Rain is considered acid rain when it contains a higher than normal amount of sulphuric and nitric acids. Acid rain is the result of natural sources, such as decaying vegetation and volcanoes, and man-made sources, especially nitrogen oxide and sulphur dioxide emissions that come from fossil fuel combustion. When these gases react with the water, oxygen and other chemicals in the atmosphere and create various acidic compounds, then this is acid rain.
Acid rain doesn't look, feel or taste any different from rain that is clean. Humans do not sustain injury from swimming or walking in acid rain. The impact on humans isn't that direct. The danger to humans comes from the pollutants, including nitrogen oxides and sulphur dioxides that cause acid rain. When these gases interact in the atmosphere, it creates fine nitrate and sulphate particles that the wind can carry long distances. People inhale these gases, which can lead to lung and heart disorders, asthma, bronchitis and premature death.
When you hear the term "wet deposition," this means acidic rain, snow and fog. The acids in the rainfall to the ground when it blows into areas that are wet. The acid water then flows over and into the ground, and it affects animals and plants as well as the water that people may end up drinking. How much it affects the area depends on the buffering capacity of the soil and the types of trees, animals and fish that live in the area and rely on the water supply.
When the weather is dry, the acid chemicals may become incorporated into smoke and dust, then fall to the ground. This occurrence is called dry deposition. The acid chemicals stick to the ground, get in your house and into your cars. When it rains, these particles and dry deposited gases are washed away, and this increases runoff. The runoff water is even more acidic.
Acid rain also increases the bioconversion of mercury to methylmercury. This accumulates in the fish that humans eat, which can be toxic to humans.