CHAPTER XXXV 

 THE PRINCIPLES OF VENTILATION 



OUR comfort or discomfort in crowded rooms and shut-up places 

 depends on the chemical purity of the air only in so far as it affects 

 the olfactory sense, but, to a vast degree, on the influence of the 

 temperature, relative humidity, and the variations of these qualities 

 of the air, which act on the great field of cutaneous sensibilit} 7 . When 

 it is stated that the chemical purity is of little account, the proviso 

 is made that the air is only altered by the presence of healthy human 

 beings, and is neither rendered poisonous by the escape of coal-gas, or 

 other noxious trade product, nor deoxygenated by the oxidative pro- 

 cesses of the soil, as it is in mines, reference being made only to the 

 discomfort and ill-health caused by the deficient ventilation of, or 

 bad methods of heating, dwelling-houses, schools, factories, theatres, 

 chapels, etc. 



The chemical purity of the air has to be considered from three 

 points of view the concentration of carbon dioxide, the concen- 

 tration of oxygen, the supposed presence of organic poison exhaled in 

 the breath. 



It is commonly supposed that any excess of C0 2 acts as a 

 poison. The truth of the matter is quite otherwise; for, what- 

 ever the percentage of C0 2 in the atmosphere ma}- be, that in the 

 pulmonary air is kept constant, as we have seen, at about 5 per cent, 

 of an atmosphere by the action of the respiratory centre. It is there- 

 fore impossible that any excess of C0 2 should enter into our bodies 

 when we breathe the air of the worst-ventilated room, in which the 

 percentage of C0 2 assuredly does not rise above 0-5 per cent., or at 

 the outside 1 per cent. The only result from breathing such an 

 excess of C0 2 is a slight and unnoticeable increase in the ventilation 

 of the lungs. The increased ventilation is exactly adjusted so as to 

 keep the concentration of CO 2 in the lungs at the normal 5 per cent, 

 of an atmosphere. 



At each breath we rebreathe into our lungs the air in the 

 nose and large air-tubes (the dead-space air), and about one-third 

 of the air which is inhaled into the lungs is " dead-space " air. 

 Thus, no man breathes pure outside air into his lungs, but air 

 contaminated perhaps by one-third or (on deep breathing) by one- 

 tenth with expired air. When a child goes to sleep with its head 

 partly buried under the bedclothes, or in a cradle with the air con- 

 fined by curtains, he rebreathes the expired air to a still greater 



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