The researchers led by Harvard T.H. Chan School of Public Health, US, found that exposures to PM2.5 and NO2 over a 10-year period increased the risk of developing colorectal and prostate cancers.

The study, published recently in the journal Environmental Epidemiology, also found that even low levels of air pollution exposure may make people particularly susceptible to developing these cancers, in addition to breast and endometrial cancers.

“Our findings uncover the biological plausibility of air pollution as a crucial risk factor in the development of specific cancers, bringing us one step closer to understanding the impact of air pollution on human health,” said Yaguang Wei, research fellow in the Department of Environmental Health at Harvard.

“To ensure equitable access to clean air for all populations, we must fully define the effects of air pollution and then work towards reducing it,” Wei said in a statement. While air pollution has been established as a risk factor for lung cancer, and a link to breast cancer risk has been emerging, few studies have looked at its effects on prostate, colorectal, and endometrial cancer risk.

Researchers analysed data from national Medicare beneficiaries aged 65 or older, collected from 2000 to 2016. All subjects were cancer-free for at least the initial 10 years of the study period. They created separate groups for each type of cancer—breast, colorectal, endometrial, and prostate—with between 2.2 million and 6.5 million subjects in each cohort.

Separate analyses looked at cancer risk under the impacts of air pollutants for various subgroups by factors including age, sex (for colorectal cancer only), race/ethnicity, average BMI, and socioeconomic status. Drawing from a variety of air pollution data sources, the researchers developed a predictive map of PM2.5 and NO2 concentrations across the US. This was then linked to beneficiaries’ residential ZIP codes to enable them to estimate individual exposures over a 10-year period.

Findings from the nationwide analysis showed that chronic PM2.5 and NO2 exposures increased the risk of developing colorectal and prostate cancers but were not associated with endometrial cancer risk. For breast cancer, NO2 exposure was associated with an increased risk, while the association for PM2.5 was inconclusive, according to the study. The researchers suggested that the mixed associations may be due to variations in the chemical composition of PM2.5, which is a complex mixture of solid and liquid particles.

When the analysis was restricted to regions where air pollution levels were significantly below national standards and the composition of PM2.5 remained fairly stable, their effect on breast cancer risk was more pronounced.?Stronger associations between exposures to both pollutants and endometrial cancer risk were also found at lower pollution levels.

“The key message here is that US air pollution standards are inadequate in protecting public health,” said study senior author Joel Schwartz, a professor of environmental epidemiology at Harvard.?"Unless all of these standards become much, much stricter, air pollution will continue to result in thousands of unnecessary cases of multiple cancers each year,” he added.