We think the problem comes from smokestacks, smelters, pulp mills, cars, and trucks. We also think it mainly comes from humans. We think the effects on the earth are trees being killed and all sorts of animals being hurt, including us. It comes mainly from smelters, pulp mills, trucks, cars, and coal, gasoline, and oil-burning power plants.
Smelters are places where metals like nickel and copper are extracted from rock.
Smelting and paper-making give off a gas called sulfur dioxide. When trucks' and cars' engines work, they give off a family of gases called nitrogen oxides. Coal burning power plants give off both of these polluting gases - sulfuric acid and nitric acid. These acids fall to the ground in many forms. Some are wet, like rain, snow, fog, and dew.