The amount of pure air required in order to keep the carbonic acid in a room at ·06 per cent., may also be ascertained from physiological data.
An average adult expires 3 ∕ 5 (·6) cubic foot of carbonic acid per hour. Now as the carbonic acid in air to be breathed must not contain more than two parts in 10,000 (·02 per cent.) in excess of what is present in external air (·04 per cent.), it follows that if x = the amount of fresh air required by an adult per hour in order to keep the carbonic acid in the room down to .06 per cent., then:—
·02 : ·6 :: 100 : x.
x = 3,000 cubic feet.
Relation of Air Required to Cubic Space of Room.—If we accept 3,000 cubic feet of air as the amount required per head per hour, this may clearly be furnished by having a large room with comparatively little circulation of air, or by having a small room with frequent interchanges. Thus, supposing the cubic space allowed to each individual is 1,000 cubic feet—that is, 10 feet in every direction—the atmosphere will require changing three times per hour.
Now, it is found that when a current of air, at the temperature of 55°-60° Fahr., is moving at the rate of less than one mile per hour, it is not perceptible—that is, produces no draught. The rate of a breeze, which is just perceptible, is 18 inches per second, or one mile per hour. As draughts are objectionable, ventilation, in the best sense of the word, means the supplying of abundant fresh air at a rate of less than one mile per hour, or warmed air at a higher rate. Air moving at the rate of 2½ miles per hour, or 3½ feet per second, is perceived as a slight draught by all, at the average temperature of our climate (about 50° Fahr.)
Where natural ventilation is employed, the difficulties of thoroughly ventilating a small space, without draught, are very great.
A change of air three or four times in an hour is all that can be borne under ordinary conditions in this country, and this necessitates a supply of 1,000 or 750 cubic feet of space respectively for each individual. And a change of this frequency is commonly not effected; the ventilating apparatus may fail temporarily, or may be wilfully stopped up, or there may be no means of ventilation; it is essential therefore to have as large a cubic space as possible. A large cubic space, does not obviate the necessity for efficient circulation of air. It is, however, advantageous, not only on account of the initial longer time before the air reaches the limit of impurity, but also because there are less draughts, and there is a larger wall surface and larger windows for unperceived ventilation.
Common Errors as to Ventilation.—(1) In relation to the cubic space of a room, it is most important to note that a lofty ceiling does not compensate for deficiencies in floor-space. One hears, “lofty” and “airy” rooms spoken of as though the two terms were necessarily synonymous. This is by no means the case. The impurities produced by respiration tend to accumulate about the persons who have evolved them, although it is true that in rooms heated by gaslight, a large amount of hot and impure air collects near the ceiling. The necessity of an abundant floor-space is shown by the fact that a space enclosed by four high walls and without a roof, will, if crowded, speedily become offensive. Twelve feet is quite high enough for large rooms in schools, hospital wards, etc., and nine feet suffices for the rooms of a private dwelling-house. There is no objection to a greater height, if it is remembered that in reckoning the practical cubic dimensions of a room, the height should only be reckoned as twelve feet. Supposing 500 cubic feet is the amount allowed per individual, then the floor-space should be forty-two square feet, which would be furnished by a room about 8½ feet long and 5½ feet wide. In barracks, soldiers are allowed fifty square feet of floor-space. In school-rooms the Education Code requires that at least ten square feet of floor-space, and at least 120 cubic feet shall be allowed for each child in average attendance.
(2) It is commonly supposed that a large room compensates for a deficient circulation of air. The cubic space of a room is really of less importance than the capacity for frequent interchanges of air. Even the largest enclosed space can only supply air for a limited period, after which the same amount of fresh air must be supplied, whether the space be small or large. Thus, supposing that as large a space as 10,000 cubic feet per head were allowed, the limit of purity would in the absence of ventilation be reached in three hours, and after that time an hourly supply of 3,000 cubic feet of air would be just as necessary as if the space were only 200 cubic feet.
(3) It must not be overlooked that the furniture in a room must be deducted from the breathing space, as the amount of air is diminished by the space occupied by the furniture. About 10 cubic feet ought to be allowed for each bed, and 3 to 5 cubic feet for each individual in a room; projecting surfaces must be allowed for by subtraction, and recesses by addition. The deductions to be made for furniture are not of any great consequence, if there is a free interchange of air; as the cubic space is of less importance than free ventilation.