VENTILATION. 255 



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 deter- 

 mined by finding the percentage of carbon dioxide in it; 

 but it must be borne in mind that to mean much this car- 

 bon dioxide must have been produced by breathing; other- 

 wise 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. 



The Quantity of Fresh Air which should be allowed for 

 each Person in a Boom. In each minute a man breathes 

 out 450 cubic inches of air containing rather more than 

 4 per cent, of carbon dioxide. This mixed with three times 

 its bulk of pure air would give a little over one cubic 

 foot containing one per cent, of carbon dioxide. Such air 

 is no longer respirable with safety. The result of breath- 

 ing it for an hour or two is headache and drowsiness; of 

 breathing it for weeks or months several hours daily, a 



What percentage of carbon dioxide produced by breathing shows 

 that the air is unfit for use? Is the proportion of carbon dioxide 

 found ordinarily in a room poisonous? Why is the percentage 

 of carbon dioxide in air usually employed in deciding whether 

 the air is fit to breathe? What must be borne in mind in deciding 

 from the percentage of carbon dioxide in it that air is no longer 

 wholesome? Is air containing much carbon dioxide fit to breathe? 

 Give an example. 



What bulk of air does a man contaminate with carbon dioxide to 

 the extent of one per cent, in each minute? Is air so contaminated 

 fit to breathe? What are the consequences of breathing it for a few 

 hours? What of breathing it for months? 



