What is the meaning of ‘moderate’ AQI in this haze? | Mumbai news - Hindustan Times
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What is the meaning of ‘moderate’ AQI in this haze?

Jan 06, 2024 04:51 AM IST

Residents complain of an uptick in respiratory infections and illnesses, even as doctors advise children, and the elderly to avoid going outdoors

Mumbai: Although the average air quality index (AQI) of Mumbai has remained moderate – between 101 and 200 – in recent weeks, and the Brihanmumbai Municipal Corporation (BMC) has claimed significant improvements in combating air pollution over the last season, concerns about the city’s air quality have acquired a fever pitch.

Mumbai, India - Jan. 4, 2024: Haze over the city skyline at Marine Drives, in Mumbai, India, on Thursday, January 4, 2024. (Photo by Anshuman Poyrekar/ Hindustan Times)
Mumbai, India - Jan. 4, 2024: Haze over the city skyline at Marine Drives, in Mumbai, India, on Thursday, January 4, 2024. (Photo by Anshuman Poyrekar/ Hindustan Times)

Photos of the city wrapped in haze, with buildings and landmarks a few hundred meters away rendered invisible, are all over social media. Residents from various parts of the city complain of an uptick in respiratory infections and illnesses even as doctors advise children and the elderly to avoid going outdoors unless absolutely necessary.

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HT spoke to experts from varied fields to understand and unpack what moderate AQI means, and what citizens can do protect themselves under the current circumstances. The experts included Ronak Sutaria, founder of Respirer Living Sciences; Rakesh Kumar, former director, National Environmental Engineering Research Institute; Dr Lancelot Pinto, pulmonologist and epidemiologist, PD Hinduja Hospital-Mahim; and Dr Rajesh Sharma, respiratory medicine expert practicing in south Mumbai hospitals.

Also read: Here's how lung cancer patients should take care during air pollution

How is AQI calculated in Mumbai?

There are 30 air quality monitoring stations in Mumbai – 16 are run by the Maharashtra Pollution Control Board (MPCB), nine by the System of Air Quality and Weather Forecasting and Research (SAFAR), and five by the BMC. These stations calculate the values of gaseous and particulate pollutants at 15-minute intervals.

Gaseous pollutants are of five types – sulphur dioxide, nitrogen oxides, carbon monoxide, ozone, and ammonia, while particulate pollutants are of two types – PM2.5 and PM10, which refer to their diameter in microns. Monitoring stations record the values of each of these pollutants in individual units – like micrograms per cubic meter (ug/m3) of air for PM2.5 and PM10 particles. These values are then converted into a sub-index using a formula, and whichever sub-index averages the highest in 24 hours is known as the AQI.

Does the AQI always accurately reflect air pollution in an area?

This largely depends on where the monitoring stations are located – a monitor in the middle of the Sanjay Gandhi national park will not give an accurate picture of a road with heavy vehicular congestion in Borivali.

As AQI refers to the 24-hour average, it is entirely possible that the air quality at a particular time and place is not accurately reflected by the number, said Sutaria. For example, the AQI in BKC on January 5 at 8pm was 193 – i.e., moderate. But the concentration of PM2.5 within the 24-hour window covered by the AQI number exceeded 300 – signifying very poor-quality air – between 6am and 8am.

How do seasons affect air quality?

Both Sutaria and Kumar emphasised that air quality is worst in the winter season, especially during the mornings, between 5am and 7am. This is because low temperatures reduce the ability of pollutants to disperse, trapping them. As temperatures increase during the day, pollutants disperse more easily, and the air quality reaches its best between 2pm and 4pm. This is why advisories recommend citizens to stay indoors in the morning during winters. This is also why air quality is not an issue during the summer months, even if the extent of pollutants present in the air is same.

Winters in Mumbai are also marked by weaker winds in the reverse direction while tall buildings hinder the mixing of winds, which aid the formation of pockets of high air pollution, said Kumar.

Why is visibility so poor? Is it entirely due to air pollution?

The presence of fog during winter months, especially in the morning hours, has a major effect on visibility, as pollutants tend to cling to moisture particles. Dust and particulate pollutants, which are given a higher weightage in the AQI, are partly responsible for this, as are small particles which behave more like gas, said Kumar. These include nitrogen oxides and hydrocarbons present in vehicular emissions, which react with sunlight and form very fine pollutants such as ground level ozone, which is a lot more harmful than ozone in the stratosphere. Long-term exposure to ground level ozone corrodes the inner lining of the throat and makes people more susceptible to respiratory infections.

Why are winters in Mumbai bad for those with respiratory illness?

High levels of air pollution and cold weather act as triggers for most patients with respiratory diseases, especially if they have an allergic condition like asthma. Inhaling cold air can lead to constriction in the throat, nasal passage, and sound box, resulting in shortness of breath, swelling in the throat, sneezing, and a stuffy nose. Smog can cause or aggravate health problems such as asthma, emphysema, chronic bronchitis, respiratory problems, eye irritation, and reduced resistance to colds and lung infections.

Additionally, indoor overcrowding during the holiday season in the winter facilitates the transmission of respiratory viruses, triggering symptoms. This results in an increase in respiratory illnesses such as bronchial asthma and allergies.

Many people are investing in air purifiers. Do you think this is a wise decision in a city like Mumbai where AQI is bad in the winter as well as the summer?

There is no real argument against air purifiers, especially given the air quality over the past two months, continuing into the present. But we need to work harder to lobby for cleaner air rather than accept air purifiers as a part of life.

What is the alternative for people who can’t invest in aur purifiers?

Masking when outdoors and in crowded places would help protect individuals. But since the most efficacious masks also tend to be the most uncomfortable, it might not be a feasible long-term solution. Avoiding road travel, opting for public transport, and consulting with a doctor for any necessary changes in medication are recommended.

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