. Since the city was built on marshy land, the resulting fog combined with smoke, and generated a thick black smog. . High levels of pollution was due to the huge population that used dung and wood as fuel in their daily lives. . Main pollutants were the industries and establishments that used steam engines run on coal. . Rice mills of Tollygunge began to burn rice husk instead of coal, leading to black soot which fell like drizzling rain.