Nepal, May 4 -- My fondest memories of growing up in Kathmandu in the 1980s are riding trolley buses, the cool shade of eucalyptus-lined streets, clear mountain views in the Northern skyline and a city that bathed in pristine air. However, in recent years, Kathmandu has topped the list of the world's most polluted cities. This April, a toxic mix of forest fires, delayed monsoon rains, emissions from aging diesel vehicles and unchecked construction dust drove PM2.5 level soaring to over 200 micrograms per cubic metre-more than 40 times the World Health Organisation's (WHO) safe limit.

This toxic air is killing more than 40,000 Nepalis every year, according to the WHO. Worryingly, this toll exceeds casualties from the 2015 earthquake. Kath...