Mumbai’s air quality differs in different pockets because of variable topography, wind movement and factors such as vehicular density, industries, biomass burning, bakeries, construction, garbage dump sites

On Sunday, Mumbaikars woke up to poor air quality. At 10 a.m., the National Air Quality Index (AQI) recorded pollution levels as high as 249, falling in the range of ‘very unhealthy’ air. By 3 p.m., the air quality had improved to ‘unhealthy’.

With many fluctuations in data, Mumbai’s air quality differs in different pockets because of variable topography, wind movement and factors such as vehicular density, industries, biomass burning, bakeries, construction, garbage dump sites, pointed out Rakesh Kumar, scientist and head, Mumbai Zonal Center, National Environmental Engineering Research Institute (NEERI).

The New Year began with a spot of good news for the residents of Chembur, commonly referred to as the ‘gas chamber’ of Mumbai. Despite, heavy industrial activity and power generation in the eastern suburb, it saw a spell of clean air thanks to calm winds, which did not carry emissions from the industrial locations, according to an analysis by Gufran Beig, scientist and project director at System of Air Quality and Weather Forecasting And Research (SAFAR), Pune – an initiative of the Indian Institute of Tropical Meteorology (Ministry of Earth Sciences).

As per SAFAR’s study, air quality at Chembur was ‘satisfactory’ or ‘good’ in the first five days of this year. Bhandup, Bandra Kurla Complex (BKC), Malad, Mazgaon, Andheri and Borivali reeled under ‘very poor’ to ‘poor’ pollution levels.

“Industrial pollution is at a point source or localised. Since the wind became calm and there was moisture in the air, the emissions from industries became stagnant and could not be dispersed over the nearby areas. Chembur can be generally clean, but for the dispersion of pollutants,” Dr Beig told The Hindu on Sunday.

Despite a lot of traffic and proximity to Churchgate and CST stations, Colaba has relatively cleaner air owing to a good green cover, Dr Beig said.

Andheri and Borivali, accessed by the arterial eight-lane Western Express Highway, have high levels of pollution. Mazgaon, which houses industrial estates of small of medium scale, had the worst air quality, falling in the range of ‘very poor’ every day till January 9.

“The transport sector is the biggest contributor to Mumbai’s air pollution, followed by industrial pollution, wind-blown dust and lastly bio-fuel emissions. Mumbai has heavy air which catches the emissions. Dust from unpaved roads gets aggravated due to heavy traffic. In the slum areas, such as Dharavi, Mankhurd, Deonar, air pollution is high due to the burning of bio-fuels such as coal and wood for cooking. The situation is better than in Delhi as a lot of people in Mumbai slums are connected to LPG. Over 70 per cent of the people in Dharavi have LPG but the slum itself is so huge and spread over such a vast area that even the lesser percentage using bio-fuels makes for high levels of pollution,” Dr Beig said.

The government’s AQI considers eight pollutants: PM10 (particulate matter up to 10 micrometers in size, result of dust); PM2.5 (particulate matter up to 2.5 micrometers in size, result of vehicle emissions, coal burning); NO2 (nitrogen dioxide, result of vehicle emissions and power plants); SO2 (sulphur dioxide, result of burning of coal, industrial and vehicle emissions); CO (carbon monoxide, result of vehicles run on diesel, gasoline); O3 (ozone, a layer of gas shields us UV rays, but harmful if present in lower atmosphere); NH3 (ammonia, emitted in agricultural processes) and Pb (lead, can be released into soil, air and water through soil erosion, volcanic eruptions, sea spray and bushfires).

In Mumbai, PM2.5 and PM10 remain the dominant pollutants, with PM2.5 being more harmful of the two, as it can get directly deposited to the bloodstream through inhalation. Vehicular pollution is the main cause of PM2.5, whereas PM10 results from dust. Areas with dug up or unpaved roads and construction projects see a high concentration of PM10.

Experts say it would be wrong to see the problem in terms of largest contributors to air pollution. “What has an impact is the air pollution at the ground level and our exposure to it. We can say that it is vehicle, construction activities, re-suspension or road dust, biomass/garbage burning, as large sources. In smaller pockets, bakeries, crematoria also create local hotspots,” Dr. Kumar said.

It is not just the quantity of pollutants, but also their characteristic which affect the air. However, AQI does not reflect this and the data fall short of the complete truth. In terms of PM2.5, for instance, the “toxicity” of the particulate matter is very important.

“The current air quality data tells you the amount of particulate matter in the air but not the chemical character of that matter, such as whether it has sulphate, nitrate or carbon. Bio-fuels emit black carbon which is the deadliest pollutant. It results from burning of coal or wood at low temperatures, as in cooking, releasing a high amount of carbon. Industrial burning of coal, on the other hand, is a high combustion activity. Low combustion increases the toxicity of the air even though the PM2.5 level may be low. For example, if the PM2.5 level is 25, but it is composed of carbon, it is more harmful than a PM2.5 level of 100 with low carbon component. Advance research in this area is under way,” Dr Beig said.

In addition to the eight pollutants considered by AQI, SAFAR tracks other parameters, such as NOx (a generic term for mono-nitrogen oxides NO and NO₂); Methane (CH4) (a greenhouse gas); benzene (pollutant often released from oil refineries and traffic), toluene (component of petrol), xylene (used in blending petrol) and mercury (produced by coal-fired power plants, other industrial processes).

Dr. Kumar of NEERI raised concerns over increasing levels of NOx in some pockets of the city. An irritant gas NOx results from diesel-powered trucks and passenger cars.

Mumbai is blessed with the sea and the sea breeze can help dilute and disperse pollution. “Land and sea breeze help during the 24-hour period to dilute the air. However, there are areas of Mumbai which do not get enough wind and therefore dilution is limited,” Dr. Kumar said.

Seasonal changes affect the air quality of Mumbai, winter being the most polluting time, followed by summer, while monsoon is the least polluted.

“In winter there is an ‘inversion layer’ – layer of the atmosphere in which there is temperature inversion preventing the air below it from rising, thus trapping pollutants. This layer, which is up to two km away from the lower atmosphere, comes down to up to one km in winter, preventing the diffusion of pollution. It is like a cover. In Delhi it comes down to as low as 100 metres,” Dr. Beig said.

Health problems are seen when someone is exposed to pollutants consistently for a period of 24 hours. Children and old people are at a greater risk. At ‘very poor’ levels of air quality, respiratory problems are aggravated and asthmatics can get seasonal attacks.

Paving the roads to allow for absorption of water instead of completely sealing them with cement can reduce PM10. Signal synchronisation can help lower vehicle emissions, suggested Dr. Beig. “If you know the amount of time the signal stays red, you can turn off the engine of your vehicle. In case of jams, crawling traffic, the vehicle engine keeps running and congestion goes up.” Last June, SAFAR started a mobile application, which gave the air quality forecast for the next three days.

“A major cause of air pollution is congestion for which a poor pedestrian system and encroachments on roads are responsible. Public transport has become expensive and not available at most locations. Mumbai needs to very quickly augment public transport through cross-financing by way of charging fees for every kilometre travelled by private vehicles. Clean air fund as a concept was given to the government a few years back to improve public transport,” Dr. Kumar said.