Explainer: Why Mumbai’s air quality has gotten so bad, and continues to worsen

One of the key factors contributing to Mumbai's worsening air quality is the slowdown of surface winds from highs of 12-13 kmph to just below 4-5 kmph. (Photo credit: Kristina DC Hoeppner via Wikimedia Commons)

The withdrawal of the monsoon and onset of winter usually marks the start of smog season in north India. This week, however, Mumbai made headlines for having air quality that was worse than even Delhi's.

On Tuesday, Mumbai recorded an air quality index (AQI) of 113, according to the System of Air Quality Forecasting and Research (SAFAR). It deteriorated to 119 on Wednesday, before plummeting to 166 on Thursday.

The ensuing smog reduced visibility, delaying local trains. The poor air quality around Chhatrapati Shivaji Terminus (CST) area even resulted in a health advisory being issued. It stated: “Everyone may feel discomfort. People must avoid prolonged outdoor exposure for it may cause respiratory illness.” In comparison, Delhi recorded an AQI of 83 and 115 and 108 on Tuesday, Wednesday and Thursday, respectively.

It isn’t a new phenomenon, though. Mumbai’s air quality has been steadily deteriorating since 2019. An analysis by Respirer Reports for the January-March quarter shows that PM 2.5 levels alone have risen from 50.2 micrograms per cubic metre in 2019 to 80.6 micrograms per cubic metre in 2023, a jump of 60.5 percent. The acceptable annual standard for PM 2.5 is 40 micrograms per cubic metre. Similarly, the PM 2.5 concentration was up during the October-December quarter of 2022, compared to 2021.

It’s affecting the AQI. This is a measure of air quality based on the concentration of particulate matter, including PM 2.5, and other pollutants. An AQI between zero and 50 is considered good, 51-100 is satisfactory, 200-300 is poor, 300-400 is very poor, and 400-500 is severe.

In the first quarter of 2023 alone, Mumbai recorded 27 poor air quality days, compared to six poor air quality days in the corresponding period in 2022. There were, in fact, no good air quality days in the first quarter of 2023.

Factor this in

Mumbai, a coastal city, has long benefited from the natural sea breeze to help keep pollution levels in check. However, environmental experts now believe that this is no longer enough, as global warming is changing wind and weather patterns.

One of the key factors contributing to Mumbai's worsening air quality is the slowdown of surface winds. Normally, a coastal city like Mumbai would experience a wind reversal every three to four days, with wind speeds reaching up to 12-13 kmph. This helps to disperse air pollutants and particulate matter. However, in recent years, wind speeds have dropped to just below 4-5 kmph, resulting in pollutants getting trapped in the air.

Another major contributor to Mumbai's air pollution, according to the Mumbai Climate Action Plan 2022, is vehicular emissions, which account for 20 percent of the total emissions in the city. Solid waste also plays a role, accounting for 8 percent of total emissions.

A ward-wise analysis shows that air pollution levels are particularly critical in Mankhurd, Govandi, and Deonar. These areas are home to a large number of industrial units and have high levels of traffic congestion.

Further, rampant construction activities (buildings, metro and road work) in the city have impacted air quality too. Emissions from construction activities, and the resultant dust, gets trapped in the air due to the slow wind speeds. This is especially concerning given the gap of 10 days or so between wind reversal cycles.

The Centre’s National Clean Air Programme (NCAP) has set a target of 40 percent reduction in particulate matter concentration in cities covered under the program by 2026. This is an ambitious goal, but it is necessary to improve air quality in Indian cities and protect public health. Mumbai’s air quality, however, has been on the decline. To meet the NCAP goal will require a concerted effort from all stakeholders, including the government, businesses, and the public.