104 
PROF. T. CARNELLEY, MR. J. S. HALDANE, AND DR, A. M. ANDERSON 
perceive no trace of difference in odour between the room and outside air in point of 
freshness. (2) That the presence of organic matter is, on an average, perceptible to 
the sense of smell when the coincident carbonic acid, due to respiratory impurity, 
reaches 2 vols. per 10,000, or a total of about 6 vols. 
In almost all the houses and schools visited we took a note of the odour perceived 
on entering the room ; and, although as a general rule the odour was some indication 
of the condition of the air in the room, yet this was by no means invariably so. In 
some cases an extremely close and almost overpowering odour was detected when the 
carbonic acid amounted to only 7 or 8 volumes per 10,000, while in other cases 
the smell was only slight with as much as 17 volumes, and in one case as much as 20 
volumes. In these latter instances the organic matter was only slightly above the 
limit we have allowed. 
The smell is, in fact, greatly influenced by the temperature, and also by the 
humidity of the air, as De Chaumont himself points out. The state of cleanliness 
of the persons in the room, and of the room itself, has a most important influence on 
the smell, quite independently of the amount of carbonic acid. There may also be 
other strongly smelling substances in a room which do not appreciably affect the 
chemical composition of the air. Our observations in the Infirmary wards (page 84) 
were very instructive in this respect. Thus, in one ward, where the excess carbonic 
acid was 20 volumes, there was a very perceptible odour. A few hours later the 
excess in the same ward was 3'4 volumes, but the ward felt much fresher, and the 
odour was barely perceptible. 
For similar reasons the feeling of closeness is not a safe guide as to the amount of 
organic matter in a room. The combustion of gas in a room will produce a high 
carbonic acid and a feeling of closeness, but, as shown above, it will have little effect 
on the organic matter. It should be stated that De Chaumont’s results apply to 
rooms at night in which lights were not burning, whereas, in almost all the one- and 
two-roomed houses we visited, an oil lamp was kept burning. 
The standards of purity adopted above are practical limits, which should easily be 
maintained by proper methods, and at not too great a cost. They are not fixed so 
low as might be desirable, but they are as low as practicable with the present 
methods of ventilation, unless, indeed, expense is no object. 
As our experiments were all made between the end of November and the end of 
April, the standards deduced from them apply strictly only to the winter months. 
They might, therefore, be lowered to 8 volumes (4 volumes in excess) of carbonic acid 
in the case of houses, and 10 volumes in the case of schools, during warmer weather, 
when the injurious effects of draughts would be in great part eliminated. 
(4.) The excess carbonic acid is due to respiration and combustion. The organic 
matter is due to respiration and the combustion of coal, oil, and possibly (to a slight 
extent) gas; also to dust. The carbonic acid and organic matter may, therefore, be 
taken as a measure of the influences contaminating the room about the time of the 
