Bengaluru, Dec. 22 -- When IntrCity SmartBus first decided to measure the air its passengers were breathing, the results were unsettling. Inside some of its long-distance buses, PM2.5 levels were five to six times higher than what the World Health Organization (WHO) considers safe, even though the cabins were sealed off by air-conditioning. PM2.5 is fine particulate matter, less than 2.5 microns in diameter, and is a pollutant.

"The AC was on, people felt fine. But the data told a very different story," said Manish Rathi, co-founder and chief executive officer (CEO) of IntrCity SmartBus. As the buses moved out of cities, through industrial belts and construction-heavy corridors, fine particulate matter seeped in steadily, turning what sh...