Thorough ventilation is necessary because close, ill-ventilated indoor spaces weaken the system and induce disease.

Original passage

The lungs are constantly throwing off impurities, and they need to be constantly supplied with fresh air. Impure air does not afford the necessary supply of oxygen, and the blood passes to the brain and other organs without being vitalized. Hence the necessity of thorough ventilation. To live in close, ill-ventilated rooms, where the air is dead and vitiated, weakens the entire system. It becomes peculiarly sensitive to the influence of cold, and a slight exposure induces disease. It is close confinement indoors that makes many women pale and feeble. They breathe the same air over and over until it becomes laden with poisonous matter thrown off through the lungs and pores, and impurities are thus conveyed back to the blood.