RESPIRATION 305 



too much stress was formerly laid on the chemical purity of the 

 air in rooms. The chemical purity is nevertheless a very important 

 index of the chances of infection through the air from person to 

 person in a room. The more air is passing through the room the 

 less the chances of infection become ; and for this reason as high 

 as possible a standard of chemical purity is desirable where a 

 number of persons, some of whom may be carriers of infection, 

 are present. A reasonable standard to aim at under these circum- 

 stances is that the excess of CO2 in the air of the room should not 

 be over .02 per cent unless lights are burning, or that about 50 

 cubic feet of air per person and per minute should be supplied. 

 This standard can easily be maintained in ordinary houses with 

 natural ventilation ; and even in the case of crowded buildings a 

 similar standard can be attained by the right application of 

 modern engineering methods. 



When air becomes very warm the regulation of body tempera- 

 ture becomes dependent on increased evaporation from the skin 

 and not merely on variation in the blood flow through it. If mus- 

 cular work is being done this point is soon reached if the air is 

 fairly still. The amount of moisture in the air then becomes very 

 important, as the rate of evaporation from the skin depends on the 

 amount of moisture already present in the air. In still air, or in 

 air moving at any given rate, a temperature is finally reached at 

 which in spite of profuse sweating the skin cannot evaporate 

 water quickly enough to prevent the body temperature from rising. 

 As I showed experimentally in 1905, this temperature is reached 

 when the wet-bulb temperature reaches a certain point.^ Thus in 

 still air and with hardly any clothing, the body temperature be- 

 gins to rise when the wet-bulb temperature exceeds 88 °F (3i°C). 

 It does not matter what the actual air temperature is, or the 

 actual percentage of moisture in the air, provided that the wet- 

 bulb temperature reaches 88°. Thus it was indifferent whether the 

 air temperature was 88° with the air saturated, or 133° with the 

 air very dry, provided that the wet-bulb temperature was 88°. 

 When the wet-bulb temperature was far above 88° the rate of 

 rise of body temperature was proportional to the rise of wet-bulb 

 temperature.^ 



When even moderate muscular work was being done the criti- 

 cal wet-bulb temperature was, even with almost no clothing, at 



^ Haldane, Journ. of Hygiene, V, p, 494, 1905. 



'Haldane, Trans. Inst, of Mining Engineers, XLVIII, p. 553, i9M- 



