Vehicular traffic, garbage burning and construction dust are all the usual suspects contributing to air pollution in Indian cities. But did you know that in Chennai, around 5.4 per cent of the particulate matter in the air is on account of the cheerful bakeries that dot the city?

Before you panic, just remember that the bakeries are by no means the biggest polluting sources in the city; transport and the dust rising due to vehicular traffic are the key pollutants.

Together, these vehicle-related pollutants account for over half of the total particulate matter (PM) that pollutes the Chennai air, says a recent study by the Central Pollution Control Board (CPCB). Particulate matter refers to pollutants small enough to enter your lungs and affect health. The study pinpointed that emissions from transportation and the dust thrown up by vehicular movement were the primary pollutants across major Indian cities.

Predictably, traffic-related pollution was highest in metros such as Chennai, Bangalore, Delhi and Mumbai, where there is a high vehicle population. Pune, too, has a problem, less on account of the traffic volume, but more on account of the dust becoming airborne. The CPCB found that the higher contribution of dust in certain cities was due to the poor state of their road networks. Non-metalled roads and the large quantities of stone dust and fine dust caused by construction work made the air unfit to breathe.

If the rising vehicle population is the problem in big cities, smaller cities such as Kanpur suffer due to burning of garbage, which spews 30 per cent of the particulate matter into the air. Secondary particulates, which are not directly emitted, but come about through atmospheric processes, was also a significant contributor to air pollution in this city.

The data threw up some other interesting findings as well. The poor power situation in Chennai has resulted in the widespread use of diesel generator sets, which are the third-worst polluters in the metro. In other cities their contribution to overall air pollution is much less.

In Chennai, diesel generator sets are responsible for 15.6 per cent of the particulate matter in the air, but less than 10 per cent in the other cities.

Nevertheless, the good news is Chennai still has good air quality in terms of all PM parameters, as does Bangalore. That’s more than you can say for cities such as Mumbai, Delhi, Kanpur and Pune. The data shows that in residential areas, PM standards relating to particles smaller than 10 micrograms, are exceeded over 90 per cent of the time in all cities, barring Chennai and Bangalore.

But what is particularly worrying for the more polluted cities is that the standards for really small particulate matter (less than 2.5 micrograms), which pose a serious health hazard, were exceeded 100 per cent of the time.