The formal term for acid rain is “acid deposition,” which can be divided into two main categories: “wet deposition” and “dry deposition.”

Wet deposition refers to acidic substances that reach the ground through precipitation, such as rain, snow, fog, or hail.

Dry deposition refers to acidic gases and particles that settle onto the ground, vegetation, and other surfaces when precipitation is absent.

Chemically, water with a pH of 7.0 is defined as neutral; values below 7.0 are acidic. The atmosphere naturally contains carbon dioxide, which dissolves in rainwater and forms weak carbonic acid, giving natural rainwater a pH of approximately 5.6. Other natural sources, including volcanic emissions and nitrogen oxides produced by lightning, can also contribute to atmospheric acidity. However, acid rain is primarily associated with sulfur dioxide and nitrogen oxides released by human activities, especially fossil fuel combustion.

Acid rain generally refers to precipitation that is more acidic than normal rainwater, typically with a pH below 5.6. The precise threshold used may vary by region and monitoring program; there is no universal official definition requiring a pH of 5.0 or lower. Environmental agencies monitor precipitation acidity to assess the effects of atmospheric pollution.

The main human health risks associated with acid rain come from the pollutants that cause it. Sulfur dioxide and nitrogen oxides can react in the atmosphere to form fine particles that irritate the respiratory system and worsen conditions such as asthma. These pollutants and related particles can also contribute to cardiovascular and other respiratory problems.

An indirect environmental impact of acid deposition is that it can increase the release of certain metals, such as aluminum, from soil into lakes and streams, harming aquatic life. Acidification can also affect the movement of some metals through ecosystems and food chains. For example, mercury can accumulate in aquatic food webs and, when consumed in contaminated fish, can damage the brain and nervous system. However, acid rain is not generally considered a direct cause of toxic metals accumulating in all fruits, vegetables, and animals, and the health effects depend on the specific metal and level of exposure.

By Michael Chiu, MWS, CI – Updated and Revised in 2026