New Delhi: Long-term exposure to air pollution may be linked to millions of cancer cases worldwide, with the risks extending beyond lung cancer to cancers of the breast, thyroid, prostate, colon, kidney and other organs, according to a new study published in Nature Health.
The study—led by researchers from the Chinese Academy of Sciences with collaborators from Fudan University in China, Boston University in the US and other institutions—analysed 109 million cancer cases across 952 locations in 34 countries between 2000 and 2020.
Researchers examined exposure to three common air pollutants—fine particulate matter or PM2.5, nitrogen dioxide (NO2) and ground-level ozone—and compared their link to cancer rates in different populations.
They found that for every increase of 10 micrograms per cubic metre in PM2.5, the overall cancer risk was 16 percent higher. A similar increase in NO2 was associated with an 11.7 percent higher risk, while ozone was associated with a 4.23 percent increase.
The researchers stressed that these figures show a link between higher pollution levels and higher cancer rates across large populations. They do not prove that air pollution directly caused cancer in every individual.
According to Dr Shyam Aggarwal, chairperson of the Department of Medical Oncology at Sir Ganga Ram Hospital, New Delhi, the study adds to evidence that the health effects of polluted air may go beyond the lungs.
“While outdoor air pollution is already classified as a cause of lung cancer, this research suggests a broader impact on cancer incidence overall,” he told ThePrint.
How did the researchers find the link?
The researchers also considered that cancer can take years to develop and looked at pollution exposure over several years rather than comparing pollution levels with cancer cases in the same year.
Using these comparisons, they estimated that about 8.82 million cancer cases were linked to PM2.5, 6.67 million to NO2, and 2.59 million to ozone over the 20-year study period.
One of the study’s limitations is that pollution exposure was estimated using large environmental datasets, including satellite-based measurements. The researchers also could not completely account for other factors that can affect cancer risk, such as smoking, occupation, diet, socioeconomic conditions and other lifestyle factors.
Still, the scale of the analysis adds to growing evidence that air pollution may play a role in cancers beyond the lungs.
Also Read: More than 80% cities under National Clean Air Programme fail on air quality norms
How can air pollution affect cancers beyond the lungs?
Air pollution has long been recognised as a cancer-causing exposure. The International Agency for Research on Cancer has classified outdoor air pollution and particulate matter as carcinogenic, with the strongest evidence linked to lung cancer.
The new study, however, found links between long-term PM2.5 exposure and several cancers outside the respiratory system.
These included cancers of the thyroid, breast, prostate, colon and rectum, ovaries, testes, kidney, bladder and pancreas. NO2 was also linked to lymphomas, cancers that develop in the lymphatic system, and multiple myeloma, a cancer of plasma cells, a type of white blood cell that helps the body fight infections.
The researchers said pollution may contribute to cancer through processes such as long-term inflammation, oxidative stress and changes in the body’s immune response.
Dr Pragya Shukla, head of the Department of Clinical Oncology at the Delhi State Cancer Institute (DSCI), said the impact of air pollution goes well beyond respiratory diseases and lung cancer, which she described as “only the tip of the iceberg”.
She said global cancer data shows that air pollution may also increase the risk of several cancers outside the respiratory system.
“By triggering systemic inflammation and severe oxidative stress throughout the body, airborne toxins actively contribute to the development of colorectal, gastrointestinal, pancreatic, liver and breast cancers,” Shukla added.
She said the findings highlight the “true carcinogenic potential of the air we breathe” and the need for stronger action to reduce pollution.
Are current air quality limits enough?
The study found that cancer risks were not meaningfully higher at pollution levels at or below the World Health Organization’s 2021 air quality guidelines.
The WHO recommends an annual average PM2.5 concentration of no more than 5 micrograms per cubic metre. The corresponding limits for NO2 and ozone are 10 and 60 micrograms per cubic metre.
But the population-weighted median PM2.5 level across the locations studied was 17.6 micrograms per cubic metre, more than three times the WHO guideline.
This means many people included in the study were living in areas where long-term PM2.5 exposure was above the recommended level.
While the study does not focus on India specifically, its findings are particularly relevant for countries such as India, where many cities regularly record PM2.5 levels far above WHO guidelines.
Women appear to face higher risks
The study also found that women generally faced higher pollution-linked cancer risks than men, especially for cancers linked to PM2.5 and NO2.
The difference was particularly noticeable for cancers affecting the digestive and respiratory systems and for hormone-related cancers such as breast cancer.
For breast cancer, a 10 microgram per cubic metre increase in NO2 was linked to about a 20 percent higher risk among women. The 20 percent figure, the researchers noted, was far higher than earlier estimates, which had put the increase closer to 2 percent.
The researchers said biological differences between men and women could partly explain this finding. One possible explanation is the interaction between oestrogen, pollution-related inflammation and oxidative stress.
But they said more research is needed to understand the reasons for this difference.
(Edited by Sugita Katyal)