About a third of global asthma cases reported in 2019 were related to prolonged exposure to pollution due to PM2.5 or fine particulate matter, a finding that researchers said provided "sufficient evidence" on the link between air pollution and asthma.

The review of 68 studies conducted from 2019-2023 across 22 countries, including South Asian ones, showed that for every 10 micrograms per cubic metre increase in PM2.5, the risk of developing childhood or adult asthma increases by over 21 per cent.

The respiratory condition is marked by recurring symptoms of wheezing, coughing and breathlessness and can severely impair quality of life.

"We estimate that globally in 2019, almost a third of asthma cases are attributable to long-term PM2.5 exposure, corresponding to 63.5 million existing cases and 11.4 million new cases," first author Ruijing Ni, from the Max Planck Institute for Chemistry, said.

While evidence has suggested long-term exposure to fine particulate matter pollution to be a risk factor for developing asthma, the researchers said that inconsistencies in earlier studies left potential health risk of this exposure to debate.

However, their analysis, published in the journal One Earth, found that "long-term exposure to PM2.5 significantly increases asthma risk in both children and adults and is associated with (nearly) 30 per cent asthma cases globally".

There is now sufficient evidence and a high level of confidence in the link between long-term exposure to ambient PM2.5 and asthma, the researchers said.

Children formed majority of the affected -- over 60 per cent, the study found. Lungs and immune function are known to fully mature until early adulthood, which is why children are considered more vulnerable to air pollution, as exposure to it can lead to inflammation and over-reactivity in airways.

Further, previous research is also limited on how PM2.5 pollution affects people in low- and middle-income countries where populations are typically exposed to higher levels of air pollution and bear a greater burden of PM2.5, the researchers said.

Using epidemiological models, the team estimated that about 0.12 million excess deaths were due to asthma. These deaths were found to exclusively occur in adults, most of them in India and China, followed by those in countries in Southeast Asia, Africa and the Middle East.

The authors noted that despite relatively lower levels of PM2.5 exposure, Northern America and Western Europe also contributed significantly to asthma prevalence and incidence.

"Our findings highlight the urgent need for policymakers to enforce stringent legislation to continuously combat air pollution, while personal protective measures, such as wearing masks, can also help reduce individual exposure and mitigate the risk of asthma," said corresponding author Yafang Cheng, the director of the Max Planck Institute for Chemistry.

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