What happens when sulfur dioxide and nitrogen dioxide emissions travel hundreds of miles? Dissolved in water vapor, they can cause rain and other precipitation to be far more acidic than normal. This problem is called acid rain.

Trees at the top of North Carolina's Mount Mitchell look dead and barren. Scientists believe that the effects of acid rain and bugs called woolly adelgids combined to devastate the trees. Forests in Europe have also been ravaged by acid rain.

Acid rain also pollutes lakes and streams. Some lakes are now so acidic that fish can't live in them.