Portland had an air quality index of over 200 on Friday — more than that of any other major city in the world, according to IQAir, a Swiss air quality technology company that operates a real-time air pollution monitoring platform.
Seattle ranked second, with an AQI of around 190.
Vancouver, British Columbia, came in third, and San Francisco took fourth place. Both cities had AQIs above 180.
An AQI measurement of zero to 50 is considered good. One of 50 to 100 is moderate. An index of 100 to 150 is unhealthy for sensitive groups, and over 150 is unhealthy for everyone. Over 200 is considered very unhealthy.
Officials urged residents in polluted areas to stay indoors as much as possible while air quality remains poor.