Air pollution can shorten lives by almost 10 years in the Indian capital, Delhi, the world's most polluted city, says a report by a US research group.
The study adds that the average Indian life expectancy is shortened by five years at current air quality levels.
India's 1.3 billion people live in areas where the "annual average particulate pollution level" exceeds the WHO safe limit of 5µg/m³, it says.
Bad air kills millions in India every year.
The smog-filled air, which usually covers Indian cities during the winter months, contains dangerously high levels of fine particulate matter called PM2.5 - tiny particles that can clog lungs and cause a host of diseases.
The Air Quality Life Index by the Energy Policy Institute at the University of Chicago (EPIC) notes that some 510 million people who live in northern India - nearly 40% of India's population - are "on track" to lose 7.6 years of their lives on average, given the current pollution levels.
However, reducing pollution levels to WHO standards would mean that an estimated 240 million people in the northern Indian state of Uttar Pradesh would gain 10 years in life expectancy.
EPIC says that since 2013, about 44% of the global pollution has come from India - currently the second most polluted country in the world.
The report says that more than 63% Indians live in areas that exceed the country's own air quality standard - which says that 40µg/m³ is safe. But in 2019, India's average particulate matter concentration was 70.3µg/m³ - the highest in the world.
"It would be a global emergency if Martians came to Earth and sprayed a substance that caused the average person on the planet to lose more than two years of life expectancy," Michael Greenstone, one of the authors of the report said.
"This is similar to that situation that prevails in many parts of the world, except we are spraying the substance, not some invaders from outer space," he added.
EPIC says that particulate pollution is the "greatest threat to human life" in India in terms of life expectancy and since 1998, this particulate pollution has increased by 61.4%. This makes it more lethal than smoking which reduces life expectancy by about 2.5 years.
The increase in air pollution over the past two decades in India has been primarily due to industrialisation, economic development and the skyrocketing use of fossil fuel. The number of vehicles on the country's roads has increased about four-fold, the report says.
It acknowledges government efforts to fight air pollution - the National Clean Air Programme (NCAP) aims to reduce harmful particulate matter by 20 to 30%.
"If India were to sustain this reduction, it would lead to remarkable health improvements," the report says, adding that a 25% reduction would increase the national life expectancy average by 1.4 years, and by 2.6 years for Delhi's residents.