Air pollution injury on plants has been carefully documented in recent years. In some areas, injury on certain species has been serious and has represented a limiting production factor. This type of damage is more noticeable, but probably not as great as the sub-lethal chronic injury that occurs over much larger areas. Chronic damage is more difficult to diagnose and evaluate. Diagnosticians should be aware that pollutants are mixed in the atmosphere and that these mixtures may produce symptoms that vary from those caused by a single compound under laboratory conditions.
Injury produced by certain pollutants along with sources is given as follows:
| Pollutant | Source | Symptoms |
| Ozone | Nitrogen dioxide and hydrocarbons emittted from automobiles, industrial combustion, oil refineries, and many lesser sources react with sunlight to form ozone. During electrical storms, ozone is produced and can be brought down from the upper atmosphere by strong down drafts. | Four different kinds of symptoms can result from ozone exposure. The most common symptom is localized thickening and pigmentation of the cell walls causing sharply defined small dot-like lesions. General upper surface bleaching is another common type of injury. Large bifacial necrotic areas ranging from white to red may develop if all the tissue through the leaf is killed. Some species show only general chlorosis or chlorotic mottling or chlorotic flecks. |
| Sulfur Dioxide | Emmitted during combustion of many fuels, especially coal and petroleum. Also released during smelting operations. | Accumulation of sulfite in tissues produces a general chlorotic appearance of the leaf and a silvering or bronzing of the undersurface. Acute injury from absorption of lethal quantities of sulfur dioxide appears as marginal or inter coastal areas of dead tissue with a gray- green water soaked appearance, which usually dries to a bleached ivory color but may turn brown, red or black. The necrotic areas may fall out and after much of the leaf is affected, it will shed. |
| Fluoride | Released from manufacturing processes involved in the production of aluminum, steel, ceramics, and phosphorous chemicals and fertilizers. | Necrosis is the characteristic symptom of fluoride injury occurring on broad-leaved species at the leaf tips and margins where the fluoride accumulates. First sign of injury is a dull-green water soaked discoloration of these tissues within 24 hours or several days depending on concentration. These water soaked tissues turn light to dark brown within 48 hours during hot weather. Cool temperatures may delay symptoms as much as several days. Symptoms on needles of pine and other conifers consist of dead tissue beginning at the tip and progressing toward the base. Injured tissue first is chlorotic and turns buff to reddish- brown. |
| Nitrogen Oxides Peroxyactyl (PAN) | Produced by high temperature combustion. | Many plants develop a silvering of the lower leaf surface with PAN. Leaves of sensitive species develop a slightly oily or waxy appearance two to three hours after exposure. Glazed symptoms develop gradually with the advanced bronzing stage following after two or three days. Very young and the most mature leaves are resistant. |
Many other substances may be released to the atmosphere and produce damage to plants. These include ethylene (usually from incomplete combustion), herbicides, chlorine gas, ammonia, particulates (such as heavy metals or sulfuric acid mist) and hydrogen sulfide. These all produce characteristic symptoms.
Plants vary in their susceptibility to different pollutants. This is indicated in the following chart:
| Pollutant | Plants that are susceptible: |
| Ozone | alfalfa, beans, oats, onion, peanut, potato, radish, spinach, tomato, petunia, grape, carnation, and pine. |
| Sulfur Dioxide | alfalfa, bean, cotton soybean, sweet pea, verbena, zinnia, apple, pear, pine okra, spinach, turnip, and sunflower. |
| Fluoride | apricot, gladiolus, grape, peach, pine, and tulip. |
| Nitrogen Oxides | azalea, bean, hibiscus, lettuce, and sunflower. |
Those observing and diagnosing air pollution injury should be aware that many things may resemble symptoms produced by air pollutants. Care should be taken in diagnosis.
