The rates of sulfur and nitrogen oxide emissions generally increased between 1900 and 1970 because of increased coal use, industrialization, emissions from electric power plants, and increased fossil fuel burning for transportation. After the passage of the Clean Air Act in the United States in 1970, emissions controls were put in place which led to decreases in sulfur oxide emissions.
The extent of effects from acid rain is still uncertain, and the matter is under further investigation. There is currently no evidence of a widespread decline in United States forests due to acid rain. However, acid rain is contributing to problems with forest health in certain localized regions where trees are exposed to highly acidic fogs and cloud masses on a frequent basis. There also may be subtle effects of acid rain on retention and cycling of nutrients through the forest and in the forest soils. It is possible as well that growth of young plants may be harmed by acid precipitation, leading to potential difficulties with normal forest regeneration.