Long-term exposure to low levels of air pollution increases the risk of lung cancer, according to a study published in The Lancet Oncology.

A European team of researchers led by Ole Raaschou-Nielsen from the Danish Cancer Society Research Center analyzed the impact of long-term exposure to air pollution on risk of lung cancer.

The research shows that the risk of lung cancer increased even at air pollution levels cleaner than the air quality standards set by the European Union.

The air pollution exposure measured in the study included nitrogen oxides and particulate matter with a diameter of less than 2.5 micrometers (PM2.5) and less than 10 micrometers (PM10), from sources including traffic, industry and domestic heating.

The study revealed that the risk of lung cancer rose by 18% in every 5 micrograms per cubic meter of PM2.5 pollution. With PM10 pollution, there was a 22% rise in the risk of lung cancer for every increase of 10 micrograms per cubic meter.

To reach their conclusions, the researchers carried out a meta-analysis, which means analyzing the combined results of many studies. It involved some 313,000 people around nine European countries, pulling data from 17 cohort studies.

Of the 313,000 people included in this meta-analysis from the ESCAPE project, 2,095 developed lung cancer after 13 years.

The study authors say: "The association between particulate air matter pollution and the risk for lung cancer persisted at concentrations below the existing European Union air quality limit values for PM10 and PM2.5."

The study authors add:

"We found no threshold below which there was no risk; the results showed a picture that 'the more the worse, the less the better.'"

The concentration of the air pollution was estimated at the participants' home addresses using land-use regression models and participants were monitored for new lung cancer diagnoses in local and national cancer registries. They also distanced the influence of air pollutants from other factors such as occupation, smoking and diet using statistical modeling.

The researchers say:

"At this stage, we might have to add air pollution, even at current concentrations, to the list of causes of lung cancer and recognize that air pollution has large effects on public health."

Lung cancer is one of the most common cancers and has a poor prognosis, the researchers note. They add: "Active smoking is the main cause, but occupational exposures, residential radon and environmental tobacco smoke are also established risk factors.

"Ambient air pollution - specifically particulate matter with absorbed polycyclic aromatic hydrocarbons and other genotoxic chemicals - is suspected to increase the risk for lung cancer."

The authors conclude: "Our study overcomes several limitations of previous studies, having a large sample size, broad European coverage, retrospective exposure assessment, adjustment for a wide range of potential confounders, and incident lung cancer as the outcome.

"Particulate-matter air pollution is ubiquitous, and on the basis of our results, further reductions [in it] can be expected to reduce the number of lung cancer cases in Europe."