A small amount of carbon monoxide may well exacerbate a pre-existing lesion, cause discomfort or a slight impairment of athletic achievement, but it has yet to be proven to cause serious disease.

Sulphur dioxide has, however, been more incriminated as a health hazard and its level, in the air, has correlated well with the incidence of acute attacks of bronchitis, a respiratory disease.

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Mortality rates are higher than in countries having a similar climate but less sulphur dioxide pollution, and recognition of this relationship has been the main impetus behind the drive to reduce the release of this gas by the combustion of sulphur-containing fuels.

Of the liquids liberated in a vapor phase by an industrial society, probably, the hydro­carbons predominate. Although atmospheric pollution is reduced by natural processes, and although the hydrocarbons as a group are usually toxic only at high concentrations, there is slight health effects associated with their excessive release.

This factor can be mini­mized by ensuring that petrol is filly burnt. Some hydrocarbons can react photo chemically in the atmosphere to produce toxic substances which, for example, irritate the eye. Petrol, however, also contains additives such as tetraethyl lead which will be discussed later.

Grit and dust particles are generated by a wide range of industrial processes. When large particles (over about 10 mm in diameter) are inhaled, they are usually trapped in the mucus of the upper respiratory tract and readily eliminated from the body.

Smaller par­ticles, however, frequently reach the lungs, where some will be trapped in mucus, but oth­ers will be engulfed by cells, the phagocytes, which serve to remove foreign bodies. In this case, the effects on the body depend on the nature of the dust absorbed.

In the case of biologically inert particles there will be no damage, even if the lungs accumulate some amount of material, as is the case with city dwellers.

Although coal dust particles are fairly inert, high concentrations of dust are not in public interest, and prolonged exposure to massive amounts can lead to the potentially disabling pneumoconiosis, sometimes, found in coal miners.

More general health hazards are produced by silica and asbestos dusts. Silica particles are liberated into the atmosphere by quarrying stone working and abrasive pro­cesses, and inhalation for some years may result in a condition in which lung tissue is replaced by cancerous tissue. Asbestos dust can cause a somewhat similar disease, and with some types of asbestos, a form of lung cancer is caused.

Here, the required exposure is so low that the environmental risks are apparent: this cancer has been found not only in asbestos workers or dockers alone, but also in their wives, whose main contact is presum­ably through the handling of clothes which have been contaminated with particles.

Beryl­lium dust is responsible for another lung disease, which is, however, fortunately not malig­nant. Although commonest amongst the workers directly involved, occasional neighborhood cases occur through pollution.