Fires belch out fine particulates known as PM2.5. These tiny particles are 2.5 microns or smaller in width — about one-thirtieth the width of a strand of hair. That means they’re small enough to be inhaled deeply into the lungs, and for that reason they are the riskiest particles to human health, the Environmental Protection Agency says.

In the past two decades, over 80 percent of smoke in the United States was human-caused, the researchers found. In 2018, human-caused fires accounted for more than 67 percent of all PM2.5 emissions.

Agricultural fires were more common in the Southeast, while human-sparked wildfires dominated in the West and caused large swaths of smoke pollution. This was especially true in California, where levels of PM2.5 smashed recommended thresholds during the Camp and Mendocino Complex Fires.

The scientists looked at the estimated population exposed to PM2.5 and linked that data with other research on how smoke exposure contributes to early deaths. Overall, they attributed 20,000 premature deaths to human-sparked fires in 2018 and another 7,400 to 2003 fires.

“Now we know that humans can play a pivotal role in reducing PM2.5 concentrations, we should be putting policies, regulations, and management plans in place to reduce human-ignited fires,” said Therese Carter, a public health researcher at George Washington University who co-wrote the study, in a news release.

The researchers conclude that there is “considerable potential” for humans to mitigate smoke exposure and the burden it places on public health. They call for more research on how prescribed burns and other fire management tactics might limit wildfires.