Why is the sky yellow in northeastern US? Canadian wildfire smoke triggers air quality alerts in Massachusetts

Canadian wildfire smoke from Ontario turned skies yellow over Boston and the northeastern US.

Massachusetts Department of Environmental Protection (MassDEP) issued an air quality alert through Thursday for high PM2.5 levels.

Sensitive groups including people with heart or lung disease, older adults, children, and active individuals are advised to reduce outdoor exertion.

Over 830 wildfires were burning in Canada as of midday Wednesday per the Canadian Wildland Fire Information System.

Air quality alerts are in effect from New York to Washington, DC and west to the Great Lakes.