Despite the poor conditions, some residents continued to spend time outdoors, while others wore face masks as the smoke drifted across the city.

The deteriorating air quality coincided with a heat alert, with temperatures reaching 33C, close to the record for 15 July. Authorities cancelled outdoor children's programmes and opened cooling centres, while Environment Canada warned that smoky conditions were unlikely to ease before Friday.

Howard Shapiro, Toronto's Associate Medical Officer of Health, compared exposure to the smoke to passive smoking, warning that the fine particles contain a mixture of harmful chemicals capable of affecting anyone, regardless of age or underlying health. The smoke originated from 148 active wildfires burning across northwestern Ontario, where firefighters continue to battle multiple large blazes.