We hear people talk about ground-level ozone as an air pollution problem. On high-ozone days, we’re cautioned against outdoor activity for the good of our lungs, and elevated ozone can impact plants – damaging the leaves of plants sensitive to ozone. Ozone is not emitted directly, but forms in the air when nitrogen oxides, largely from auto exhaust and power plants, react with hydrocarbons on hot, sunny days with little wind.