The amount of carbonic acid in air is usually fairly proportional to that of the other respiratory products. It may therefore be taken as a measure of the impurity of the air. There are, however, certain fallacies in this test. In soda water manufactory, for instance, there would be a comparatively harmless excess of carbonic acid. In dirty rooms, and in hospitals and other institutions where rooms are not vacated for a considerable period, the amount of organic matter present is often in excess of what would have been anticipated, judging by an estimation of the carbonic acid. This is strikingly shown by some valuable researches at Dundee, which are summarised in the following table. If we take the average amount (in excess of outside air) of carbonic acid, organic matter, and micro-organisms respectively in houses of four or more rooms as unity, then in one or two-roomed houses or tenements we have as follows:—
| HOUSES OF FOUR ROOMS AND UPWARDS. | TWO-ROOMED HOUSES. | ONE-ROOMED HOUSES. | |
|---|---|---|---|
| Carbonic acid | 1 | 1·5 | 2·0 |
| Organic matter | 1 | 1·6 | 4·4 |
| Micro-organisms | 1 | 5·1 | 6·7 |
It is evident that in these cases the carbonic acid did not increase in the same proportion as the organic matter and micro-organisms, and that it alone does not form a sufficient test of the impurity of any given atmosphere. The amount of carbonic acid, however, is a valuable and convenient test of the condition of the air of a room, and the problems of ventilation, of which examples are given on page [137], are based on its amount.
The Standard of purity is somewhat difficult to fix. The external air ought only to contain 4 parts of carbonic acid to 10,000 parts; but it is almost impossible to maintain this degree of purity in inhabited rooms. The experiments made by Drs. Parkes and De Chaumont showed that when the carbonic acid is ·06 per cent., or in the proportion of 6 parts in 10,000 of air, the air begins to be perceptibly stuffy (page [125]); this may therefore be taken as the limit of impurity. Pettenkofer has adopted the limit of ·07 per cent.[7]
The problem then is to discover the amount of pure external air (containing ·04 per cent. of carbonic acid) that will be required to pass hourly through a room, for every person in that room, in order to keep the carbonic acid at the ratio of ·06 per cent.
This may be ascertained by actual observation of the air of rooms in which a given number of persons are placed; or by calculations from physiological data.
As the result of numerous experiments on the atmosphere of prisons, barracks, etc., where the amount of fresh air supplied per hour is exactly known, it is found that in order to keep the carbonic acid at ·06 per cent., 3,000 cubic feet of pure air are required per head per hour; 2,000 cubic feet keep the carbonic acid at ·07 per cent.; 1,500 cubic feet at ·08 per cent.; and 1,200 cubic feet at ·09 per cent.
For the removal of the products of combustion of gas, an additional supply of air is required, for the amount of which, see page [116].
Where a number of sick persons are collected, as in hospitals and workhouses, a much freer supply of air is required. Much depends, however, on the cleanliness of the wards, and on whether the ventilation is constant in character. In St. Thomas’s Hospital, the space allotted to each ordinary patient is 1,800 cubic feet, and to each patient in the fever wards 2,500 cubic feet. Thus, by changing the air of the wards twice in the hour, an abundant supply of fresh air is ensured. The mortality after operations, and in all fevers, is much diminished by a free supply of air.
Soldiers are allowed 600 cubic feet of space per head in their sleeping rooms, which involves a change of the air five times per hour, in order that the carbonic acid may be maintained at ·06 per cent. The limit of overcrowding for lodging-houses is usually fixed at 300 to 500 cubic feet, but this is too little.