A very large number of causes of contamination of air by physical and chemical agents are given by Dr. Parkes in his work on practical hygiene.

Among the mineral substances which are suspended in the atmosphere are particles of coal, sand, steel or other metal, and everything which is included in the term dust, flocks of cotton, flax, hemp, all of which may be either inhaled or swallowed. It is perfectly well known that many trades are distinguished as unhealthy in consequence of the inevitable mixture with the air of various products which are connected with manufacturing processes. The effect of the air of mines appears to be particularly deleterious to the health of the miners. It was stated many years ago, by the chief medical officer of the Privy Council, that 30,000 miners in England break down prematurely every year from pneumonia and bronchitis. At that time one exception only was given: the colliers of Durham and Northumberland, where the mines were well ventilated, and the miners did not suffer from pulmonary affections, excepting in an ordinary degree. The evidence of the extent to which the air of mines is contaminated with coal-dust is of a very emphatic kind. In the next illustration a specimen of a miner's lung is given, showing the enormous accumulation of coal-dust in the lung structure.

Stone-masons and metal-workers, also workmen in potteries, grinders, button-makers, cotton-spinners, match-makers, and others have all been mentioned as suffering from the effects of the contaminated air which they habitually breathe.

Horses are often looked upon as animals which, to a great degree, are exempt from the action of air contaminated in the manner described, but the evidence in proof of this belief is extremely meagre, in fact it is mainly negative; indeed it does not seem to have occurred to anybody that the condition which we have just illustrated of the coal-miner's lung would be found in the lungs of the horses working in the same pits if it were looked for, and there cannot be any doubt whatever that horses working in positions where the air is largely mixed with dust, or otherwise contaminated with mechanical impurities, would exhibit traces of injury from these causes in their pulmonary organs on post-mortem examination.

Occasional outbreaks of disease have occurred among horses grazing in the neighbourhood of brick-kilns and smelting-works, and chemical investigation has demonstrated the existence of poisonous products in the air which the animals had to breathe, and also in the pasture on winch they were feeding, and it is quite possible that in many instances of unexplained outbreaks of affections of the respiratory organs the cause might be found in the condition of the atmosphere, the presence in it of either mechanical or chemical matters.

Substances of a much more deleterious character than ordinary dust undoubtedly obtain an entrance into the air; these come under the head of organic impurities.