Trehan compared the lungs of a 55-year-old man living in Himachal Pradesh with those of a 52-year-old man living in the capital. While the former's lungs appeared pink, the Delhiites's lungs were darker in colour. Trehan has explained this as a side-effect of breathing in air laced with pollutants.

"The men came to the hospital last week for heart surgery. I was shocked to see the difference in the colour of their lungs," said Trehan, who has also shared his findings with CM Arvind Kejriwalin a bid to highlight the urgent need to curb rising pollution levels. The CM later re-tweeted the images.

Trehan has also supported Delhi government's odd-even formula but added that long-term measures must be looked into. "Overloaded trucks cause 10 times more pollution than those ferrying the allowed weight. The law must deal strictly with defaulters. There should be limitations on diesel-run vehicles as well. Also, people must be made aware of the urgency of the situation so that they can make small changes in their own lives like carpooling," he added.

Dr VikasMaurya, senior consultant of respiratory medicine at BLK super-specialty hospital, said he too has seen many patients from Delhi with lungs as damaged as a smokers'. "Pollution is emerging as a silent killer in Delhi. It weakens lungs, affects heart function as well as overall performance levels of young and otherwise healthy individuals," he said. He added that pollutants like particulate matter enters the respiratory tract causing inflammation in the mucosa and long-term damage.

PM10 or respirable particulate matter equal to or less than 10 microns, can cause coughing, wheezing, asthma attacks and bronchitis. The levels of PM2.5 (tinier and more easily inhaled particulate matter), which is a more dangerous pollutant, too were four or five times the safe standard of 60 micrograms per cubic metre in the city, said another expert.