Sulphur and nitrogen in fuels are particularly harmful because, when burned, they form sulphur dioxide and oxides of nitrogen, which dissolve in rain water to produce:

SO₂ + H₂O → H₂SO₃ (sulphurous acid), and NOₓ + H₂O → nitric/nitrous acids. These acids fall as acid rain, which corrodes monuments and metals, kills plants, and acidifies lakes. (Note: smog and the greenhouse effect involve different mechanisms; ozone is a separate atmospheric concern.)