Despite its splendor, this highland paradise is far from worry free. Natural visibility from the highest peaks of the Smokies should be around 100 miles, but pollution has decreased it to an average of 20 miles. The man-made pollution results from fossil fuel burning in industrial and urban areas which releases sulfur dioxides and nitrogen oxides. About half of the nitrogen oxides released into the atmosphere come from automobile exhaust. Forty-five of the top 50 polluting power plants are east of the Mississippi, and air currents carry pollutants to the highest peaks of the Smokies, the first barrier to pollutants that have been carried from the industrialized Ohio and Tennessee river valleys in the jet stream. The rates of pollution at the higher elevations in the Smokies tend to be among the highest in the continental United States.
Nitrogen oxides and volatile organic compounds combine in the presence of sunlight to form ozone, which damages plant life and sometimes reaches levels hazardous to human health. In the presence of moisture, these substances become weak sulfuric and nitric acids, commonly known as acid rain. These acidic compounds threaten the plants and high-level streams within the park in the form of rain as well as clouds, which bathe the peaks in pollutants, particles, and gases.