Not in the best of worlds... but, in areas that use coal as a fuel, especially large inducstrial furnaces, a by-product of the coal burning is sulphur dioxide. This can react with oxygen and water to form sulphuric acid, which falls to the ground along with rain water, which is the so-called acid rain.
In rain forests or other pristine, non-industrial areas, the rain is about as pure of water as one can get...