Prolonged exposure to air pollution is significantly associated with an increased risk for lung cancer, according to the results of a new meta-analysis.

The study adds "substantially to the weight of epidemiological evidence" that links the 2 phenomena, say the authors, led by Ole Raaschou-Nielsen, PhD, from the Danish Cancer Society Research Center, in Copenhagen.

The researchers, who synthesized data from 17 cohort studies in 9 European countries, found that the risk exists even when the levels of particulate matter (PM) air pollution were below the current European limit values.

Sources of PM air pollution include traffic, industry, and domestic heating.

"We found no threshold below which there was no risk; the results showed a picture that 'the more the worse, the less the better,' " said Dr. Raaschou-Nielsen in a press statement.

"Particulate matter air pollution contributes to lung cancer incidence in Europe," the study authors conclude.

The study, which is one of the largest studies of its kind, included data on more than 300,000 individuals and 2095 lung cancer cases and had a median follow-up of 12.8 years. It was published online July 10 in the journal Lancet Oncology.

"At this stage, we might have to add air pollution, even at current concentrations, to the list of causes of lung cancer," write Takashi Yorifuji MD, PhD, from Okayama University Graduate School of Environmental and Life Science, and Saori Kashima, PhD, from Hiroshima University, in Japan, in an accompanying editorial.

Air pollution is not recognized by medical practitioners as a cause of lung cancer, the pair say. For instance, it is not listed in the iconic Harrison's Principles of Internal Medicine as a cause.

It may also be time to "recognise that air pollution has large effects on public health," the Japanese editorialists state.

Evidence for an association between air pollution exposure and lung cancer is "accumulating," they say. Still, the lung cancer risk associated with air pollution is much lower than that associated with smoking. For instance, in the current study, there was a hazard ratio [HR] of 1.22 (per 10 μg/m³ increase in PM 10 ). By contrast, other research indicates that the relative risk [RR] of developing lung cancer is 23.3 for currently smoking men and 12.7 for currently smoking women compared with nonsmokers. However, the editorials point out that 'everybody is exposed to air pollution." Thus, "the public health effect is quite large."

The editorialists also point out that the World Health Organization has estimated that smoking caused 5.1 million deaths and 71% of lung cancer worldwide in 2004, whereas air pollution caused 1.2 million deaths and 8% of lung cancer worldwide in the same year.

Greater Risk for Adenocarcinomas

In the new study, the authors assessed the impact of long-term exposure to nitrogen oxides and particulate matter with a diameter of less than 2.5 micrometers (PM 2.5 ) and less than 10 micrometers (PM 10 ) on the risk for lung cancer.

Air pollution concentration was estimated at individuals' home addresses using land-use regression models. Participants were tracked for new lung cancer diagnoses in cancer registries. The researchers also applied statistical modeling to control for other factors like smoking, diet, and occupation.

The analysis found that for every increase of 5 micrograms per cubic meter of PM 2.5 pollution, the risk for lung cancer rose by 18%, and for every increase of 10 micrograms per cubic meter in PM 10 pollution, the risk increased by 22%.

The risk/HRs were even higher for the lung cancer subtype, adenocarcinomas. The same increments of PM 10 and PM 2.5 were associated with HRs for adenocarcinomas of the lung of 1.51 and 1.55, respectively.

However, other studies have not found that there is a pronounced relation between air pollution and any particular lung cancer subtype, the authors and editorialists point out.

No association between lung cancer and nitrogen oxides was found.

The HRs for lung cancer were similar with and without restriction to study participants below most of the predefined public safety threshold values, say the authors. This suggests that "exposure of populations to particulate matter air pollution even at concentrations below the existing European Union air quality limit values for PM 10 (40 ug/m³) and PM 2.5 (25 ug/m³) might increase the risk for lung cancer," they write.

The new study is "sophisticated and overcame several limitations of previous air pollution studies," the editorialists say.

They point out that earlier studies examined the effect of air pollution on lung cancer by assessing the geographical correlations between air pollution concentration data in communities and aggregate data on lung cancer. However, this method is subject to exposure misclassification and confounding, especially by tobacco smoking.

The new study takes advantage of the fact that researchers subsequently tried to reduce these systematic errors by shifting to case-control or cohort studies with area-level exposure assessment.

Dr. Raaschou-Nielsen and colleagues took the next step by conducting a meta-analysis of these newer studies.

The new study also benefited from a high follow-up rate and adjustment of potential confounders, including a set of smoking variables, the editorialists emphasize.

The study was funded by the European Community's Seventh Framework Programme. The authors have disclosed no relevant financial relationships.