Why Mumbai Had Poor Air Quality In October

While vehicular emissions have long been recognised as a substantial source of air pollution, the ongoing construction projects have further deteriorated the ambient air quality. Additionally, certain climatic conditions further trapped the pollutants for a prolonged period, as we explain later, to cause alarming Air Quality Index (AQI) levels in the city.

What is PM 2.5 and PM 10

Particulate matter measuring less than 10 micrometre in diameter has the ability to penetrate deep into the respiratory system. Among these particles, those with a diameter of less than 2.5 micrometre, known as PM 2.5, pose the most significant health risk. PM 2.5 particles are capable of infiltrating the lungs and occasionally entering the bloodstream, resulting in serious health consequences.

PM 2.5 and PM 10 are critical components of the Air Quality Index (AQI). A network of 25 Continuous Ambient Air Quality Monitoring Stations (CAAQMS) across the city monitor these particulate matter levels along with other parameters to determine the ambient air quality. According to the World Health Organization (WHO), the permissible annual limit for PM 2.5 levels in the air stands at 5 microgram per cubic metre (µg/m³) of air, although the National Ambient Air Quality (NAAQ) Standards define a higher threshold of 40 µg/m³ annual and 60 µg/m³ daily limit. IndiaSpend previously reported that if India were to meet these WHO air quality standards, it could potentially extend the average lifespan of its citizens by four years.

In 2019-20, 31% of PM 2.5 was from transport, up from 16% in 2015-16, revealed a study by System of Air Quality and Weather Forecasting and Research (SAFAR), at the Indian Institute of Tropical Meteorology, Pune. But this was before a number of construction projects commenced in the city, according to Gufran Beig, the founder of SAFAR and Chair Professor in National Institute of Advanced Studies (NIAS). Construction activities in Mumbai contribute 8% to the total particulate matter and the Mumbai Metro project alone is responsible for 3.2% of the suspension dust in the city, as of 2022. Moreover, there has also been a 11% increase in total vehicles between 2019 and 2021.

Most affected localities

Between October 1 and October 21, air quality monitors in Chakala (Andheri East), Mazgaon, and Navy Nagar (Colaba) registered the highest PM 2.5 levels within the city. Between October 14-20, the daily PM 2.5 concentration in Chakala soared to a level that was nearly three times the CPCB limit, measuring 171 µg/m³, while Mazgaon saw a level over twice the CPCB limit, with a daily average of 128 µg/m³. Data for the second and third weeks of October for the Colaba monitoring station are unavailable. However, during October 3-7, the daily average PM 2.5 concentration exceeded twice the CPCB limit, reaching 129 µg/m³.

Further, monitors in areas such as Mazgaon, Khindipada (Bhandup West), and Bandra-Kurla Complex (BKC) reported PM 2.5 levels that were twice as high as those recorded in October 2022. As for PM 10, Vile Parle, Deonar, and Chakala reported levels that were double the permissible limit set by the CPCB.

“Regional mapping of activities and the development of hyperlocal action plans hold significant importance. Ensuring that smaller, local construction activities adhere to the regulations and guidelines set by the Brihanmumbai Municipal Corporation (BMC) is crucial," emphasises Anumita Roychowdhury, executive director at Centre for Science and Environment (CSE). For instance, in January 2023, after receiving several complaints from residents, the BMC shut down illegal commercial units in Andheri East. Such small clusters of illicit operations can substantially impact the local PM levels.

Vortex because of climatic conditions

Mumbai, with its three sides surrounded by the sea, has historically benefitted from coastal winds that help disperse pollutants, explains Beig. “Despite the high emission levels, Mumbai is usually better than other cities like Jaipur, Ahmedabad or Delhi.” However, in October of this year, a confluence of factors negated this advantage, he says. This resulted in the entrapment of pollutants from various sources within the city.

“After the monsoon receded, the coastal winds, which typically aid in pollutant dispersion, became slow," explains Beig. "Additionally, there was a significant temperature differential between the Sahyadri hills and the plain region, near Navi Mumbai. The winds descending from the hills were considerably cooler than those in the plains, giving rise to a vortex effect. In this region, airborne dust was trapped and subsequently pushed towards Mumbai. Given that surface winds in Mumbai were already stagnant, particulate pollution found itself trapped within the city for prolonged periods."

Beig anticipates that with the city's recent temperature decrease and the conclusion of the post-monsoon stagnation of coastal winds, the AQI will return to its typical levels.

Increase in AQI in the city during winters

The city has witnessed a consistent rise in the average AQI over the winter months. “In 2022, the modulated large scale wind pattern due to rare triple dip La Niña conditions led to unusually calm winds in October to beginnning of December resulting in record-breaking air pollution levels in the city during that period,” Beig says. With the absence of the La Niña influence, and if emissions are kept below standards set by the municipality, “it is anticipated that pollution levels should remain below the previous peak levels,” he adds. “As long as the carrying capacity (the maximum emissions an area can take without surpassing the standards) is not breached, it is less likely to encounter significant air quality issues.”

The Mumbai Climate Action Plan 2022 notes key challenges in managing air quality, including the lack of enforcement and regulation in the implementation of Construction and Demolition Rules. In March 2023, the BMC issued a strategy for air quality mitigation, and on October 25, it released a fresh 27-point guideline for tackling the ongoing issue.

“Mumbai was one of the first cities that came up with a Climate Action Plan” says Milind Mhaske, director of Praja Foundation, a Mumbai-based NGO. “The document lists a set of essential parameters for monitoring and managing air quality. However, these guidelines need to be enforced and decisive actions need to be taken when these standards are breached. Effective leadership demands proactive responses to significant environmental shifts like these, rather than merely reacting to crises by issuing reactive guidelines.”

IndiaSpend contacted BMC ward councillors to request information regarding the measures taken to address the problem and to provide information on construction and development activities that are not in compliance with regulations. We will update this story when we receive a response.

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