dehydration and fatigue and impair attention, judgment, balance and reaction time. Research on California workers shows that heat increases injuries seemingly unrelated to temperature, including falls or vehicle crashes — patterns consistent with diminished cognition and attention.

Official counts likely understate the problem, since heat is often the contributing factor rather than the injury named on the claim form.

Canadian evidence shows how easily this burden can hide in plain sight. A peer-reviewed study linked 374,078 accepted workers’ compensation claims in Québec with daily temperatures from 2003 to 2010.

The study found that, for every 1 C increase in the........

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