Clearing the air on dirty art.

Impenetrable security systems at museums and galleries don't stave off the sly attack of one culprit. Air-pollution damage to artworks may accumulate more stealthily than conservationists had thought. A new study could give art exhibitors incentive to step up protection against harm wrought by dirty air.

As a high school student in Pasadena, Calif., Leon M. Bellan began working with researchers at the California Institute of Technology in Pasadena to assess the visibility of dirt on art. He printed nearly microscopic black dots at various densities. He then recruited 30 subjects to tell him which samples appeared dirty. His laser prints mimic microscopic airborne pollution better than do the arrays of larger, individually visible dots used by previous researchers for such studies, says Bellan.

Research published in 1959 and 1976 had concluded that people can see soot on a white backdrop when it covers 0.2 percent of the surface area. But Bellan, now a Caltech undergraduate, and his colleagues found that people don't perceive the microscopic particles until 12 percent of a sample is covered. If a clean area appears next to the dirty region, such as when a frame is removed, pollution becomes more noticeable. Observers then detect dots covering 3.6 percent of the surface. Bellan, Lynn G. Salmon of Caltech, and Glen R. Cass, now at the Georgia Institute of Technology in Atlanta, report their findings in the May 15 ENVIRONMENTAL SCIENCE & TECHNOLOGY.

The results indicate that observers aren't sensitive to gradual darkening, so curators may not install adequate systems to protect the works, Salmon says. Meanwhile, even less noticeable pollutants such as ozone and nitric acid attack the art, initiating damaging chemical reactions, she says.