Seattle, Los Angeles and San Francisco also recorded some of the world’s worst air pollution, according to IQAir, a Swiss air quality technology company that operates a real-time air pollution monitoring platform.
Since Thursday, Portland’s air quality index has been the highest among all major world cities, per IQAir data. The city’s AQI, or air quality index, has remained over 200, which is considered very unhealthy.
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.
On Saturday, Vancouver, Canada, ranked second on IQAir’s list, with an AQI of 199. Seattle was third, with an AQI of around 180.
Officials have urged residents in polluted areas to stay indoors with the doors and windows closed as much as possible while air quality remains poor.
This story has been updated to reflect Saturday’s data.