Acid rain describes sulfuric and nitric acids deposited from the atmosphere. Often associated with precipitation, the term also applies to dry acidic materials. These acids commonly result from sulfur dioxide and nitrogen oxides reacting with moisture and other substances in the atmosphere. Although there are natural sources for these chemicals, much attention has been given to man-made sources, such as coal power plants. Acid rain is problematic due to acidification of soil, rivers, and lakes beyond the tolerance range of plants and animals. Acid rain can also erode man-made structures.
Water resists rapid changes to pH --- a measure of the acidity of a substance with lower numbers indicating stronger acidity. However, even this resistance is overcome by prolonged and persistent exposure to acid rain. Ecosystems within lakes and rivers may be vulnerable to acidification of the water in surprising ways. For example, mayflies die off at a pH of 5.5, while trout and perch can survive in much more acidic water. However, with the decline of mayflies and other insects, trout may have insufficient food to support their population. At pH 5 many fish eggs fail to hatch and juvenile fish tend to be more susceptible to acidity, impairing the fish population's continued health.
Direct contact with acid rain can weaken trees and destroy their leaves. This is especially true in high altitude forests where the trees are frequently immersed in an acid cloud. Acid rain can also harm trees in a more subtle fashion by reducing nutrient levels and increasing the level of toxic substances in the soil. The buffering capacity of soil varies greatly between different soil types, resulting in greater damage to forests in some areas than others, even though the acid rain exposure may be similar.
Many people take great pride in maintaining the appearance of their vehicle, but acid rain can literally erode the vehicle's protective coating. To counter these effects, automobile manufacturer's have begun coating new vehicles with acid-resistant paints.
Limestone and marble construction materials are especially damaged by acid rain. This is due to the calcite mineral content in these materials that is easily dissolved away. This damage is readily seen in older stone buildings and monuments where carvings placed in the stone have eroded. Not all stone is susceptible. Granite and sandstone have a chemical composition that does not react with acid rain, though some types of sandstone contain carbonate, which will react.
Physical contact with acid rain, either as droplets falling from the sky or from swimming in an acidic lake, has little direct impact on the health of humans. However, the pollutants responsible for the formation of acid rain are associated with an increase in respiratory disease and other illnesses. These pollutants may even infiltrate indoor spaces causing problems ranging from asthma to premature death. Laws such as the Clean Air Act strive to reduce the amount of pollution in the air. The U.S. Environmental Protection Agency reports that between 1980 and 2009, the national average for sulfur dioxide in the air decreased by 76 percent, and nitrogen dioxide decreased by 48 percent.