When sulfur and nitrogen dioxides (both common by-products of coal and other fossil fuels) are emitted into the atmosphere, they often fall back to Earth as acid rain. While normal rainfall has a very slightly acid pH of about 5.6, acid rain can be so highly acidic that it is capable of literally melting rocks. In parts of the world with a great deal of atmospheric pollution, rain, snow and fog with a pH lower than that of vinegar has been reported.

This is a problem for urban Europe for many reasons, but perhaps most notably because the statues and buildings that have stood for thousands of years are now, suddenly, disintegrating due to the very crush of people who want to see them. Calcium containing stones such as marble, granite and limestone are particularly vulnerable, now noted in many historical districts as causing statues and buildings to quite literally flake away as bits of gypsum.
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