Air pollution can cause harmful effects to agricultural crops and other species of vegetation. Air pollutants that are pathogenic to plants, arise from a variety of natural and man-related phenomena. They can be divided into primary and secondary types. Primary pollutants originate at the source in a form toxic to plants; secondary pollutants develop as a result of reactions among pollutants. Apart from sulphur dioxide, the principal pollutants of serious concern are fluorides, ozone, photochemical smog products, ethylene and various aerosols occurring as suspended particular matter. Gases such as chlorine, hydrogen sulphide, oxides of nitrogen, and ammonia may constitute a hazard or cause damage to vegetation on occasion in localized areas. Suspended particular matter, consisting of small particles of smoke, sulphuric acid mist, soot and various metallic dusts, may be transported by wind for considerable distances from strong sources such as urban and industrial areas. These pollutants accumulate on leaf surfaces and clog the stomata to produce direct damage or retardation in growth.
The deleterious effects of our pollution on vegetation has been recognized for more than 100 years. Ozone and hydrogen chloride (from the LeBlanc soda process) where two of the first air pollutants to be recognized as phytotoxic. Concern over the effects of air pollutants on plants has grown with the increasing damage caused.
The types of pollutants injurious to plants may be classified as necrotic, chlorotic, or atrophic. Specific effects include changes in cell-wall permeability, plasmolysis, changes in tissue pH, interference with cell-wall synthesis, acceleration of respiration, inhibition of enzymes, and a reduction in the rate of photosynthesis. A particular cause of concern is the damage to forests by acid rain, due to sulphur emission. Transboundary agreements on reduction of annual sulphur emission by at least 30% of the 1980 level by 1993 had been agreed by 18 countries in Europe as of June 1984.