When certain air quality monitoring sites go offline, air pollution often increases, according to an analysis by the EPA’s Office of the Inspector General.

A series of analyses by government and academic researchers found that when some air quality monitors that gather daily data went down, levels of fine particulate matter, or soot, went up by about 4 percent. For sites that gather data once every three days, soot pollution increased by 9 percent, according to the report published in September 2025.

Nearly 36 percent of air monitoring sites that gather data more intermittently than daily had worse air quality on average when they were offline, the report states.

“While the results of our analyses do not indicate malicious behavior at any specific site, they demonstrate that there is a risk of underreported air pollution,” it states.

The EPA publishes its air monitoring schedule online, giving operators of polluting sites an opportunity to “time peak emissions for when a monitoring site is offline.” Also, a huge volume of air quality data and limited EPA capacity allows state and local governments to “strategically turn off monitoring sites on days that they expect high pollution,” potentially allowing them to avoid more strict regulations.

The report recommends limiting the distribution of the quality monitor scheduled and using other air quality monitoring techniques to see whether a site might be underreporting pollution.