The simultaneous occurence of extreme levels of ozone and particulate matter have increased significantly in the past decade due to wildfires in the western US

The area of the western US hit by the unusually high co-occurrence of two air pollutants because of wildfires has more than doubled in the past decade, exposing millions more people to dirty air.

California and other western states have seen historic forest fires in the past five years that have claimed lives, destroyed property and forced evacuations. Now there is evidence that the human cost reaches much further than the blazes’ immediate vicinity.

After personally experiencing an increase in smog and smoke in recent years, Daniel Swain at the University of California, Los Angeles, and his colleagues explored the role wildfires play.

Two types of air pollution – tiny particulate matter called PM2.5 and ozone – are both linked to human health concerns, but they tend to peak at different times of the year. If there is a significant level of wildfire activity, however – which in the western US can occur between July and September – it is possible to see simultaneous peaks in the two pollutants. Such a co-occurrence is thought to have a disproportionately more severe health impact than either pollutant in isolation.

Swain and his colleagues looked at an area of the western US stretching from Washington in the north to California in the south, and extending as far east as Montana and New Mexico. They divided the area into 111-kilometre-wide squares. Using data they had previously gathered, supplemented by new satellite data, they looked for what they term extremes in the levels of both PM2.5 and ozone between 2001 and 2020.

Over their 20 year study period, the number of squares experiencing the co-occurrence of the two pollutants more than doubled, from 18.9 per cent to 44.6 per cent. The largest areas affected were seen in hot, dry summers with many fires: 2015, 2017, 2018 and 2020. “It’s a very large increase over a short period of time,” says Swain. 

The number of people affected increased too. On one day – 21 August 2020 – about 46 million people were exposed to peaks in both pollutants.

“It’s a public health crisis, in the sense that likelihood of direct harm to an individual is somewhat low but the cumulative harm to the millions and millions of people who are exposed repeatedly is very high,” says Swain.

The study, along with past research, suggests that an increase in atmospheric ridges of high pressure sitting in place is both driving the start of fires and exacerbating the impact of the resulting air pollution by trapping it.

Swain says the research confirms how widespread the human health impacts are. “Most of the people exposed to these dangerous air pollution episodes are not living in places directly threatened by the flames themselves. People who are ‘safe’ from fires are not safe from the air pollution effects even if they live hundreds of even thousands of miles away,” he says.

Air pollution: US wildfires now lead to air pollution events affecting millions | New Scientist