Researchers have identified a link between nitrogen dioxide (NO2) and the occurrence of breast cancer in studies of women in North America and Europe. Credit: Unsplash
Air pollutants not only increase the risk of lung cancer, but they could also be a risk factor for breast cancer. This is what is revealed by a synthesis of the international literature carried out by researchers from Inserm, CNRS, and the University of Grenoble Alpes. Among the three main pollutants studied, nitrogen dioxide has the highest level of evidence. According to available data, around 1,700 cases of breast cancer are attributable each year in France to exposure to atmospheric pollutants. These results are the subject of a publication in the journal Environmental Health Perspectives.
Certain atmospheric pollutants, and in particular suspended particles and atmospheric pollution as a whole, are recognized as carcinogenic by the IARC (International Agency for Research on Cancer); this recognition is linked to their demonstrated effects on lung cancer. However, in recent years, epidemiological work has suggested that atmospheric pollutants could also influence the occurrence of breast cancer, which is the most common cancer in Europe.
Breast cancer is one of the cancers with the best known risk factors, with a contribution from factors related to reproductive life (for example the age of the first period), alcohol consumption and probably exposure to environmental factors and in particular those disrupting the estrogen axis, as well as genetic polymorphisms such as those linked to genes BRCA.
In this study, carried out by the team of Inserm research director Rémy Slama at the Institute for the Advancement of Biosciences (IAB, Inserm / CNRS / UniversitéGrenoble Alpes), the scientists were interested in the role of certain atmospheric pollutants on the occurrence of breast cancer.
The objective was to synthesize the results concerning the dose-response relationship between pollution and the occurrence of breast cancer, taken from all the available publications (what is called a meta-analysis, which provides an average of the relationships described by each study. taking into account their precision). Moreover, for the pollutants for which the association was the clearest, the scientists also wanted to provide an estimate of the number of cases of breast cancer that could be attributable to it in France, as well as an estimate of the associated economic costs.
Stronger effect for nitrogen dioxide
The study made it possible to identify for the three pollutants considered, namely particles in suspension with a diameter less than 10 microns (PM10), suspended particles with a diameter less than 2.5 microns (PM2,5) and nitrogen dioxide (NO2), respectively 27, 32 and 36 associations linked to the occurrence of breast cancer, all reported in women in North America and Europe. It is for nitrogen dioxide that the synthesis of studies was most clearly in favor of an adverse effect on the occurrence of breast cancer.
Nitrogen dioxide is mainly emitted by fossil fuel combustion processes, such as those in vehicle heat engines and district heating.
The estimated effect of nitrogen dioxide was higher for hormone-dependent breast cancers (whose tumors express estrogen and progesterone receptors), although not all studies were able to consider this endpoint. . Researchers estimate that around 1,700 cases of breast cancer, or around 3% of cases occurring annually in France, could be attributed to this exposure and other pollutants associated with nitrogen dioxide. The associated economic cost, cumulating tangible costs (those linked to treatments) and intangible costs (those linked to death, loss of quality of life and patient suffering) is of the order of 600 million to one billion Euros per year.
For the other two pollutants considered (PM10 and PM2.5), the level of evidence was lower, without it being possible to exclude an adverse effect. “Carrying out a large meta-analysis like this one is an approach which has the advantage of synthesizing all the scientific literature on the question, and therefore of obtaining particularly robust results. In this case, for nitrogen dioxide, the analysis focused on a set of 36 studies totaling more than 120,000 cases on 3.9 million subjects ”, underlines Rémy Slama.
The observational approaches used in this work do not, however, exclude the contribution of other pollutants whose atmospheric concentrations are closely correlated with those of nitrogen dioxide. Within the complex mixture of air pollution, certain components are known for their carcinogenic or estrogen-disrupting activity, such as polycyclic aromatic hydrocarbons (PAHs). These could therefore also be relevant from the point of view of the development of breast cancer, given the involvement of the estrogenic pathway in certain breast cancers.
Methodological biases to be taken into account
The study of the links between atmospheric pollution and breast cancer poses numerous methodological challenges. The main ones are those of the quality of the characterization of exposures, confounding bias and publication bias – the bias related to the fact that a study that highlights an association is more likely to be published than a study. not showing any association.
This meta-analysis takes them into account by carrying out analyzes restricted to studies presenting the most precise measure of exposure, to those taking into account the main risk factors for breast cancer (some of which are potential confounding factors). A so-called “trim-and-fill” analysis is also carried out: by describing the distribution of the estimates resulting from the published studies, it makes it possible to identify whether these over-represent the studies in favor of a link and correct for any publication bias. . The effect observed in studies carried out in Europe did not appear to be weaker than in North America.
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