The city's air, usually much better than national capital Delhi, is expected to decline sharply on Diwali night when it is likely to fall to from 'moderate' category to "poor".
This is when, a government air monitoring system says, the air Mumbai breathes could increase the likelihood of respiratory symptoms in sensitive individuals and put the elderly and children at risk.
But what makes this worse is the steady increase in the share of PM 2.5 pollutants - 100 times thinner than a human hair which can reside in the lungs and aggravate respiratory conditions - would spike.
The worry is that by the time Mumbai is done celebrating Diwali, 60 to 70 per cent of the suspended particulate matters would be of the PM 2.5 variety.
The Pune-headquartered System Of Air Quality And Weather Forecasting And Research, or SAFAR, said the recent spell of rains in the city, however, may just have ensured that the city's air quality was going to be the best ever on any Diwali over the last three years.
"This time the air quality will not be bad due to weather conditions... Surface winds, which were calm, have picked," Dr Gufran Beig, director SAFAR said. The winds ensure that the pollutants do no stagnate and choke people.
Mumbai Mayor Vishwanath Mahadeshwar said banning firecrackers - as was ordered by the Supreme Court for Delhi - is not the only solution to combat air pollution. "There are other things such as vehicular traffic that also contribute to pollution. But (in future) if a ban is needed, the state government should order it," the senior Shiv Sena leader told NDTV.
The government-funded body also ranked Mumbai localities by the expected air quality over the next five days.
According to this list, Nerul and Bandra-Kurla complex are at the bottom of the list and south Mumbai's Worli on the top. The other localities ranked by Safar, ranked by the quality of air are, Bhandup, Malad, Colaba, Chembur, Andheri, Borivali and Mazgaon.