Highlighting a new PAHlution hazard.

While acid rain's effect on wildlife is a growing public concern, the phototoxicity of other pollutants accompanying those acidic combustion byproducts has not been seriously evaluated, according to Jacques Kagan at the University of Illinois in Chicago. Research by Kagan and his co-workers suggests this has been a serious omission. In studies they conducted using tadpoles, mosquito larvae, brine shrimp, water fleas and flat-head minnows, concentrations of polycyclic aromatic hydrocarbons (PAHs) that had previously been considered nontoxic turned out to be quite lethal when the aquatic animals were exposed to them in the presence of light.

This calls into question the role of acid rain in the death of lakes in the northeastern United States and elsewhere. Kagan believes that PAHs might prove to be as much to blame as their more infamous acidic companions. What's more, though PAHs have hardly been ignored, most research on them has focused on their potential as carcinogens. Kagan says that in doing this, science has overlooked a large measure of their toxicology. In fact, he points out, several PAHs registering the most acute phototoxicity in his studies are noncarcinogenic.

Kagan's work grew out of a discovery reported last year that anthracene, a simple PAH, was toxic to tadpoles only in the presence of sunlight or ultraviolet light. Tadpoles kept in the dark were unaffected by an anthracene solution -- even an anthracene solution that had previously been irradiated with light. Kagan's follow-up studies have examined seven more of the simples PAHs, including two of the most common -- fluoranthene and pyrene. Test concentrations of PAHs in water usually ranged from 0.001 to 6.67 parts per million, and light exposures typically lasted a total of 30 minutes.

Ultraviolet light made some PAHs lethal to water fleas at mere parts-per-billion levels, with fluoranthene and pyrene the most toxic. Similar effects were observed in other test species. For mosquito larvae, the most phototoxic PAH was pyrene. Though mildly toxic in the dark, it became 1,500 times more so when the larvae were also irradiated with ultravoilet light.

Kagan notes that the most phototoxic PAHs he studied were very strong absorbers of light in the precise spectral bands emitted by their ultraviolet lamps, a finding that hints at a possible factor for screening out other phototoxic PAHs.