-- Smoking is far and away the leading cause of lung cancer, a disease that will kill an estimated 160,000 people in 1997. Other suspected causes are exposure to arsenic, asbestos, radon, second-hand smoke, and possibly, air pollution, which may play a part in causing the 20% of lung cancer cases that occur in nonsmokers.

Now a new study bolsters the argument that air pollution can indeed increase the risk of the cancer, according to a letter in this week's issue of Nature. And the researchers made the discovery by comparing the growth of pollution-sensitive lichens -- scaly combinations of algae and fungi that can be found growing on rocks and trees -- to lung cancer death rates in a large section of Italy with a population of 4 million.

"The relationship between lung cancer and atmospheric pollution remains controversial despite 50 years of discussion, partly because studies are frequently restricted to small, well-monitored areas" wrote Cesare Cislaghi of the Institute of Medical Statistics at the University of Milan, and Pier Luigi Nimis of the Department of Biology at the University of Trieste in Italy.

"Our results strongly support a relationship between air pollution and lung cancer." they said.

The researchers looked at the number of lichen species in 662 locations, and compared them to lung cancer mortality data from 1981 to 1988 from the Italian National Institute of Statistics. They found that the biodiversity -- the number of lichens -- varied depending on the presence of common components of air pollution, such as sulfur dioxide, nitrogen oxides and dust. And they found that lichen biodiversity was linked to lung cancer mortality in men under age 55.

There was no link between lichen biodiversity and other mortality due to other illnesses, such as larynx (voicebox) cancer or chronic bronchitis. Nor was there a link to lung cancer rated in women, who have different smoking habits from men and who, at least in Italy, are much less likely to develop lung cancer. Women make up 13% of lung cancer deaths in Italy, while in the U.S. the number is closer to 45%, and rising.

"The relative risk associated with pollution exposure is small, but the affected population is large, and thus the impact of pollution in terms of cancer mortality is important." the researchers concluded.