Since the atmosphere is in some respects foremost among the environmental factors with which man is concerned, it is not surprising that much attention has been directed to its composition and possible influence on human well-being. There is no doubt that under certain conditions the air which we breathe may become vitiated; but the charge of certain types of bodily malaise to "bad air" has often been made more easily than it could be substantiated. Malaria affords a specific illustration of the failure of tradition to sustain the alleged air-borne causation of the disease in harmony with the literal significance of its name. With the development of sanitary science, many of the reputed dangers of air supposedly contaminated in various ways have been dispelled, or have been unmistakably transferred to something other than the products of respiration.
The idea that air in overcrowded, unventilated quarters may harbor dangerous components has