In a recent study published in the Journal of the National Cancer Institute, researchers identified air pollution as a contributor to breast cancer.
Using a spatiotemporal model (which includes data collected over space and time), researchers analysed a large US-wide sample to investigate links between air pollution and breast cancer risk in women.
Researchers observed an 8 percent increase in breast cancer risk for a 10 mg/m3 increase in estimated historic PM2.5 concentrations over a 10–15-year period.
PM (particulate matter) is a mixture of solid particles and liquid droplets found in the air. PM 2.5 are fine inhalable particles, with a diameter that’s 2.5 micrometres and smaller. According to the US Environmental Protection Agency, PM 2.5 poses the greatest risk to health.
The study included 1,96,905 female participants without a prior history of cancer in six US states and two metropolitan areas and identified 15,870 breast cancer cases in the follow up period. Researchers estimated annual average PM 2.5 concentrations for a five-year period prior to enrolment of the candidate.
Pollution and India
The AQLI (Air Quality Life Index) data reveals that residents in India, Bangladesh, Nepal and Pakistan are expected to lose about 5 years of life expectancy on average, if existing levels of pollution persist. Since 2013, about 59 percent of the increase in world pollution has come from India alone, according to a study by the University of Chicago published in September 2023.
All of India’s 1.4 billion people live in areas where the annual average particulate pollution levels exceed WHO guidelines. The WHO's recommended 24-hour average guideline for PM2.5 was 5 micrograms per cubic meter (µg/m3). As of 2022, India’s national average particulate matter concentration was 53.3 μg/m3 according to World Air Quality Report.
Among women, gynecologic cancers (including breast cancer) accounted for 51 percent of all cancers, according to an Indian Council for Medical Research (ICMR) report released in 2022.
Bearing all this in mind, Indian oncologists say that the new US study identifying PM as a possible cause of breast cancer is not entirely unexpected, as PM is already associated with increased risk of lung cancer.
“Low-dose long-term exposure to PM would result in increased incidence of other cancers like lung, and esophagus,” said Dr Ruquaya Ahmad Mir, Senior Consultant - Surgical Oncology, Apollo Cancer Centres.
However, other experts point out the link between physiological and genetic characteristics of Asians and Caucasians and cancer risk. “Racial and ethnic differences between Caucasians and Asians may also contribute to the different risk profile for breast cancer in these populations,” said Dr Uma Dangi, Consultant, Medical Oncology, Fortis Hiranandani Hospital, Vashi.
The frequency of BRCA1/2 genetic mutations range from 2.9 percent to 24 percent among Indian familial breast cancer patients according to earlier studies. In the general U.S. population, it is estimated that about 1 in 400 to 1 in 800 individuals carry a BRCA1 or BRCA2 mutation. This equates to roughly 0.125 percent to 0.25 percent of the population according to the US National Cancer Institute.
The US study recruited adults aged older than 50 years, who are typically menopausal or post-menopausal, precluding analyses of the effect of PM on cancer among pre-menopausal women.
According to Dr Dangi, breast cancer usually occurs in women over 50 in western countries, which could be reason why this age group was chosen for the study. “The older the candidate, the longer their exposure to environmental toxins, and thus the risk of developing cancer is higher,” Dr Dangi said.
The US study identified the association between particulate matter pollution and breast cancer for estrogen receptor–positive (ER+) tumours, but not for estrogen receptor–negative (ER-) tumours.
In ER+ breast cancer, cancer cells have receptors that can bind to estrogen, a hormone, promoting cell growth. Hormonal medicines like Tamoxifen prevent cancer cells from using estrogen to grow and spread and lead to a better outlook.
ER- breast cancer lacks these receptors and doesn't respond well to hormonal treatments. It tends to be more aggressive and grows faster, requiring chemotherapy and targeted treatment.
Dr. Dangi said the incidence of estrogen-positive breast cancer is higher in the west. Comparatively, India has a higher number of triple negative cases.
This type of cancer cells are estrogen- and progesterone-negative and also don’t make any or much of the HER2 protein (the cells test "negative" on all three tests.) These cancers tend to be more common in women younger than 40, who have a BRCA1 mutation. This type of cancer tends to grow and spread faster, has fewer treatment options, and tends to have a worse prognosis. The BRCA1 gene protects one from breast cancer, and a mutation of this increases one’s chances of getting the disease.
Experts added that more studies must be conducted to come to conclusions regarding the Indian population.
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