Acid rain is a term that encompasses several ways in which acids fall from the sky and cause environmental damage. These acids come from air pollution, primarily sulfur dioxide and nitrogen oxide gases. They can fall as acid precipitation, or fall back directly as acidic particles and gases. Acid rain damage can range from polluted waterways to corroded statues and buildings to direct effects on human health.
The worst acid rain effects on ecology are seen in waterways, such as lakes, streams, and marshes. The most sensitive areas are those that are located in watersheds in which the soil is not very effective at neutralizing acidic compounds. When this happens, the water becomes more acidic. This means it has a lower pH. In addition, aluminum is released into the water from the soil, and is highly toxic to many forms of aquatic life.
Some plants and animals can tolerate acidic waters, but others will die as the pH decreases. While some lakes are naturally acidic, the pH of most streams and lakes is between six and eight. When the pH goes down to five, most fish eggs will not hatch. Some adult fish will die at lower pH levels, causing some acid lakes to be entirely devoid of fish. Even if the fish survive, they may be physically stressed and unable to effectively compete for habitat and food.
More recent research has identified acid rain effects in the shallow waters of the coastal ocean. The ocean overall is not greatly affected, but the affects of acid rain are magnified in waters near the coast. They result in lower pH and a reduction in carbon storage.
The lowered amount of carbon means that organisms such as corals, sea urchins, and some types of plankton lose the ability to make their hard outer shells. These types of organisms are necessary to provide food and living conditions for other ocean creatures. Their death could have serious effects on ocean ecosystems. For instance, corals form reefs that provide habitat for a substantial number of marine organisms.
The deposition of nitrogen from the atmosphere has effects on both freshwater and ocean ecology. It can cause massive overgrowth of algae. Some of these can be toxic, and directly affect humans by contaminating shellfish. A common effect of algal growth is to use up all of the oxygen in the water. This can cause the formation of dead zones.
Forests are another ecosystem that manifest acid rain effects. This is a combination of the direct effect on the leaves and needles of the trees, and changes in soil chemistry and microbiology. This can happen especially in high mountain regions, where trees are surrounded by fog and clouds that have more acid than the local rainfall. This can cause the loss of essential nutrients in the leaves.
In addition, acid rain causes nutrients in the soil to be washed away, so they are unavailable for plants. The subsequent release of aluminum is toxic to trees and plants. A lowered pH can also kill beneficial soil microorganisms.
It is thought that acid rain alone does not cause the death of trees in forests. Scientists think it predisposes them to other stresses, such as insect damage, drought, disease, or cold weather. Acting in concert with these other factors, some forests in areas with heavy amounts of acid rain have died completely.
Acid rain effects on human materials, such as statues, have been substantial. In addition to the damage caused by acid rain, the dry deposition of acidic particles has been a significant factor in the degradation of these items. Buildings and statues that are particularly vulnerable are those made of limestone and marble. In many areas, structures have been significantly corroded, and gravestone markings have flaked away. Metal items, such as bronze and copper, can also be corroded by acid rain.
Human health can be directly impacted by acidic particles. Particulate matter can lodge in the lungs. The exposure to particulate matter in the air has been correlated with an increased mortality from heart and lung diseases. Also, such compounds increase the tendency towards bronchitis and asthma in exposed individuals.