Air pollution is the biggest threat to human health in India as it reduces life expectancy by an average of five years, a United States-based think tank said on Tuesday.

The Energy Policy Institute at the University of Chicago made the statement as part of its Air Quality Life Index for India.

The institute said that child and maternal malnutrition in India reduce average life expectancy by about 1.8 years, while smoking reduces life expectancy by 1.5 years.

The organisation said that Delhi is the most polluted mega city in the world, and that air pollution in the national capital shortens lifespan by nearly ten years. It calculated the reduction in life expectancy on the basis of what the average lifespan would have been if the World Health Organization’s guidelines on fine particulate pollution had been followed.

The global health body’s guidelines state that the annual average PM2.5 concentration should not be higher than 5 micrograms per cubic metre.

Particulate matter smaller than 2.5 microns (or about a ten-thousandth of an inch) is particularly dangerous to human health. Such particles are small enough to travel deep into the respiratory system, potentially impairing lung function.

The Energy Policy Institute said on Tuesday that India accounts for about 44% of the world’s increase in air pollution since 2013. It said that a permanent nationwide reduction of 25% in particulate pollution would increase life expectancy by 1.4 years in India, and by 2.6 years in Delhi.

If current levels of pollution persist, the average life expectancy of residents of Bihar, Chandigarh, Delhi, Haryana, Punjab, Uttar Pradesh and West Bengal will be reduced by 7.6 years on average, the report said. However, it said that the country’s entire population lives in areas where annual average particulate pollution exceeds the World Health Organization’s guidelines.

Meanwhile, at the global level, 7.4 billion persons live in areas where PM2.5 levels exceed the World Health Organization’s benchmark, the institute said. The figure accounts for 97.3% of the world’s population.

It said that particulate pollution is the greatest risk to human health worldwide.

“While particulate pollution is set to reduce global average life expectancy by 2.2 years, first-hand cigarette smoke...reduces global life expectancy by about 1.9 years,” the report added. “Alcohol use reduces life expectancy by 8 months, unsafe water and sanitation, 7 months, HIV/AIDS, 4 months, malaria, 3 months, and conflict and terrorism, just 9 days.”