Delhi's Chief Minister Arvid Kejriwal said Sunday that India's air pollution has become "unbearable" as his city's air quality has moved into the "hazardous" category. Delhi residents have been urged to say inside. 

More than 150 flights have been impacted due to reduced visibility from the smog. Pollution levels hit 810 micrograms per cubic meter early Sunday. The World Health Organization (WHO) has claimed that the safe daily maximum is 25 micrograms. 

The pollutants themselves are called PM2.5, with exposure to the particles causing chronic disease such as asthma, heart attacks and bronchitis.

On Friday, over five million masks were handed out to help Delhi students to deal with the pollution. Schools have been closed until Tuesday. The population of the Delhi metropolitan area is over 20 million. 

A study by the Massachusetts-based Health Effects Institute claims that 1.2 million Indians were killed by air pollution in 2017. More people die in India from pollution than from car accidents or from malaria. The study says that pollution reduces the lifespan of the average South Asian child by two-and-a-half years. 

Out of the 30 most polluted cities in the world, 22 of them are in India. China also faces a major air pollution problem, shaving an estimated $38 billion off its economy each year.