Pregnant women exposed to two common pollutants may give birth to babies with heart deformities, says epidemiologist Beate Ritz of the University of California at Los Angeles. She collected daily air pollution measurements from 30 locations around L.A. and correlated them to local birth registries. For women in areas with the highest levels of carbon monoxide and ozone, the risk of having a child with certain heart malformations and valve defects was triple that of women living in the least noxious neighborhoods. In moderately polluted areas, the risk was double. "The dose-response relationship is just about the closest you can get to a smoking gun in this kind of study," says Ritz.

Normally, two children in a thousand are born with such heart defects, which can be life threatening and require corrective surgery before age 1. In the most polluted areas, the incidence jumped to six per thousand. "The effects are probably much greater even than we're detecting because the worst cases result in miscarriages," Ritz says. She can't yet say which pollutants are to blame, as carbon monoxide levels may just be a marker for other harmful particles or chemicals, but the intensity of the effect shocked her: "Carbon monoxide is principally an indicator of car exhaust, and nearly every urban center in the country has exhaust levels that would be dangerous to pregnant women."