Regarding the map, I am sure you have done this with input from everyone responsible in India. So, I guess it is all right. However, studying and working at Bombay for years and running a small chemical company myself out of Gujarat right now, I feel the chocolate color of extreme severity should have been extended further north from Mumbai and then bend westward at least up to the Arabian sea before it meanders northbound again. This is more so since you have shown extreme corrosivity up to Calcutta (from the neighborhood I originally come from!).
I think extreme corrosivity results from the combination of salinity in the atmosphere, humidity and the industrial pollution. Mumbai/Surat/Baroda coast line is famous for this combination. On the east coast, Orissa may be sea coast but that much industrial pollution compared to West coast is simply not there. Come to Calcutta and you do not even have a sea coast. It is all sweet river Ganga (or Ganges as the Brits called it in lieu of the proper Hindi or Bangla pronunciation) that flows through. The city's maximum pollution comes from automotive exhaust this days than anything else (just been back from there this January, had to be on bed rest at home for the injury and could not even breath every time had to visit the doctor).
In addition to the automotive exhaust, some industrial pollution is there coupled with people's traditional habit of burning natural brush as fuel along with dry cow dung which give out some smog. But it is nowhere near the severity of the type of chemical water and air pollution you can find in Entire Maharastra and Gujarat coastline, in the middle of which Mumbai lies. So, if I were you, I would either enlarge the severity bar or reduce it at least partly between Hyderabad and Calcutta.
One more thing amazing to me (in fact outright faulty) is that you have represented Delhi and surrounding region as moderately corrosive. That used to be the traditional case for which Iron pillar still survives I suppose. But modern Delhi is in a mess. There is a joke among us never to fly through Delhi as we might hit another aircraft due to the heavy smog all through the year. True, it does not have the saline weather. But the air pollution from auto exhaust coupled with the bad air from Refinery near Taj Mahal and all have made Delhi a sick place in the north. The current atmospheric corrosion ought to be a lot higher than it used to be, judging from the report I get and from the personal visit 3 years back, I am ready to take bet on this. In fact, some of the coastal Kerala without any industry is nearly similar in terms of atmospheric corrosion and lot better in terms of air breathability.
Anyway, these are my personal views again. The people staying at India should know better.
Gautam
