But this month, Seattle has routinely outpaced Beijing, sometimes with air five times as dirty, reaching an AQI of 150 to 200 for hours or days running, including most of Monday and Tuesday this week.

Meanwhile, Beijing’s rate this week was as low as the 30s, though it rose during the day to top 100. In July, however, the city averaged 44, the seventh-lowest average since 2008.

The reason for the shift? China’s crackdown on bad air, seen as a lag on quality of life and economic development, and widespread wildfires across the west of North America for another summer, in British Columbia, Washington, Oregon, and California. With the winds in the right (or wrong) direction, Seattle, Spokane, and other major cities in the region get covered in smoke and visible ash.

This ratio isn’t a blip. China added new regulations and stepped up enforcement to move business and individuals away from coal, burned both in homes and in industrial operations that surround Beijing, and largely towards natural gas for heating and powering factories. This has led to less pollution, but also hardship—and cold homes—for households during the transition. Natural-gas supplies in China are tight despite a huge increase in domestic production.

Meanwhile, global climate change and further residential settlement into forested areas across North America mean an increasing likelihood of wildfires of the scope seen on average across the last decade on the West Coast. People are the cause of most wildfires—usually by accident—and climate change has led to hotter, drier summers. A long history of preventing fires has also led to woods full of the equivalent of kindling that can go up like an explosion and that burn hotter than fires that occur naturally.

Seattle and other Northwest cities are likely to see these levels of pollutants routinely in future summers, based on winds.

The AQI measures various air pollution sources, picking the highest number among them. With fires and smog, the high numbers almost always count particles smaller than 2.5 microns in diameter in a volume, which can penetrate the lungs and enter the bloodstream. These particles’ diameter are about 3% of the width of a human hair. The so-called PM2.5 reading counts the number of such particles as micrograms per cubic meter of air.

At the highest levels, breathing air outside on a single day can be the equivalent health risk of smoking a couple dozen cigarettes.

At 51 to 100, the risk is mostly for those with a high sensitivity. From 101 to 150, experts recommend children, the elderly, and at-risk individuals with asthma and other conditions avoid exertion. Cross 150 into the “unhealthy” label, and public health officials recommend everyone take care. Above 200, and the situation starts to worsen, with everyone at risk of serious health conditions. The 500 mark is well beyond reasonable concern.