NEW YORK, May 16 (Reuters Health) - It's no news that cigarette smoke is bad for the lungs, but researchers may be a step closer to understanding exactly how smoking increases the risk of lung cancer. In a new study, one type of chemical in cigarette smok

But the findings do not mean that these chemicals, known as polycyclic aromatic hydrocarbons (PAHs), are the only culprits in lung cancer, one of the study's authors told Reuters Health in an interview. Cigarette smoke contains many chemicals, several of which may be harmful, according to Dr. Gerd P. Pfeifer, of the Beckman Research Institute of the City of Hope in Duarte, California. PAHs are released into the environment by gasoline and diesel engines, but the largest source of the chemicals is cigarette smoke. The compounds have been suspected of increasing the risk of lung cancer because exposure to PAHs can mutate the p53 gene, which helps prevent tumors from developing, Pfeifer said. When this tumor-suppressing gene becomes mutated, as happens in about 60% of all cases of lung cancer, damaged cells are able to grow and become cancerous. In laboratory experiments, Pfeifer and his colleagues exposed human lung cells to a number of PAHs found in cigarette smoke. After exposing the cells to the chemicals, the researchers analyzed the p53 gene to see how it had been damaged. Then they compared these genetic changes to a database of mutations that are known to occur in lung cancer. The genetic damage in the study "matches pretty well with the mutations in lung cancer," Pfeifer said. The study provides important evidence that cancer-causing substances in cigarette smoke are major causes of lung cancer, Dr. Stephen S. Hecht, of the University of Minnesota Cancer Center in Minneapolis, notes in an editorial that accompanies the study. "Avoiding exposure to these compounds is the key to decreasing lung cancer incidence," Hecht writes.

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