Pure water has a pH of 7.0. However, normal rain is slightly acidic because carbon dioxide (CO2), sulfur and nitric oxides dissolve into it forming weak carbonic, sulfuric and nitric acids, giving the resulting acidic mixture a pH 4.1 to 6.7 (pH 4 is one thousand more acidic than pH 7). Limestone and marble are alkaline stones (CaCO3) and are sensitive to acidic liquids. When used outside it will etch and scour causing surface roughness and giving the appearance of fading. Urban rain will dissolve exposed alkaline stone at least 50 times faster than unpolluted rain. University research has found that by minimising water contact with the surface an impregnating sealers can reduce but not stop this phenomena. A topical coating will make the situation worse as it will entrap rain water enhancing the acidic reaction.