Heavy wildfire smoke sitting on top of the Portland region prompted a statewide air quality alert early Thursday afternoon.

Oregon's Department of Environmental Quality and a handful of regional air quality agencies issued the Sept. 10 advisory for most of the Willamette Valley and Southern Oregon. DEQ officials expect the advisory to stay in effect until at least Monday afternoon.

The alert means people who have medical conditions affected by bad air quality should remain indoors and avoid strenuous activity. Air quality issues caused several regional schools and organizations to cancel activities Thursday and Friday.

Areas closest to fires — such as Marion and Clackamas counties, Eugene, the Rouge Valley and Roseburg — are feeling the worst of the smoke impacts. Air quality is expected to get worse for Portland and its suburbs Thursday night and into Friday, according to DEQ.

At the same time, Swiss air-tech firm IQAir reported Thursday that Portland had the worst air quality of any major city in the world. The company makes the AirVisual app that allows people to monitor their local air quality.

According to the IQAir ranking, Portland and San Francisco, also under a cloud of thick wildfire smoke, top a list of major cities with air quality issues. The list includes Jakarta, Dubai, Delhi, Jerusalem and Mexico City.

The company's Top 10 ranking of bad air quality cities (right now):

1 Portland

2 San Francisco

3 Jakarta, Indonesia

4 Dubai, United Arab Emirates

5 Delhi, India

6 Lahore, Pakistan

7 Nur-Sultan, Kazakhstan

8 Seattle

9 Los Angeles

10 Kuwait City, Kuwait