PORTLAND, Ore. – The city's air quality was downgraded from moderate to unhealthy for all Tuesday evening and remained unhealthy for sensitive groups Wednesday.

The Environmental Protection Agency says everyone should reduce prolonged or heavy exertion, and consider staying inside.

Portland's Air Quality Index (AQI) was 154 on Wednesday evening. A rating higher than 150 is considered unhealthy for everyone.

Related: Check the current AQI

A rating higher than 200 is considered "very unhealthy." AQI measures how many particles are in the air.

For two days last week, Portland's air quality was classified as unhealthy for everyone due to smoke from more than 100 wildfires in British Columbia being blown south to the Rose City.

Portland's AQI of 160 on Aug. 4 was worse than some of the cities most notorious for poor air quality in the world. The hazy smoke made it difficult to see more than a few glimpses of the otherwise blue-sky day, giving metro residents a taste of life in a much more polluted city.

Photos: Wildfire haze in Portland

Compare that to Shanghai, which had an AQI of 117, which is only unhealthy for sensitive groups such as kids and people with respiratory problems. Beijing was experiencing relatively clear air Friday with an AQI of just 60, which is moderately good air quality.

To the south, Mexico City's air quality index was 93, also considered moderately healthy. Los Angeles also had moderate air, with an AQI of 61.

Before the wildfire smoke rolled in, Portland's air quality index was projected to be 50.

The smoke from the British Columbia fires rolled into the Portland area Tuesday evening, starting as a low-lying smog and filling the sky by Wednesday. Thousands of people in western Canada have been evacuated due to the wildfires that have been raging for more than a month.

A fire is also burning in the Mt. Jefferson wilderness.

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The wildfires have caused air quality around Seattle to deteriorate to “among the worst in the country,” state officials told KGW's sister station KING5.

View wildfires, air monitor reports

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Air Quality Index Rating System (photo: Oregon DEQ)
Air Quality Index Rating System (photo: Oregon DEQ)