Kathmandu, Nov. 8 -- Basudev Paudel is in his late fifties and at his age, he believes that regular morning walks are important for keeping healthy. From his home in Kathmandu's Chabahil, Basudev's walks take him around Pashupatinath Temple and the Guheshwori area. Three members of the Paudel family followed suit, but they never gave much thought to the quality of the air they were breathing in.

Sunil Paudel, Basudev's son, stopped going on morning walks last year after learning that Kathmandu's air quality was reaching unsafe levels. Rather than benefits, morning walks could have serious health impacts, Sunil said.

The air in Kathmandu, already dubbed one of the most polluted cities in the world, gets worse once the winter sets in. And...