According to the World Air Quality Report 2019 compiled by IQAir Air Visual, 21 of the world's 30 cities with the worst air pollution are in India.
Ghaziabad, a satellite city of the capital New Delhi, is ranked as the world's most polluted city, with an average PM 2.5 concentration measurement of 110.2 in 2019 - nine times the level which the US Environmental Protection Agency regards as healthy.
PM 2.5 includes pollutants such as sulphate, nitrates and black carbon, with exposure to such particles having been linked to lung and heart disorders.
India is one of five countries regarded as having ‘unhealthy’ levels of pollution, alongside Afghanistan, Mongolia, Pakistan and Bangladesh.