Air pollution shortens lives worldwide by more than two years, researchers reported Tuesday.

Across South Asia, the average person would live five years longer if levels of fine particulate matter met World Health Organization (WHO) standards, according to a report from the University of Chicago's Energy Policy Institute.

In the Indian states of Uttar Pradesh and Bihar, home to 300 million, lung and heart disease caused by PM2.5 pollution reduces life expectancy by eight years, and in the capital city of New Delhi by a decade.

PM2.5 pollution -- 2.5 microns across or less, roughly the diameter of a human hair -- penetrates deep into the lungs and enters the bloodstream.

In 2013, the United Nations (UN) classified it as a cancer-causing agent.

The WHO says PM2.5 density in the air should not top 15 microgrammes per cubic metre in any 24-hour period, or 5 mcg/m3 averaged across an entire year.

Almost all populated regions in the world exceed WHO guidelines, but nowhere more so that in Asia: by 15-fold in Bangladesh, 10-fold in India, and nine-fold in Nepal and Pakistan.

Surprisingly, PM2.5 pollution in 2020, the most recent data available, was virtually unchanged from the year before despite a slow-down in the global economy and Covid lockdowns.

Compared to other causes of premature death, the impact of PM2.5 pollution is comparable to smoking tobacco, more than three times that of alcohol use, and six times that of HIV/AIDS, the report said.