Remedially, oxygen may be considered as it exists diluted in the atmosphere, or as prepared artificially for inhalation with a definite proportion of air.

Pure fresh air of the elevated country or the coast is of well-known efficacy in all conditions of debility, of chronic catarrh and chronic dyspepsia; sea-air especially contains more ozone than the air of land, and is of value to those who have lived in towns and followed sedentary occupations. On the other hand, patients with weak chests and readily congested lungs are better in a less rare and less ozonized atmosphere, since a large proportion of ozone may excite in them irritation of mucous membrane (Cornelius Fox). During epidemics of influenza an unusual amount of ozone has been verified in the air, while in cholera epidemics it has been almost absent. The choice of a climate for any given case is, however, generally influenced by other considerations than the mere amount of oxygen to be obtained; the subject need not, therefore, be fully considered in this place. The chief cases in which theory indicates, and experience justifies, the use of oxygen-inhalation, are those of asphyxia and of venous congestion occurring in the course of phthisis, asthma, or emphysema.