# Page:Popular Science Monthly Volume 81.djvu/392

Owing to the fact that the percentage of ${\displaystyle {\ce {CO2}}}$ is the usual test of ventilation and that only a very few parts per 10,000 in excess of fresh air are permitted by the English Factory Acts, it is generally supposed that ${\displaystyle {\ce {CO2}}}$ is a poison and that any considerable excess has a deleterious effect on the human body. No supposition could be further from the truth.
The percentage of ${\displaystyle {\ce {CO2}}}$ in the worst ventilated room does not rise above 0.5 per cent., or at the outside 1 per cent. It is impossible that any excess of ${\displaystyle {\ce {CO2}}}$ should enter into our bodies when we breathe such air, for whatever the percentage of ${\displaystyle {\ce {CO2}}}$ in the atmosphere may be, that in the pulmonary air is kept constant at about 5 to 6 per cent, of an atmosphere—by the action of the respiratory center. It is the concentration of ${\displaystyle {\ce {CO2}}}$ which rules the respiratory center, and to such purpose as to keep the concentration both in the lungs and in the blood uniform (Haldane); the only result from breathing air containing 0.5 to 1 per cent, of ${\displaystyle {\ce {CO2}}}$ is an inappreciable increase in the ventila-