Acid rain is rain, or any other form of precipitation, including dust, that is unusually acidic. It commonly has a pH range of 4.2 to 4.4, in contrast to approximately 5.6 for normal rain, which is slightly acidic because it absorbs carbon dioxide, and 7.0 for pure water.
The main cause of acid rain is the emission of sulfur dioxide and nitrogen oxides into the atmosphere from human activity, especially the burning of coal and other fossil fuels for electric power generation, motor vehicle operation, and steel and other industrial production, but also from animal agriculture. Natural causes include volcano eruptions, natural fires, lightning, earthquakes, and some microbial activity. These compounds react with water vapor in the atmosphere to produce sulphuric_acid and nitric acid.
Acid rain has a variety of harmful effects. Moreover, these effects can spread over wide areas rather than just being close to the sources of the emissions because the sulphur dioxide and nitrogen oxides can be blown long distances by the wind.
For example, it can damage the soil by stripping it of calcium and other important nutrients and release aluminum and other metals in forms toxic to both plants and animals into it, and eventually into water bodies. Trees and other plants also suffer damage from the effect of acid rain, which can wash away protective layers on their leaves and needles, thus making them more vulnerable to extremes of temperature, diseases and insects.
Acid rain can also damage lakes and other fresh water bodies by making them more acidic, thereby harming insects, mollusks, fish, amphibians and other aquatic organisms, and eventually affecting other species, including some birds. Acid rain can result in thinner eggshells in some species of birds, resulting in the hatching and surviving of fewer chicks.
Acid rain can directly affect human health by causing or aggravating respiratory illnesses such as asthma and chronic bronchitis. This is in addition to the damage that the pollution that causes acid rain can inflict on human health via the lungs, including from the ultra-fine particles and from the ground-level ozone caused by nitrogen oxides.
Moreover, acid rain can also cause economic and cultural damage, including accelerating the deterioration of painted surfaces and the corrosion of bridges and other steel structures as well as promoting the weathering of stone buildings and a loss of detail on statues and gravestones.
Substantial progress has been made on reducing acid rain in the developed countries by reducing the emission of the pollutants that cause it. Ways in which this is being accomplished include by conserving energy, replacing fossil fuels with renewable energy, installing filters in fossil fuel electric power plants and automobile exhaust systems, and replacing automobile trips with travel by public transportation, walking and bicycles.