Four out of 10 of the cities in the world with the worst air pollution are in India, the World Health Organization says.
Gwalior, Allahabad, Patna and Raipur have high levels of a type of pollution known as particulate matter (PMs), a study found.
The capital, Delhi, featured as the 11th most polluted city.
Tiny particulate matter can cause stroke, heart disease, lung cancer, and chronic and acute respiratory diseases.
The WHO says more than seven million premature deaths occur every year due to air pollution. Three million of those deaths are due to outdoor air quality.
The WHO study examined air quality in 3,000 cities, towns and villages across 103 countries.
The findings are based on country reports and other sources between 2008 and 2013, although some nations did not contribute data.
Delhi had an annual average PM2.5 measurement of 122, the study found.
The PM2.5 micro-particles are classed as a Group 1 carcinogen by WHO and, as well as damaging the lungs, are a cause of heart disease and other life-threatening conditions.
Maria Neira of WHO praised India’s government for developing a national plan to deal with the problem.
“Probably some of the worst cities that are the most polluted ones in the world are not included in our list, just because they are so bad that they do not even have a good system of monitoring of air quality, so it’s unfair to compare or give a rank,” she told Reuters news agency.
The dirtiest air was recorded at Zabol in Iran, which recorded a PM2.5 measure of 217.