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A list of the harmful effects of atmospheric pollutants upon a plant and animal life and upon inorganic substances would be a very long one if fully developed. We can only suggest some of these effects. For humans in cities, both sulphur dioxide and hydrocarbon compounds, altered by photochemical reaction to produce sulphuric acid and ethylene, respectively, are irritants to the eyes and to the respiratory system. Nitrogen dioxide is also an eye and lung irritant when present in sufficient quantities. Photochemical alteration of nitrogen oxides leads to production of ozone, which acts as an irritant in smog and would be lethal if it occurred in large concentrations.

For persons suffering from respiratory ailments, such as bronchitis and emphysema, the breathing of heavily polluted air can bring on disability and even death. In a recent London fog, that of December 1962, more than 300 deaths were attributed to the breathing of polluted air. Particularly hard hit were the very young and the old. There is also a suspected linkage between breathing of atmospheric pollutants and lung cancer, since the incidence of that disease is higher in cities than in other areas. It is thought that the accumulation of atmospheric hydrocarbon compounds on lung tissues may predispose to the onset of lung cancer. Carbon monoxide is a cause of death when inhaled in sufficient quantities. Everyone knows that the carbon monoxide from automobile exhaust will kill in a short time when breathed in a closed garage. Carbon monoxide levels are a general indicator of the degree of air pollution for vehicular exhausts, but concentrations rarely reach sufficient levels in the open air to be a threat to life. Nevertheless, the long-continued inhalation of small amounts of carbon monoxide is suspected of harmful effects, not as yet evaluated.

Ozone in urban smog has a most deleterious effect upon plant tissues and in some cases has caused the death or severe damage of ornamental trees and shrubs. Sulphur dioxide is injurious to certain plants, and is a cause of loss of productivity in truck gardens and orchards in polluted air. Atmospheric sulphuric acid in cities has in some places largely wiped out lichen growth. Building stones and masonry are susceptible to the corrosive action of sulphuric acid derived from the atmosphere. Metals, fabrics, leather, rubber, and paint deteriorate and discolor under the impact of exposure to urban air pollutants. Ozone, in particular, is deleterious to natural rubber, causing it to harden and crack. The sulphuric acid produced from sulphur dioxide corrodes exposed metals, particularly steel and copper. Not the least of the economic losses from pollution are simply from soilage of clothing. automobiles, furniture, and interior floors, walls, and ceilings. The cleaning bill totaled for a large city is truly staggering, when calculated to include the labour and cleaning agents expended by householders.

Lead and other toxic metals in the polluted atmosphere are a particular source of concern for human health in the future. The lead-bearing particles from auto exhausts tend to concentrate in the grass, leaves, and soil near major highways. There is good reason to suppose that humans ingest lead particles directly from the air and that these may prove to be a health hazard. Although lead poisoning from atmospheric sources has not yet been documented in humans, there is now evidence that it has caused the deaths of animals in city zoos.

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