Sulfur and nitrogen oxides (often abbreviated SOx and NOx, respectively) are common by-products of fossil-fuel combustion. SOx forms from sulfur impurities in fuels, especially coal and some petroleum products. NOx forms from nitrogen impurities in fuels and from nitrogen in the air used

Some forests in areas subject to acid rain have also declined. For example, many trees in central European forests have died; however, these forests have also been subjected to other forms of pollution, including ozone. The health of forests in high-mountain areas of the eastern United States that receive very acidic fog as well as acid rain has also declined, but some scientists believe that these effects may also be related to other pollutants and a natural aging cycle. There is evidence, though, that depletion of soil-bound calcium and magnesiumÑneeded as nutrients by plantsÑis widespread in forest areas of the eastern United States and Europe exposed to acid rain; research in Sweden points to soil acidification as the reason behind the loss of soil-bound calcium and magnesium. Some scientists suspect that this soil-nutrient depletion may be having long-term, cumulative, negative effects on the health of these forests.

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