A view of air pollution in West Delhi. Photo: Jean-Etienne Minh-Duy Poirrier/Flickr, CC BY SA 2.0
- Scientists have been able to show for the first time that air pollution can cause lung cancer in people who have never smoked.
- The pollutant PM2.5 does not directly cause cancerous mutations in lung cells but ‘awakens’ dormant mutations that already exist in lungs.
- Crucially, the researchers also showed that blocking a molecule called IL-1β could prevent cancers from taking root.
New Delhi: Scientists have been able to show for the first time that air pollution can cause lung cancer in people who have never smoked, a finding that will send alarm bells ringing among the millions of people who live in areas where pollution is higher than acceptable limits.
The research – conducted by UK scientists at the Francis Crick Institute and the University College London – found that exposure to particulate matter (PM2.5) in the air promotes the growth of cells in the lungs which carry cancer-causing mutations. It was presented by Prof. Charles Swanton at ESMO Congress – an oncology platform – on September 10.
The researchers’ key finding is that air pollution does not create cancerous mutations, but “awakens” dormant mutations that can cause cancer. Crucially, they were also able to identify a way to prevent the awakening of these mutations.
According to the researchers, smoking remains the biggest risk factor for lung cancer but outdoor air pollution causes roughly 1 in 10 cases of lung cancer in the UK. “An estimated 6,000 people who have never smoked die of lung cancer every year in the UK, some of which may be due to air pollution exposure. Globally, around 300,000 lung cancer deaths in 2019 were attributed to exposure to PM2.5,” the Francis Crick Institute said in a press release.
The findings have great significance for India. A recent study found that three Indian cities – Delhi, Kolkata and Mumbai – are among the top 20 cities with the highest exposure to PM2.5, which are tiny particles around 3% of the width of a human hair. The study also found that people in South Asia have the highest average exposure to the pollutant.
While examining data from over 400,000 people, the researchers also found higher rates of other types of cancer in areas with high levels of PM2.5. They speculate that air pollution could promote the growth of cells carrying cancer-causing mutations elsewhere in the body.
Establish the connection
The findings rewrite the way cancer is understood. According to the BBC, the “classical view” of cancer starts with a healthy cell which acquires more and more mutations in its DNA until it reaches a tipping point. Then it becomes a cancer and grows uncontrollably.
Many environmental agents, such as UV light and tobacco smoke, cause damage to the structure of DNA, creating mutations which cause cancer to start and grow. But cancerous mutations have also been found in healthy tissue. “But no evidence could be found that air pollution directly mutates DNA, so scientists looked for a different explanation,” the Francis Crick Insitute said in a press release.
Therefore, they investigated the theory that PM2.5 causes inflammation in the lungs which can lead to cancer. Inflammation wakes up normally inactive cells in the lungs which carry cancer-causing mutations. The combination of cancer-causing mutations and inflammation can trigger these cells to grow uncontrollably, forming tumours.
To test the hypothesis, the study examined a type of lung cancer called epidermal growth factor receptor (EGFR) mutant lung cancer – which develops in people who have never smoked. They exposed mice which had EGFR mutations in their cells to air pollution levels that are normally found in urban areas. They found that cancers were more likely to start from these cells, compared with those mice who were not exposed to air pollution.
“The researchers showed that blocking a molecule called IL-1β, which normally causes inflammation and is released in response to PM2.5 exposure, prevents cancers from forming in these mice,” the Francis Crick Institute said.
The research team think that the model presented in their study could be responsible for the early stages of many different types of cancer, where environmental triggers awaken cells carrying cancer-causing mutations in different parts of the body.
The lead investigator of the study, Stanton, said that it has “fundamentally changed” how lung cancer in people who have never smoked is viewed.
“Cells with cancer-causing mutations accumulate naturally as we age, but they are normally inactive. We’ve demonstrated that air pollution wakes these cells up in the lungs, encouraging them to grow and potentially form tumours,” he said.
The study’s co-first author Emilia Lim said even “small changes” in air pollution levels can affect human health. She said that 99% of the world’s population lives in areas which exceed annual WHO limits for PM2.5, “underlining the public health challenges posed by air pollution across the globe”.
The researchers also said that finding ways to block inflammation caused by air pollution may prevent many cases of cancer in the world.
What does it mean for India?
Indians are exposed to some of the highest levels of PM2.5 pollution in the world. The main sources of PM2.5 emissions are vehicular combustion, industrial combustion, waste and stubble burning.
A study published in July 2021 found that globally, about four million people die because of breathing ambient air with unsafe levels of PM2.5. A quarter of those deaths occur in India. Another recent report found that air pollution cut the life expectancy of Indians by five years.
While the WHO has set the annual average levels of PM2.5 over an area should not exceed 5 µg/m3, India’s Central Pollution Control Board has a much more relaxed limit – 40 µg/m3. However, the air pollution in several cities far exceeds these relaxed limits.
India has put in place the National Clean Air Programme (NCAP) in 2019 to reduce PM2.5 pollution in 132 cities by at least 20% by 2024, as compared to 2017 levels.
Though the environment ministry said last week, that 95 of the 131 cities covered under NCAP have shown an improvement in air quality. But the ministry is only considering PM10 emissions and not PM2.5 emissions.
There is also a concern that India’s policies through the NCAP and the Finance Commission focus “exclusively” on cities. This excludes the vast swathe of rural India that is as affected by air pollution and does not holistically address all sources, Bhargav Krishna, a fellow at the Centre for Policy Research, told The Wire recently.
He added that India should set stronger ambient air quality standards and think about “airsheds” that encompass larger geographies of rural and urban India.