Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions. Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but acid rain has a pH level lower than this and ranges from 4–5 on average.

Formation of Acid rain :

Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.

Effects of Acid Rain :

1. Acidification effect human nervous system, respiratory system and digestive system.

2. Acid contaminates with potable water and enter man’s body. The heavy metals released by acid rain may cause potential threat to human health.

3. Acidification of drinking water reservoirs and concurrent increase in heavy metal concentration may cause injurious effects.

4. Acidic water can also leach aluminium from the soil. This run­off carry dissolved aluminium to lakes, rivers and streams. This cause death of fish by clogging its gills and deprives it of oxygen.

5. Snails, clams, oysters etc. having their shells of calcium carbonate are among the forest animals to die in acidic lakes.