A tall chimney can protect a local community from the acute effects of air pollution by ejecting the poisons at, say, 75 metres above ground. However, this does not make the pollution disappear. The pollutants will only travel downwind to spread to other places nearby.
In the case of copper smelters, which emit sulphur dioxide (SO2) along with other poisonous metals and gases, the SO2 emissions will get converted into toxic-laden dust (particulate matter) and acid gases, that will travel as a descending plume in the airshed.
So in Thoothukudi, if the prevailing wind blows east at a modest speed of 1 km/hour, the plume from Sterlite's smokestack will spread over Thoothukudi town within four hours. Thoothukudi town centre is about 6 km from Sterlite's chimneys. Four hours after the first release of poisoned air from Sterlite's smokestack in, say February or March when the wind is still blowing east, the poisons will be manifest as mild to severe health effects in Thoothukudi town.
Ideally, your air should not have any detectable quantities of SO2. SO2 is destructively acidic. It will tear into the soft tissues of your lung and turn it into jelly and drown you in your own juice. This is not an exaggeration.
Sterlite is legally permitted by TNPCB to discharge 1.2 tonnes of SO2 per day into Thoothukudi's air. But smelters and copper refineries are complex machineries. The SO2 that is emitted from chimneys is never at a steady rate. It goes up and down depending on a variety of factors.
A minor change in, say, the sulphur content of the copper concentrate, can cause a spike in the emission of sulphur dioxide. Such unintended gas releases will be vented through the chimney and end up far away in Thoothukudi town. And then you have the case of intentional releases.
The best copper smelters have hiccoughs – when a copper smelter has a digestive problem in one of its many furnaces, it will have to release the excess gases generated through the chimney. Such releases can last from less than a minute to tens of minutes. If it lasts more than a few minutes, that is sufficient to suffocate populations living downwind of the factory.
The way the best in the world deal with such emergencies is by siting copper smelters far away from human habitation so that the toxic fumes descend on lands that don't matter. In the case of Sterlite's smelter, any accidental release through the smokestack will descend on Thoothukudi town depending on atmospheric conditions.
Sterlite's first cardinal sin was picking the wrong site. An ultra red category, air pollution-intensive industry should not have been located in close proximity to a densely populated urban area.
Fugitive emissions: The ones that got away
Toxic gases are like criminals. Just as only a small fraction of the criminals get caught, only a fraction of the toxic gases from factories are trapped by pollution control devices.
Fugitive emissions of toxic gases from a factory happen at or near ground level, and are difficult to control. Every time a furnace is opened for charging, gases escape. Vehicular movement results in resuspension of road dust. Joints, flanges, valves, storage sheds, containers storing and transporting chemicals, the act of loading and unloading material are all sources of fugitive emissions.
The US Environmental Protection Agency estimates that fugitive emissions of sulphur dioxide can be up to 6% of the total sulphur charged as part of copper concentrate into furnaces. For Sterlite, that could mean up to 90 tonnes of sulphur released as SO2 and other compounds.
People living in the immediate vicinity are more likely to be harmed by these ground level emissions than the smokestacks. That is why you have the people from Kumareddiapuram, Therku Veerapandiapuram and Meelavittan complaining of clothes getting dirty when they are hung out to dry, about black dust on leaves, of dust-caked feet, of films of dust on water, and of the first rains bringing down black water from the roofs.