An erupting volcano will release gases into the atmosphere, the largest portion of which is water vapor. Other gases include carbon dioxide (CO2), sulfur dioxide (SO2), hydrochloric acid (HCl), hydrogen fluoride (HF), hydrogen sulphide (H2S), carbon monoxide (CO), hydrogen gas (H2), NH3, methane (CH4), and SiF4. Harmful concentrations of volcanic gases usually do not extend further than 10 km from the volcano. However, some gases are transported away from the eruption on ash particles while others form salts and aerosols. Volcanic gases are also produced when water is heated by magma. Gases also escape from travelling pyroclastic flows, lahars and lava flows, and may also be produced from burning vegetation.
Acid rain can be produced when high concentrations of these gases are leached out of the atmosphere. High concentrations of CaF2 can burn vegetation and other material on contact. Fluoride and chloride can contaminate water. Livestock have died from drinking such contaminated water. Fluoride and chloride can also be irritating to the skin and eyes of animals, and can damage clothes and machinery. Carbon monoxide and carbon dioxide are usually produced in small amounts. However, large amounts of these gases will sometimes build up in low lying areas and can asphyxiate livestock and harm vegetation.
When Katmai erupted in 1912, acid rain damaged clothes that were drying outside on a line 2000 km away from the erupting volcano in Vancouver, British Columbia. Carbon dioxide built up at the bottom of tropical Lake Nyos was released from the bottom of the lake when the lake overturned. Fifteen hundred people were killed and 10,000 people burned in this disaster.