https://phys.org/news/2018-06-delhi-reels-summer-smog-indian.html

Smog spikes during winter in Delhi, already one of the world's most polluted cities, where air quality eclipses the World Health Organization's safe levels on any given day.

Pollution levels usually ease in summer, providing some relief from the smog as temperatures soar to mid-40 degrees Celsius (113 Fahrenheit).