The characteristic symptom of ozone toxicity to plants is the appearance of necrotic spots on leaves. A definite positive correlation between the extent of leaf damage and the duration of exposure of plants to ozone levels exceeding 4 p .p.h.m. has been established (see Moore, 1975). No such correlation could be demonstrated in the case of sulphur dioxide.
Episodes of potentially phytotoxic ozone concentration can occur in many rural areas. Both additive and synergistic effects can occur when ozone is superimposed over mixtures of SO2 and NO2.
The concern for ozone depletion has greatly increased in view of the use of chlorofluorocarbon compounds as aerosol propellants and refrigerants which can reduce the ozone content of stratosphere layer, thereby permitting some more of the ultraviolet radiation to reach the Earth.
Such changes could spell an adverse impact on agriculture, climate and human health. Researchers working with the US National Oceanic and Atmospheric Administration have collected data on ozone contents of the atmosphere from 60 stations around the globe and have concluded that the ozone shield that protects the earth from harmful solar radiation has thinned out slightly (by about two per cent) over the USA and U.K. during the period 1970-74.
Exposure to high concentrations of photochemical oxidants in the atmosphere has been found to cause injury to rice plants in the form of reddish- brown flocks appearing on leaves (Nakamura and Ota, 1978). This kind of injury is not caused by nitrogen oxides or sulphur oxides but has been experimentally shown to be caused by exposure to ozone or other oxidants.
The injury symptoms appear in the form of numerous small, punctate, brown or bleached chlorotic flocks on the upper surface of leaf lamina. Exposure to oxidants also decreases growth and yield of rice plants.
The limit of 50 parts of ozone billion, as a running eight-hour average recently proposed in some countries e.g. UK, is intended primarily as a precautionary measure to avoid adverse effects on health. Inflammation and changes in lung function have been observed in people exposed even to 80- 100 ppb ozone for several hours. The proposed standard is stricter than both the European Commission (EC) and WHO guidelines.
Most of the ozone at ground level is formed by the action of sunlight on two classes of pollutant emitted mainly by automobiles: nitrogen oxides and volatile organic compounds. The levels of ozone at many monitoring sites all over Britain regularly exceed 50 ppb. Compliance with the proposed standard will require either cutting the present emissions of VOCs by 75-80 percent or a reduction of NOx emissions by 95 per cent.
The results of recent research at the Imperial College’s Centre of Environmental Technology have indicated that asthma patients experience more severe symptoms on days of elevated ozone levels. In each case the reported symptoms were about 15 per cent higher on such days.
High levels of ozone and toluene in urban air have been causing ever more asthma sufferers to seek hospitalization. The relative humidity and temperature also play some part, while sulphur dioxide often does not if the concentrations remain regularly low. The effects of nitrogen oxides are difficult to determine.