Pollution from car exhaust, factory emissions, fuel combustion and other sources can hang a brown cloud over some cities. Air pollution not only contributes to respiratory diseases in humans and damages buildings, it can also affect plants. The effects of air pollution on plants develop over time and can't be undone. Some plants are more susceptible to pollution damage than others according to Fred Davis, a chemist from Kent State University.
Chemicals such as sulfur dioxide, ozone, fluorides and peroxyacyl nitrate damage the leaves of plants. If enough leaves are damaged, the entire plant will die. Sulfur dioxide, a by-product of burning fossil fuels such as oil, coal and gasoline, causes changes in the colors of leaf tissue, which may turn white, brown or yellow.
Some sulfur dioxide converts to sulfuric acid, which eats holes in the leaves. Ozone damage on leaves appears as mottled spots, which may be yellow, black or brown. If the damage by ozone is severe enough, the plant will drop its leaves altogether. Fluoride damages the edges of plants and causes them to turn brown or black. Peroxyacyl nitrate causes a condition known as silver leaf, in which the underside of the leaves turn silvery white or bronze.
Dr. Kent reports that nitrogen dioxide, a byproduct of combustion from car engines or open fires, can slow the growth of plants. Fortunately, rainfall transforms nitrogen dioxide into nitric acid, which adds nitrogen to the soil and actually benefits plants.
However, carbon monoxide is less benign. This component of car exhaust is poisonous to humans and will stunt the growth of plants. Some evergreens will drop their leaves completely when exposed to carbon monoxide.
Air pollution weakens plants and makes them more susceptible to insect infestation. The University of Colorado reports that pine trees stressed by air pollution are more susceptible to damage from pine bark beetles. A 2008 Newsweek story reported that pine beetles had destroyed 22 million acres of pine trees in Canada and more than 1.5 million acres in Colorado.