For her study, Das chose three most polluted zones, one area with near-zero pollution and eight plant species to assess the impact.

"I chose Salt Lake PNB area, Ultadanga Muchibazar, and Shyambazar as three polluted zones and compared them to a near-zero pollution zone in the remotest area of Kestopur where there is no source of emission. The eight plant species chosen were mango, peepul, jarul, neem, kanchan, bougainvillea and debdaru. These are the commonest streetside variety of trees, found in almost every corner of the city," Das said.

The results were startling.

She examined the content of chlorophyll, ascorbic acid, peroxidase and phonolic of trees -- the most important ingredients for life sustaining photosynthesis. In polluted zones, these ingredients were found to have sharply declined. "The components at Salt Lake PNB were much less than what a plant should have in a pollution-free zone. It started dropping even further as I moved from PNB to Muchibazar in Ultadanga and Shyambazar five-point crossing," said Das.

"For dust particles in the air, you can blame the high concentration of diesel vehicles in the city. They always emit a huge amount of dust particles, a major cause of concern for environmentalists worldwide. Most advanced countries have put strict curbs on the number of diesel vehicles on the road. In our country, these smoke-and-dust belchers easily outnumber petrol vehicles," said city-based auto emission consultant S M Ghosh.

Environmentalists also blame the lax enforcement of green rules in the city for the crisis. "It is a pity that less than 20% vehicle owners obtain pollution-under-control certificates. Without enforcement, whatever stringent emission standards you follow, the result is a big zero," said environment crusader Subhas Datta.