When levels of carcinogenic pollutants in Delhi’s air started breaching the 300-mark on a daily basis around four winters ago, Mumbaikars were unconcerned because they believed their proximity to the sea would forever prevent such a situation. But for the last couple of years, the benevolent sea breeze seems unable to ‘sweep out’ the unhealthy mix of dust and particulate matter (PM2.5 and PM10) hanging over Mumbai.

The city has witnessed one of its most polluted Octobers, with the air quality index (AQI) rising way above 300 in some of the western suburbs.

For a couple of days, Mumbai’s particulate levels were higher than those in Delhi – one of the most polluted cities in the world. People with sensitive airways now need to rush to their doctors to update their medicine doses; a paediatric hospital said the number of children coming in with respiratory complaint s has shot up by 30% in a month.

“The biggest change on the pollution front in Mumbai is the timing of the smog, ’ ’ said Dr Di l ip Boralkar, former member secretary of the Maharashtra Pollution Control Board (MPCB). While smog would usually lift by 9am to 9.30am, it now lingers till 11am or beyond on some days. “We don’t need air monitors and AQI readings to know that air pollution levels in Mumbai are worsening. The smog tells the whole story,’’ he said.

Too much construction at once
What has brought about this change in Mumbai’s pollution story? In the absence of any concrete studies/findings, the jury is still out. There are, of course, the usual suspects.
For one, ongoing construction that’s estimated to cover five times the area of Nariman Point, Mumbai’s landmark commercial area. Green activists also blame the metro construction which has for the last five or more years reduced busy roads, including Dr D N Road that runs along the city’s famed Art Deco precincts, to almost half their original width. Then there is vehicular pollution from Mumbai’s 12 lakh private cars – the city has the highest car density in the country with 600 cars per kilometre of road.
“Our roads have been halved, vehicles crawl as a result and take double the time to reach their destination, spewing more particulate matter into the air,’’ said pulmonologist Dr Sanjeev Mehta.
Climate change plays a role
Climate change is another factor. In 2022, experts traced the high levels of particulate matter in the city to La Nina, a climate pattern in which an abnormal drop in surface temperature over the Pacific Ocean severely affected the speed of coastal winds around Mumbai. With barely any wind blowing from the Arabian Sea, pollutants couldn’t be dispersed. While there is no La Nina this year, air pollution is worsening. This, say experts, is in part due to the October heat and the delayed retreat of the monsoons.
Ozone levels are rising
Dr Boralkar, who has been studying air pollution in Mumbai for over four decades, believes photochemical reactions have become a major contributor to air pollution and smog in the past three years. These reactions are triggered by heat: “could be the spark in the car engine needed to burn the fuel”. When nitrogen oxides are present, a photochemical reaction can lead to the creation of free radicals of oxygen. “These free oxygen radicals immediately combine with normal oxygen molecules and create ozone, an abrasive and corrosive gas,’’ said Dr Boralkar.
Harish Phuleria, associate professor from IIT Bombay’s Environmental Science and Engineering Department, said a combination of outdoor pollution caused by vehicular and construction activity and “some meteorological phenomenon” is at play. “We need to conduct studies that can pinpoint the reasons. We also need to understand if the rising air pollution problems occur across Mumbai or are restricted to certain pockets,’’ said Dr Phuleria.
Highrises may act as curtain
Experts are also considering whether highrises act as an impediment to sea breeze activity. “Highrises could be creating localised wind patterns that prevent the sea breeze from acting like it previously did,’’ said an academician who didn’t want to be named. Dr Phuleria, however, said wind patterns caused by tall buildings wouldn’t be big enough to cause meteorological changes for the entire city.
Scientists want to study the contribution of other pollutants as well, including wood burning in bakeries and crematoria, cooking in homes, restaurants and streetside stalls, as well as burning of waste in the open.
Meanwhile, the overall AQI reading for the city continues to be unhealthy at 160 even as global sister cities such as coastal New York and London have “good” readings under 30.