VENTILATION 55 



such airs give on analysis '6 per thousand of carbonic 

 acid. 



This "6 per thousand of carbonic acid is made up as 

 follows : — 



•4 per thousand of COj normally existing in the air. 

 •2 per thousand due to respiratory impurity. 



The maximum amount of respiratory impurity which can 

 exist without impairing the freshness of the atmosphere is 

 ■2 per thousand ; if the air smells rather close the amount 

 of respiratory impurity is '4 per thousand ; if the air is 

 close the respiratory impurity is equal to '67 per thousand ; 

 if very close, disagreeable, and offensive the carbonic acid 

 present is equal to "9 per thousand of respiratory impurity. 



Beyond this degree of impurity it is not possible for the 

 sense of smell to differentiate. 



It is assumed that an air which does not differ in the 

 sense of smell from the outside air is fit to breathe, but 

 that no greater vitiation should be allowed. It is on this 

 basis that -2 per thousand of respiratory impurity has been 

 fixed. 



In recent years an attempt has been made to place the 

 permissible respiratory impurity at a higher figure than 

 that given by de Chaumont. 



The explanation of this proposed change is based not only 

 on an examination of the carbonic acid present, but also of 

 the number of micro-organisms in the air, and the amount 

 of oxygen required for the oxidation of organic matter. 

 In this way it has been suggested that the amount of per- 

 missible respiratory impurity might be raised to "6 or '9 per 

 thousand. 



We do not feel justified in accepting this lower standard 

 of purity for stables, where it will be remembered the im- 

 purity is not only of respiratory origin, but also derived 

 from fffices and urine ; -2 parts of carbonic acid per thousand 

 volumes of air is, therefore, adopted as the maximum amount 

 of permissible organic impurity in the air of well ventilated 

 stables. 



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