India is the world's second most polluted country, slightly trailing only Nepal, the Energy Policy Institute at the University of Chicago (EPIC) said on November 19.

Particulate pollution is so severe that it shortens the average Indian's life expectancy by more than four years relative to what it would be if World Health Organization (WHO) air quality guidelines were met. This is up from about two years in the late 1990s due to a 69 per cent increase in particulate pollution, it said.

Concentrations in Indian states of Bihar, Uttar Pradesh, Haryana, Punjab, and the National Capital Territory of Delhi are substantially higher, and the impact on life expectancy exceeds six years. Its new air pollution index, known as the Air Quality Life Index (AQLI), finds that air pollution reduces global life expectancy by nearly two years, making it the single greatest threat to human health.

The tool gives figures like -- for an average resident of Delhi, gain in life expectancy if the WHO guidelines are met, could be up to 10.2 years. Likewise, it gives numbers of years lost to pollution for every district of India for a span of 18 years between 1998 and 2016.

What makes AQLI unique is that it converts pollution into perhaps the most important metric that exists -- life expectancy. It does so at a hyper-local level throughout the world. Further, it illustrates how air pollution policies can increase life expectancy when they meet the World Health Organization's (WHO) guideline, existing national air quality standards, or user-defined air quality levels.

This information helps informing local communities and policymakers about the importance of air pollution policies in very concrete terms. Loss of life expectancy is highest in Asia, exceeding six years in many parts of India and China; some residents of the US still lose up to a year of life from pollution.

Fossil fuel-driven particulate air pollution cuts global average life expectancy by 1.8 years per person, according to the pollution index and accompanying report produced by the EPIC.

"Around the world today, people are breathing air that represents a serious risk to their health. But the way this risk is communicated is very often opaque and confusing, translating air pollution concentrations into colors, like red, brown, orange, and green. What those colors mean for people's wellbeing has always been unclear," Michael Greenstone, the Milton Friedman Professor in Economics and Director of the EPIC, said.

Greenstone also noted: "My colleagues and I developed the AQLI, where the 'L' stands for 'life' to address these shortcomings. It takes particulate air pollution concentrations and converts them into perhaps the most important metric that exists, life expectancy."

The AQLI is based on a pair of peer-reviewed studies co-authored by Greenstone that quantify the causal relationship between long-term human exposure to particulate pollution and life expectancy. The results from these studies are then combined with hyper-localised, global particulate matter measurements, yielding unprecedented insight into the true cost of air pollution in communities around the world.

Seventy-five per cent of the global population, or 5.5 billion people, live in areas where particulate pollution exceeds the WHO guideline. The AQLI reveals that India and China, which make up 36 per cent of the world's population, account for 73 per cent of all years of life lost due to particulate pollution.

On average, people in India would live 4.3 years longer if their country met the WHO guideline, expanding the average life expectancy at birth there from 69 to 73 years. In the US, about a third of the population lives in areas not in compliance with the WHO guideline.

Those living in the country's most polluted counties could expect to live up to one year longer if pollution met the WHO guideline. Globally, the AQLI reveals that particulate pollution reduces average life expectancy by 1.8 years, making it the greatest global threat to human health.

By comparison, first-hand cigarette smoke leads to a reduction in global average life expectancy of about 1.6 years. Other risks to human health have even smaller effects: alcohol and drugs reduce life expectancy by 11 months; unsafe water and sanitation take off seven months; and HIV/AIDS four months.

Conflict and terrorism take off 22 days. So, the impact of particulate pollution on life expectancy is comparable to that of smoking, twice that of alcohol and drug use, three times that of unsafe water, five times that of HIV/AIDS, and more than 25 times that of conflict and terrorism. "While people can stop smoking and take steps to protect themselves from diseases, there is little they can individually do to protect themselves from the air they breathe," Greenstone said.