It was also hard to believe at first because, if it were so bad, surely someone would have done something. Alas, it's just not sufficiently in the public's, and thus policymakers', perception.

My first thought would be lung cancer, but the WHO says:

> Smoking is the leading cause of lung cancer, responsible for approximately 85% of all cases.

> lung cancer [caused] an estimated 1.8 million deaths (18%) in 2020

So the 15% of lung cancer deaths that weren't caused by smoking would only account for 300k deaths.

https://www.who.int/news-room/fact-sheets/detail/lung-cancer

> Particulate matter – often abbreviated as PM – is everything in the air that is not a gas. These are very small particles made up of sulfate, nitrates, ammonia, sodium chloride, black carbon, mineral dust and water that are suspended in the air that we breathe.

> Particles with a diameter of 10 micrometers (10 millionth of a metre) or less can enter deep inside a person’s lungs. But the most health-damaging particles are even smaller. Those with a diameter of 2.5 micrometers or less – abbreviated as PM2.5 – can penetrate the lung barrier and enter a person’s blood system. These are extremely fine particles: 2.5 micrometers is about one-thirtieth of the diameter of a human hair.1

> All studies of the mortality impacts of air pollution consider our exposure to particulate matter. Some studies also consider the impact of ground-level ozone. The death toll from ozone is much lower than that of PM, but it is still considerable: it’s responsible for hundreds of thousands of premature deaths every year. Other air pollutants are rarely considered in global studies.

These claims appear to be based on studies in which PM2.5 levels have been correlated (by statistical models) with increased risks of heart disease as well as respiratory problems.

I haven't been able to find anything describing exact mechanisms.

But Pope et al [1] concluded that "a decrease in the concentration of PM2.5 of 10 μg per cubic meter is associated with an increase in life expectancy of 0.77 year."

For comparison, the EPA's clean air standard for PM2.5 is 12 μg per cubic meter.

1: https://www.nejm.org/doi/full/10.1056/NEJMe0809178

2: https://www.epa.gov/criteria-air-pollutants/naaqs-table

https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.123.0... (“Projected Change in the Burden of Excess Cardiovascular Deaths Associated With Extreme Heat by Midcentury (2036–2065) in the Contiguous United States”)