Why is Delhi's air so toxic?

More than 100,000 children under the age of five die every year in India because of air pollution, according to a report released this month by Down to Earth Magazine and the Centre for Science and Environment. Remarkably, India’s noxious air is now responsible for 12.5 percent of all deaths in the country.
 
One of the worst affected cities is New Delhi, where somedays breathing the air is equivalent to smoking 44 cigarettes a day. In 2018, the capital city had an astonishing zero good air quality days.  “Delhi’s deadly air” is the subject of a new People and Power documentary out this month.

So why is Delhi’s air so polluted? And what’s being done to change that?