Subway air does extra damage.

The hodgepodge of minute compounds that drifts through the air in many environments has been linked to heart and respiratory problems. But mixtures vary in composition from place to place, and it's not known which ones are most harmful.

Research now indicates that the mix of airborne particles in subterranean transit stations is more damaging to human cells in culture dishes than are particles in street-level air. Time spent underground might therefore contribute to health risks in sensitive people, suggest Hanna L. Karlsson and her colleagues at the Karolinska Institute in Stockholm.

The researchers collected particles smaller than 10 micrometers in diameter from air in a Stockholm subway station and along a busy street above it. In the lab, the scientists exposed lung cells to solutions containing various doses of particles from one location or the other.

In similar doses, particles from subway air caused about eight times as much DNA damage as did those from street air. The former particles also produced four times as much evidence of cellular injury from particularly reactive chemicals called free radicals, the scientists report in the January Chemical Research in Toxicology.

"Probably the particles in the underground arise from friction between the wheels and the steel rails," Karlsson says. Compared with the particles in street air, subway particles contain at least 10 times more iron, most notably in the form of magnetite. That compound is a type of iron oxide that's particularly good at generating free radicals. Subway air is also 5 to 10 times as thick with particulate matter as street level air is, enhancing its harmful potential, the researchers say.