This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
In order that air be unwholesome to breathe, it is by no means necessary that it shall have lost so much of its oxygen as to make it difficult for the body to get what it wants of that gas. The evil results of insufficient air-supply are rarely directly due to that cause even in the worst ventilated room, for the blood flowing through the lungs can take what oxygen it wants from air containing comparatively little of that gas. The headache and drowsiness which come on from sitting in badly ventilated rooms, and the want of energy and general ill-health which result from permanently living in such, are dependent on a slow poisoning of the body by the reabsorption of matters eliminated from the lungs in previous respirations. What these are is not accurately known; they doubtless belong to those volatile bodies mentioned above as carried off in small quantities in each breath, since observation shows that the air becomes injurious long before the amount of carbon dioxide in it is sufficient of itself to do any harm. Breathing air containing one or two per cent, of that gas produced by ordinary chemical methods does no particular injury, but the breathing of air containing one per cent, of carbon dioxide produced by respiration is decidedly injurious, because of the other things sent out of the lungs along with it. Carbon dioxide, in any such percentage as is commonly found in a room, is not poisonous, as used to be believed, but as it is tolerably easily estimated in air, while the more dangerous injurious substances evolved in breathing are not, the purity or foulness of the air in a room is usually determined by finding the percentage of carbon dioxide in it; but it must be borne in mind that to mean much this carbon dioxide must have been produced by breathing; otherwise the amount of it present is no guide to the quantity of the more important poisonous substances present. Of course when a great deal of carbon dioxide is present the air is irrespirable, as for example sometimes at the bottom of wells or brewing-vats lowered tone of the whole body, less power of work, physical or mental, and less power of resisting disease. The ill effects may not show themselves at once, and may accordingly be overlooked or considered scientific whims by the careless, but they are there ready to manifest themselves nevertheless.
What conditions determine the supply of fresh air which should be provided to a room?
Is air ever unwholesome while still capable of supplying the oxygen which the body requires? What results from living in ill-ventilated rooms? Why? Does air once breathed become injurious before the quantity of carbon dioxide in it is poisonous? What percentage of pure carbon dioxide may be present in the air breathed without doing harm?