Contamination of the air constantly occurs, and to some extent at least is inevitable; the process of respiration, for example, has the effect of charging the atmosphere with carbonic acid (carbon dioxide), as well as various organic impurities or products of decomposition given out with the expired air. How deadly these mixtures of the products of respiration may become is illustrated by the constantly-quoted case of the Black Hole of Calcutta, in which, out of 300 men who were imprisoned, 260 died very quickly from the poison which they breathed from their own lungs. Carbon dioxide, which is one of the products of combustion, is extremely poisonous, and contamination of the air with more than 1 per cent is rapidly fatal to animals. Sulphuretted hydrogen, a gas set free in the decomposition of organic matter, is highly deleterious to animals.

Section from Upper Lobe of a Collier's Lung.

Fig. 469. - Section from Upper Lobe of a Collier's Lung.

A, Deposits of coal dust in the air-vessels (highly magnified).

Ammoniacal vapours, which are constantly present in badly ventilated stables, exercise an irritating effect on the sensitive membrane of the eyes, and the comparative rarity in the present day of ophthalmia and other serious diseases of the eyes among horses is ascribed to the improvement which has taken place in the methods of construction and ventilation.

Admixture of sewage gas with the atmosphere is probably responsible for the occurrence of many forms of derangement of the digestive organs which may often be attributed to other causes.

Emanations from marshes were for a long time regarded as being the immediate cause of certain specific fevers in man and animals, but while marsh lands are undoubtedly unhealthy, it has now been demonstrated that malaria is caused by inoculation with the germs of the disease through the agency of mosquitoes. A somewhat similar case is that of the disease following upon the bite of the tsetse fly in South Africa.

The most dangerous forms of organic contamination are undoubtedly the minute organisms, which under certain conditions produce specific diseases when introduced into the bodies of animals, including man. The different disease germs are active in different ways; thus, the germs of phthisis (pulmonary consumption) may produce disease when inhaled; the bacillus of enteric (typhoid) fever when swallowed; and the tetanus (lockjaw) bacillus when received into a wound or scratch. Pure air and water, cleanliness and sunshine, are important weapons against these invisible foes.

It may be observed that while air charged with disease germs may be inhaled with impunity by the perfectly healthy animal, it may prove fatal to the animal which is in bad condition.

Cases of the injurious effects of contaminated atmosphere might easily be multiplied, but enough evidence has already been advanced to prove the necessity for a sufficient supply of pure air as one of the factors in a condition of perfect health.