AQI readings above 100 are the key threshold for runners, golfers and others exercising outdoors as wildfire smoke spreads across the northeastern United States and parts of Canada.
The Air Quality Index runs from 0 to 500, with 101-150 labeled unhealthy for sensitive groups, 151-200 unhealthy for everyone, and anything above 200 a signal to avoid going outside.
High-exertion activity becomes riskier first because heavier breathing pulls in more pollutants; even lower-intensity activities such as golf can add exposure through long hours outdoors.
Smoke from wildfires in northern Minnesota and across the Canadian border has drifted into major cities, raising concern that poor air quality could also disrupt outdoor sports including MLB games.