California wildfires in 2007 repeatedly caused ground-level ozone to spike to unhealthy levels across a broad area, including much of rural California as well as neighboring Nevada, the scientists said.

The study, published in Geophysical Research Letters, found the unhealthy levels of ozone detected occurred mostly in rural areas. However, this finding may be a result of the computer modeling, which lacked the fine detail to zoom in on relatively compact urban areas, the researchers said.

"It's important to understand the health impacts of wildfires," lead author Gabriele Pfister said in a statement. "Ozone can hit unhealthy levels even in places where people don't see smoke."

The researchers, using a combination of computer models and ground-level measurements, studied intense California wildfires that broke out in September and October of 2007. The study found that ozone was three times more likely to violate safe levels when fire plumes blew into a region than when no plumes were present.