The Reach of Wildfire Smoke Is Going Global and Undoing Progress on Clean Air

On the heels of an exceptionally fiery and smoky summer, two new reports released Wednesday confirmed what many Americans have been already seeing and breathing.

Smoke from increasingly frequent and increasingly large fires has started to undo decades of hard-won gains in air quality, and the problem is expected to only get worse, not just in the United States but also around the world.

More than two billion people were exposed to at least a day of fire-related air pollution each year between 2010 and 2019, a report from researchers in Australia found. And in the United States, wildfires have undone about 25 percent of past progress in cleaning up air pollution in states from coast to coast.

“People have known that it’s becoming a bigger issue in the Western states,” said Marissa Childs, a fellow at Harvard University’s Center for the Environment and a co-author of the study that focused on the United States. “But I was really shocked when we were running some of these estimates and seeing that states all the way to the East Coast were being influenced.”

While her paper doesn’t include data from 2023, Dr. Childs said the wildfires in Canada and subsequent smoke over large swaths of the northern United States this year had shown “more than ever” that everyone is going to be affected by the growing problem of wildfires, no matter where they live.

Climate change is one of the driving forces behind worsening fires worldwide. As the atmosphere warms, many forests and other natural ecosystems are becoming drier and more prone to catching on fire. “It’s just so clear that, sometime in the last five to 10 years, something’s changed,” said Marshall Burke, a professor of environmental policy at Stanford University and a co-author of the report that focused on the United States. “You don’t have to cook the books.”

Together, the two studies show how wildfires are a growing health threat. Wildfire smoke can contain a variety of pollutants, including fine particulate matter, or PM 2.5, a type of air pollution made up of very small particles that can invade the lungs and bloodstream.

Thanks to the Clean Air Act, air pollution in the United States has generally improved since the 1970s. But levels of PM 2.5, which are routinely tracked by the Environmental Protection Agency and had been declining, took a marked turn back up around 2016.

Since 2016, wildfire smoke has had a statistically significant effect on PM 2.5 trends in 35 out of 48 continental states, according to Dr. Burke and Dr. Childs’s study. (The data set did not include Alaska or Hawaii.) The effect was most notable on the West Coast, where air quality has worsened drastically in recent years. But even in some New England states, smoke caused pollution levels to plateau after many years of decline.

Although the air is now cleaner in the United States than in many other parts of the world, air pollution remains a problem for public health. “It’s pretty clear that wildfire smoke is affecting a lot more people on a lot more days than it used to,” said Christopher Tessum, a professor of environmental engineering at the University of Illinois Urbana-Champaign who researches air pollution but wasn’t involved in either study.

Globally, pollution from fires is taking a bigger toll on residents of poorer countries.

The study that was led by scientists from Monash University in Australia found that each year between 2010 and 2019, every person worldwide had an average of almost 10 days of wildfire smoke exposure. The concentration of polluted air was significantly higher in poorer countries, the researchers found.

Smoke exposure between 2010 and 2019 was also higher than during the decade prior, and it underscores the prevalence and health risks of wildfires.

“We need to put a lot more resources to low-income countries to fight the fire smoke,” said Yuming Guo, an environmental expert at Monash who co-wrote the study.

The study incorporated data from both wildfires and those planned or controlled by people, such as prescribed burns. The researchers used a number of sources to collect data on pollution, and examined ground-level ozone levels in addition to levels of PM 2.5. While ozone high in the atmosphere protects us from harmful radiation, ozone close to the ground can cause breathing problems and can aggravate respiratory illnesses like asthma, bronchitis and emphysema.

Countries with hot and dry conditions that make them vulnerable to wildfires were particularly choked by PM 2.5, including those in central Africa, Southeast Asia and South America.

“Different countries experience different fire smoke,” Dr. Guo said. “So different countries should deploy different resources.”

Determining what approaches to use is going to be a complicated effort anywhere.

“It can’t be done the way that we’ve dealt with, say, industrial pollution or cars,” said Colleen Reid, a professor of geography at the University of Colorado, Boulder, who studies the health impacts of wildfires, but was not involved in either of the papers released Wednesday. “There’s not like a scrubber or a catalytic converter, some sort of technological thing you can put on a wildfire.”

“While we work on policy solutions to try and deal with wildfires, we also can protect people’s health by investing in better air quality in indoor spaces,” Dr. Reid added, noting that it was important to make sure people knew how to protect themselves outside on smoky days by wearing masks or respirators. She also emphasized the importance of tackling climate change.

“In addition to all the policies to address wildfire smoke, obviously we need significant change to decrease our greenhouse gas emissions,” she said, “so we can try to deal with the climate side of the equation that’s increasing wildfire risk.”