As many as about 5,000 inhabitants fell ill, and at least 20 of them died following this calamity. A 5-day episode of severe air pollution in London in 1952 took a toll of some 4,000 deaths.
However, although the general, adverse effects of air pollutants are widely known, our knowledge of the long-term, chronic effects on man, of repeated exposures to low concentrations of such pollutants, is very meagre (Papetti and Gilmore, 1971).
Extensive researches done by Hickey and associates (see Hickey, 1971) in 38 large American cities have brought to light that increasing concentrations of nitrogen dioxide and S02 in the air are statistically correlated with increasing death rates from chronic diseases associated with ageing and senescence, e.g., cancer, heart diseases, diabetes mellitus. Etc. Concentrations of various metals were found to be negatively correlated with such mortality rates. It is now generally agreed that air pollutants can and do cause long-term, slow and chronic effects on health in addition to their immediate and acute effects.
Some studies have indicated (see Hickey et al., 1974) that the atmospheric SO2 NO2 and lead can cause mutations in human beings. These noxious gases are ocular and respiratory irritants. Concentrations of SO2 and NO2 in the atmosphere have been shown to constitute statistically significant predictors for mortality rates for various diseases of ageing, including cancer and arteriosclerosis.
Similarly, lead concentrations were significant predictors for infant mortality rates and congenital malformations. Atmospheric NO2 can give rise to the potent mutagen nitrous acid (HNO2) and possibly bisulphites may also be mutagenic. Peroxides are already known to be mutagenic.
Some atmospheric chemical mutagens may be those which alter resting DNA in such a manner those mutations appear after subsequent replications. Replication errors could some time lead to a gradual accumulation of mutational errors.
Polluted air of big cities also contains carcinogens, e.g., benzopyrene, and other polycyclic compounds. Some sources of benzopyrene are tobacco smoke, automobile exhaust and effluents from industry and electric power stations.
Reasonably low concentrations of environmental mutagens, carcinogens or other toxic substances can, however, sometime be tolerated by animals and humans which possess certain repair mechanisms and detoxification mechanisms to cope up with adverse situations.
The various kinds of particulates in the atmosphere definitely cause soiling of various objects (such as laundry clothing) and are responsible for various nuisance effects. High dust levels in some occupational environments are also definitely harmful regardless of chemical composition but unless toxic or reactive constituents are present in the dust, no perceptible damage may be caused to vegetation.
Particulates often screen out visible and ultraviolet light rays and hence alter the temperature distribution of the air on the earth’s surface. They are also responsible for some reduction in visibility by attenuating light as it passes through the atmosphere. Visibility may be reduced either by light scattering or light absorption by the particulate matter.