Key Point:

• Urban air pollution appears to contribute a small but significant increase in lung cancer risk.

OSLO—Surprisingly few data have been gathered on the long-term health effects of air pollution, including its influence on the development of lung cancer. Furthermore, the studies that have tried to quantify the impact of air pollution on lung cancer risk have done so only in large populations, not in individuals.

To assess the long-term impact of air pollution on lung cancer risk, Per Nafstad, MD, and colleagues studied 16,209 men in Oslo.[1] Their primary finding: “Air pollution does seem to contribute to a small but significant increase in a person’s lung cancer risk,” said Dr. Nafstad, a Professor of Medicine at the University of Oslo.

27-YEAR FOLLOW-UP

The investigators were fortunate to have available to them two large databases. The first contained details on all individual cancer cases in Norway; the second included information on the average annual concentration of air pollutants at each of the participants’ home addresses. This enabled the investigators to produce lung cancer risk estimates for each individual.

The men in the study had agreed in 1972 to participate in a long-term follow-up study of cardiovascular disease. At enrollment, the men were 40 to 49 years old; most were current (56%) or former (25%) smokers. During the 27-year follow-up, 2,802 of the men developed cancer; in 418 cases, it was lung cancer. The average time to lung cancer diagnosis was 16.1 years.

Not surprisingly, smoking status had a huge impact on lung cancer risk. The relative risk for lung cancer ranged from 4.58 in former smokers to 36.47 in heavy smokers (the comparison was with those who had never smoked).

To determine what, if any, additive risk was presented by ambient air pollution, Dr. Nafstad and his colleagues focused on two common pollutants:
• Nitrogen oxides, the main source of which is vehicular emissions.
• Sulfur dioxide, which is released primarily from heating fuels.

Not only were the investigators able to estimate the pollution level at each participant’s home, but in most cases they were also able to account for any changes in residence that the men made within Norway.

Because of the long average time until lung cancer developed, statistical analysis focused mainly on the effect of air pollution exposure during the first five years of follow-up. During that time, the median levels of nitrogen oxides and sulfur dioxide at the men’s homes were 10.7 and 9.4 µg/m3, respectively. During the remainder of the study, the nitrogen oxide levels tended to increase, whereas the sulfur dioxide concentrations decreased.

A small, but nonetheless significant, increase in lung cancer risk was seen with high nitrogen oxide levels but not with sulfur dioxide levels. In comparison to the men whose average annual exposure to nitrogen oxides was less than 10 µg/m3 during the first five years of the study, those whose average annual exposure surpassed 30 µg/m3 had a 36% increase in risk. Furthermore, a dose-response relationship was found: Lung cancer risk rose by 8% for every 10-µg/m3 rise in the nitrogen oxide level.

The association between nitrogen oxide concentrations and lung cancer risk could not be explained by other variables. In fact, levels of this pollutant were quite evenly distributed during the first five years of follow-up, regardless of the participants’ smoking status, occupation, or education level.

Because there were so few cases of lung cancer among never smokers, the investigators could not quantify the extent to which air pollution increased their lung cancer risk. Among smokers, air pollution appeared to have a weaker effect on heavy smokers than on light smokers; however, this difference did not reach significance.

MARKERS, NOT CAUSE

Dr. Nafstad said that lung cancer is unlikely to result directly from exposure to nitrogen oxides or sulfur dioxide at the estimated concentrations found in the study. He and his colleagues regard levels of these pollutants as markers of the overall extent of air pollution; other constituents of the pollution may have been responsible for the observed increase in lung cancer risk.

It was not possible, he noted, for him and his colleagues to determine how air pollution may have caused or contributed to lung cancer development in some of their study’s participants. “It is possible that the air contained carcinogens, especially in the 1970s when our study began, or that airborne particles may somehow act on the lungs in such a way as to cause cancer,” Dr. Nafstad said. “But that is pure speculation,” he quickly pointed out.

—Timothy Begany

Reference
1. Nafstad P, Haheim LL, Oftedal B, et al. Lung cancer and air pollution: a 27 year follow up of 16,209 Norwegian men. Thorax. 2003;58:1071-1076.

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