When it comes to air quality, neighboring countries are in it together. In 2023, wildfire smoke from across the Canadian border became a primary source of air pollution in major U.S. cities, according to a report released this week.

The annual World Air Quality Report by IQAir, a Swiss air quality technology company, showed that U.S. residents enjoyed cleaner air in 2023 than 75 percent of the 134 countries and territories measured. However, the report also found that most of the U.S. had almost double the level of air pollution deemed acceptable by the World Health Organization, or WHO. The overall amount of unhealthy air nationwide crept up slightly from the previous year, but some cities, such as Milwaukee, saw up to a 50 percent increase. The report found that although air quality still suffered from the usual climate-change worsening culprits, such as fossil fuel industries, smoke from Canadian wildfires was behind many of these spikes.

Extended exposure to air pollution is deadly, causing more than 8 million estimated deaths worldwide every year, and has been linked to a myriad of health problems, such as respiratory diseases and cancers. Studies have shown days with higher air pollution can lower student test scores and spike emergency room visits for heart problems.

“We really want to encourage people to treat air quality just like they would treat the weather, look to see what the air quality is before you spend extensive time outdoors,” Christi Chester Schroeder, an air quality science manager at IQAir, told Grist.

For its report, IQAir collected data from over 30,000 monitoring stations around the world. Annual pollution averages for each country and territory were based on measured amounts of PM2.5, or fine particulate matter 2.5 micrometers or smaller. When inhaled, these tiny, invisible particles can enter the lungs and bloodstreamAccording to guidelines set by WHO, yearly air pollution averages should not exceed 5 micrograms of PM2.5 per cubic meter of air. U.S. residents are exposed to almost double that.

Read the full article about U.S. air quality by Sachi Mulkey at Grist.