A third to a half of ultrafine exposure comes from driving. "If you have otherwise healthy habits and don't smoke, driving to work is probably the most unhealthy part of your day," says Scott Fruin, an environmental scientist at the University of Southern California. The tiny particles can also slip into homes, especially those near busy roads. Cooking with a poorly ventilated stove can be another source of exposure to ultrafines, he says. Some fireplaces also generate particulates. However, so-called HEPA filters can help clear indoor air of particulates, including ultrafines.
Avoiding exposure. Exercising near traffic increases one's exposure, since heavy breathing draws particles deep into the body. Murray Mittleman, a cardiologist at Harvard University, suggests that people—especially those with heart conditions—try to exercise away from major roadways. Masks probably won't have much of an effect on the tiny ultrafines, says Fruin, but might stop larger particles. (Some U.S. Olympians, in fact, have been advised to wear face masks in heavily polluted Beijing.) Two government websites, airnow.gov and epa.gov/airtrends, can help people plan outdoor activities.
Still, there's no reason to live in complete fear of particulates; the risk for any individual remains quite small. That tripling of heart attack risk due to traffic exposure, for example, sounds frightening but is similar to the increased short-term risk posed by exercise or even sexual activity, says Robert Brook, a University of Michigan cardiologist. "The absolute risk—meaning the risk you have every single time you are exposed to air pollution, exercise, or have intercourse—is still very, very low," he says. "But when you multiply that by the tens of millions of people being exposed to polluted air all of the time, you end up getting large numbers of people who are affected."
Araujo, for one, isn't taking any chances. He used to bike frequently in heavy traffic. However, after seeing what happened to his mice, he says: "You won't find me stuck behind bus tailpipes anymore."