This Indian city has the world's worst air

In 2014, the World Health Organization measured air quality levels in 1,600 cities around the world, and the Indian capital city of New Delhi was found to have the highest concentration of particulate matter smaller than 2.5 micrometers, also called PM2.5.

The city's average PM2.5 level was a whopping 153, compared to 14 in New York and 56 in Beijing. The WHO's safety threshold for humans is 10. PM2.5 particles are not visible, but they go deep into the lungs, and are more likely to cause chronic health problems.

According to analysts, the large number of old vehicles on roads, burning of bio-mass, industry emissions and coal-powered power stations are mainly to blame for Delhi's pollution levels. There are roughly 8.5 million registered vehicles in the city, and 1,400 cars are being added to the streets each day.

Last week, the National Green Tribunal, a government agency that deals with environmental disputes, attempted to ban diesel-powered vehicles older than 10 years from Delhi's streets.

Beyond Delhi, the news doesn't get better for the South Asian powerhouse: 13 out of the world's 20 most polluted cities are in India, and the WHO says outdoor air pollution is the country's fifth biggest killer.