India cities dominate world air pollution list
Fourteen Indian cities are among the world's 20 most polluted, according to World Health Organization (WHO) data.
BBC analysis of the study shows the northern city of Kanpur tops the list of world cities with the highest PM2.5 levels in 2016.
PM2.5 are tiny but deadly air particles, which can increase the likelihood of respiratory and cardiovascular diseases.
Air pollution caused 4.2 million deaths globally in 2016, the study said.
Kanpur's average PM2.5 level in the same year was 173, which is 17 times higher than the WHO's safe limit.
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India's capital, Delhi, is in sixth spot with average PM2.5 levels recorded at 143.
The study added that nine out of 10 people in the world breathe polluted air.
Around 3.8 million people died in 2016 due to pollution "from cooking with polluting fuels and technologies", it said.
"It is unacceptable that over 3 billion people - most of them women and children - are still breathing deadly smoke every day from using polluting stoves and fuels in their homes. If we don't take urgent action on air pollution, we will never come close to achieving sustainable development," said Tedros Adhanom Ghebreyesus, director-general of WHO.
India has seen rapid development in the past two decades, but it has come at the cost of increasing pollution.
Weak industrial regulation means that factories do not follow pollution-control measures. Rapid construction has also contributed to high levels of PM2.5.
Last year, a public health emergency was declared in Delhi as pollution levels crossed 70 times the safe limit.
Several Chinese cities were also listed in the report as highly polluted but the data for those places was four or five years old.
Maria Neira, WHO's head of public health, said India should follow the example of China, which had made big improvements in air quality.
"There is a big step at the government level [in China] declaring war on air pollution," Ms Neira said. "One of the reasons for that is that the health argument was very strongly presented, and the fact that the citizens were really breathing air that was totally unacceptable.
"We would be very happy if we would see a similar movement now in India which is one of the countries for which we are particularly concerned."