254 THE HUMAN BODY. 



sufficient for two. Moreover, fires, gas, and lamps all use 

 up the oxygen of the air and give carbon dioxide to it, and 

 hence calculation must be made for them in arranging for 

 the ventilation of a building in which they are to be used. 



When breathed Air becomes unwholesome. 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 poison- 

 ing 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. Breath- 

 ing 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 



What conditions determine the supply of fresh air which should 

 be provided to a room? 



Is air ever unwholesome w r hile still capable of supplying the 

 oxygen which the body requires? What results from living in ill- 

 ventilated rooms? Why? Does air once breathed become injuri- 

 ous 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? 



