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 levels 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 bloodstream. According 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.

For most of 2023, PM2.5 levels across the country averaged about 9.1 micrograms per cubic meter of air, with the worst air concentrated in large cities including Washington, D.C., and New York City. The report showed pollution levels spiked in the summer, when hot, stagnant air and sunshine can interact with pollutants to create pockets of unhealthy air. In Washington, D.C., and Chicago, PM2.5 levels more than doubled in June, up to over five times WHO guidelines. Columbus, Ohio, was the most polluted U.S. city for the second year in a row. 

But the IQAir report also contained good news for the U.S.: Aggressive wildfire mitigation efforts seem to be working, which led to a less severe fire season and cleaner air on the West Coast as compared to previous years. In Portland, Oregon, PM2.5 levels dropped by almost 40 percent, while Los Angeles saw a 10 percent decrease. Of the 25 most populated cities in the U.S., Las Vegas had the cleanest air.

According to Schroeder, “A big theme of this year’s report was transboundary haze,” a term that describes when smoke travels across borders. This past summer, Canada endured its worst wildfire season on record. As the blazes tore through 5 percent of the country’s forests, they created huge plumes of soot that drifted into the eastern U.S., blanketing New York City in an orange haze and impacting air quality as far south as Florida.

“The wind is the most efficient transportation system on earth,” said Joel Thornton, a professor of atmospheric chemistry at the University of Washington. Even though large wildfires have become an unsurprising reality, Thornton found that last year’s in Canada were unprecedentedly bad. As forests continue to be unseasonably drier and warmer due to climate change, the stage is set for these fires to get even worse, he said. “It’s a harbinger of what’s to come.” 

Last month, the Environmental Protection Agency finalized new standards for air pollution, bringing the annual average limit down from 12 micrograms of PM2.5 per cubic meter of air to 9 micrograms. The new target still exceeds the WHO’s guidelines of 5 micrograms, but could still bring huge improvements. According to the Biden administration, the new rules would prevent an estimated 4,500 premature deaths every year and save billions in health costs. To reflect their tightened standards, the EPA also updated the Air Quality Index, a handy color-coded scale that runs from green (“good”) to maroon (“hazardous”). 

Experts like Thornton say that wildfires may hamper efforts to meet the EPA’s new standard, even as government regulations, such as the Clean Air Act, have made U.S. air safer than most of the world’s. “Wildfires are basically wiping out a lot of that progress,” Thornton said. A 2023 study published in Nature found that wildfire smoke undid almost 25 percent of air quality improvements since 2000. 

Currently, the EPA does not take pollution levels from wildfires into account in its regulatory actions, as part of an “Exceptional Event Rule” that kicks in when natural disasters skew environmental data. As the weather warms and fire season inches closer, fire management strategies may become key to sparing communities from blazes and unsafe air alike.

This story was originally published by Grist with the headline It’s official: US air quality got worse in 2023 on Mar 22, 2024.