Once it was thought that only the chemical constituents of the air were important in distinguishing good air from bad air. More recently it has been recognized that the physical qualities of the air are also important. Dusts and pollens may be primarily responsible for symptoms like those of hay fever and asthma from which many people suffer.
When a room contains a great many people without arrangements for free ventilation they begin to feel drowsy and fatigued. This sensation results from a combination of the warmth and the accumulation of a certain amount of dioxide which is exhaled in the air from the lungs.
Once all sorts of diseases were believed to be due to bad ventilation and to gases coming from marshes and sewers. Now it is known that these diseases are actually due to germs which are transferred from one person to another. Moreover, the marshes were breeding places of mosquitoes and other insects which carried diseases.
Once it was thought night air was unhealthy. Night air is just as healthy as day air, provided it is not contaminated by germs or infested with insects which carry germs. Everyone knows now that the wind comes up and the mosquitoes fly toward nightfall.
In cold weather there is a tendency to overcrowding and lack of ventilation. This means a greater likelihood of infections being transferred from one person to another. Unsatisfactory ventilation may be a factor in the increase of coughs, colds, and other respiratory diseases during the winter;