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).