Smoke blanketed large areas of Ontario and Quebec on Tuesday, while an orange haze hung over much of the north-eastern US, according to BBC.
Some cities including Toronto and New York had among the worst air quality in any city in the world overnight.
Much of the smoke is coming from Quebec, where 160 fires are burning.
Canadian officials say the country is shaping up for its worst wildfire season on record.
Experts have pointed to a warmer and drier spring than normal as the reason behind the trend. These conditions are projected to continue throughout the summer.
Environment Canada issued its strongest air quality warning for Ottawa on Tuesday, deeming it a "very high risk" to people's health.
In Toronto and its surrounding areas, the air quality was classified as "high risk".
Meanwhile, the US Environmental Protection Agency (EPA) classified the air quality in much of the north-east as "unhealthy" especially for people who already have respiratory issues.
In total, some 100m people around North America are thought to be under a form of air quality warning.
RHM/PR
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