Some fish kills correlate with major acid rain events. To date there is no definitive proof of acid rain damaging the Park's forests, but changing soil conditions raise concern. Still, using the natural world as an uncontrolled, possibly irreversible, experiment does not benefit the ecosystems. Ozone results when nitrogen oxides and hydrocarbons react in sunlight. Park ozone levels occasionally reach unsafe levels.
Ozone can make breathing difficult, especially for people with respiratory problems.
Ozone causes visible damage to sensitive Park plants including black cherry and milkweed. Long term effects remain unknown. Park ozone levels often remain at high levels longer than in nearby urban communities. Air stagnation episodes in the Smokies occur frequently, trapping and concentrating pollution. The Park's ozone, nitrogen, and sulfur levels are among the nation's highest.