outside.
This is _ventilation_.

_Ventilation_, therefore, is the maintenance of the air in a confined
space in a condition conducive to health; in other words, "ventilation
is the replacing of the impure air in a confined space by pure air
from the outside."

=Quantity of Air Required.=--What do we regard as impure air? What is
the index of impurity? How much air is required to render pure an air
in a given space, in a given time, for a given number of people? How
often can the change be safely made, and how? These are the problems
of ventilation.

An increase in the quantity of CO2 [carbon dioxide gas], and a
proportionate increase of organic impurities, are the results of
respiratory vitiation of the air; and it has been agreed to regard the
relative quantity of CO2 as the standard of impurity, its increase
serving as an index of the condition of the air. The normal quantity
of CO2 in the air is 0.04 per cent, or 4 volumes in 10,000; and it has
been determined that whenever the CO2 reaches 0.06 per cent, or 6
parts per 10,000, the maximum of air vitiation is reached--a point
beyond which the breathing of the air becomes dangerous to health.

We therefore know that an increase of 2 volumes of CO2 in 10,000 of
air constitutes the maximum of admissible impurity; the difference
between 0.04 per cent and 0.06 per cent. Now, a healthy average adult
at rest exhales in one hour 0.6 cubic foot of CO2. Having determined
these two factors--the amount of CO2 exhaled in one hour and the
maximum of admissible impurity--we can find by dividing 0.6 by 0.0002
(or 0.02 per cent) the number of cubic feet of air needed for one
hour,==3,000.

Therefore, a room with a space of 3,000 cubic feet, occupied by one
average adult at rest, will not reach its maximum of impurity (that
is, the air in such a room will not be in need of a change) before one
hour has elapsed.

The relative quantity of fresh air needed will differ for adults at
work and at rest, for children, women, etc.; it will also differ
according to the illuminant employed, whether oil, candle, gas,
etc.--an ordinary 3-foot gas-burner requiring 1,800 cubic feet of air
in one hour.

It is not necessary, however, to have 3,000 cubic feet of space for
each individual in a room, for the air in the latter can safely be
changed at least three times within one hour, thus reducing the air
space needed to about 1,000 cubic feet. This change of air or
ventilation of a room can b