Sulfur dioxide (SO₂) and nitrogen dioxides (NOx) both contribute to the problem of acidification.

These gases, emitted by thermal power plants, transport and other industries, can react with water vapor in the atmosphere to form acidic compounds: sulfuric acid in the case of SO2 and nitric acid for NOx.

Once formed, these acids can be carried over long distances by the wind. When they encounter rain, snow or hail, they form acid rain. 

Acid rain causes:

• the acidification of lakes, killing the microorganisms that provide nourishment for the fish, which then also die out;
• the acidification of soil, thus endangering the plant life that draws water from the soil;
• damage to forests.