158 VENTILATION purity. to which the carbonic acid may be allowed to rise in good Standard ventilation. It is generally admitted that the air in which people dwell and sleep should not under any circumstances be a u owe( j to contain more than ten parts in 10,000. This has been accepted as the permissible proportion by Car- nelley, Haldane, and Anderson, after an extensive examin ation of the air of middle and lower class dwellings. 1 De Chaumont, judging by the rough and unsatisfactory test afforded by the sense of smell, concluded that the air of a room ceased to be good when it contained eight volumes of carbonic acid in 10,000 of air, 2 and recommends that six parts in 10,000 be taken as the maximum permissible in good ventilation. Parkes, in his Manual of Hygiene, quotes observations which point to an equally exacting standard as desirable. The rate at which an adult respires carbonic acid varies widely with his condition of repose, being least in sleep, greater in waking rest, and very much greater in violent exercise. As a basis on which to calculate the air necessary for proper ventilation we may take the production of carbonic acid by an adult as 6 cubic feet per hour. 3 Hence he will produce per hour, in 6000 cubic feet of air, a pollution amounting to one part of carbonic acid in 10,000 of air. If the excess of carbonic acid were to be kept down to this figure (1 in 10,000), it would be necessary to supply 6000 cubic feet of fresh air per hour ; if the permissible excess be two parts in 10,000, half this supply of fresh air will suffice ; and so on. We therefore have the following relation between (1) the quantity of air supplied per person per hour, (2) the excess of carbonic acid which results, and (3) the total quantity of carbonic acid present, on the assumption that the fresh air that is admitted contains four parts by volume in 10,000 : Eate of sump tion of Air supplied per Carbonic Acid Adult per Hour. (Parts by Volume in 10,000). Excess due to Total Respiration. Quantity. 1000 6 10 1200 5 9 1500 4 8 2000 3 7 3000 2 6 Thus, to preserve the lowest standard of purity tolerated by sanitarians, ventilation must go on at the rate per person of 1000 cubic feet per hour, and 3000 cubic feet per hour are required to preserve the higher standard on which some authorities insist. Parkes advises a supply of 2000 cubic feet of air per hour for persons in health and 3000 or 4000 cubic feet for sick persons. The English Barracks Improvement Commissioners 4 require that the supply be not less than 1200 cubic feet per man per hour. Pettenkofer recommends 2100 cubic feet, and Morin 5 con siders that the following allowances are not too high : Hospitals (ordinary) 2000 to 2400 cubic feet per hour. Do. (epidemic) 5000 Workshops (ordinary) 2000 Do. (unhealthy trades) 3500 Prisons 1700 Theatres 1400 to 1700 Meeting halls 1000 to 2000 Schools (per child) 400 to 500 Do. (per adult) ! 800 to 1000 1 Phil. Trans., 1887, vol. clxxviii. B, p. 61. In school-rooms well ventilated by mechanical means these authors found 13 parts of car bonic acid in 10,000 of air, which they consider a limit permissible in rooms of that class, though not in dwelling-rooms. 3 Proc. Roy. Soc., 1875, vol. xxiii. p. 187. 3 This estimate is based on the observations of Pettenkofer, Angus Smith, and Parkes. 4 Report, 1861. 6 See Etudes sur la Ventilation, Paris, 1863 ; also Proc. Inst. Mecli. ny., 1867, p. 63. Gas lights add to the vitiation of the air of rooms at a rate which may be roughly estimated by treating one cubic foot of gas burnt per hour as nearly equivalent to one adult person, so far as the production of carbonic acid is concerned. Thus an ordinary burner, giving a light of about twenty candles and burning four cubic feet of gas per hour, uses the air of three or four men. The purity of the air of a room depends, of course, to some extent, on the proportion of its cubic capacity to the number of inmates. The influence of capacity is, however, often overrated. Even when the allowance of space is very liberal, if no fresh air be supplied, the atmo sphere of a room quickly falls below the standard of purity specified above ; on the other hand, the space per inmate may be almost indefinitely reduced if sufficient means are provided for systematic ventilation. Large rooms are good, chiefly because of their action as reservoirs of air in those cases (too common in practice) where no sufficient provision is made for continuous ventilation, and where the air is changed mainly by intermittent ventilation, such as occurs Avhen doors or windows are opened. It must be borne in mind, too, that no room is hermetically sealed. In the absence of proper inlets and outlets casual ventilation goes on through every chink and cranny, and even by diffusion through the plaster of the walls. The ventilation given in this way is generally most inade quate ; but a large room has at least the advantage over a small one that it offers more chances for the casual en trance of fresh air, as well as a larger wall-surface through which diffusion may occur. It has also the advantage that a greater volume of air may more easily be passed through it than through a small room in a given time, without causing disagreeable draughts. A general idea of the cubic capacity per inmate, allowed by law or by custom in certain cases, is given in the table below : Hospitals 1 200 cubic feet (and upwards). Middle class houses j 1000 Barracks ; 600 Good secondary schools j 500 London board schools Workhouse dormitories London lodging-houses One-roomed houses 6 ... ( ubic rapacity of room 130 300 240 212 To realize the need of provision for ventilation, it is only necessary to compare these figures with those already given for the rate of consumption of air. Taking the lowest permissible degree of purity (10 parts of carbonic acid in 10,000), we see that, if no fresh air were allowed to enter, the dweller in a middle-class house would make the atmosphere of his room unfit for breathing after occupying it for 1 hour, and that the sleeping rooms of the poor would fall below the standard in 13 minutes. The atmosphere of rooms is changed partly by diffusion, Vuntil. but chiefly by actual currents of air. The experiments of tio11 ^: Pettenkofer have shown that air passes to a very sensible , u j sl< extent through the substance of brick walls. In houses cmTell built of stone the movement of air through the walls must of air. be insignificant ; but, as regards individual rooms, what is chiefly important in this connexion is the percolation through dry plaster, causing an exchange of atmosphere to occur between the inside of the room and the space within the lining of the wall, which is generally in com munication with other parts of the building and with the external air. In order that the atmosphere of a room should be changed by means of air currents, three things are necessary, (1) an inlet or inlets for the air, (2) an outlet or outlets, and (3) a motive force to produce and maintain the current. One might think it needless to 6 Mean of 29 measurements by Carnelley, Haldane, and Anderson,
loc. cit.