filters in removing solid particles from the air, and in thus
purifying it before its entrance into the lungs.

In the immediate neighborhood of three Philadelphia high schools,
having an approximate enrollment of over 8000 pupils, is a huge
manufacturing plant which day and night pours forth grimy smoke and
soot into the atmosphere which must supply oxygen to this vast group
of young lives. If the vital importance of nose breathing is impressed
upon these young people, the harmful effect of the foul air may be
greatly lessened, the smoke particles and germs being held back by the
nose filters and never reaching the lungs. If, however, this principle
of hygiene is not brought to their attention, the dangerous habit of
breathing through the open, or at least partially open, mouth will
continue, and objectionable matter will pass through the mouth and
find a lodging place in the lungs.

There is another very important reason why nose breathing is
preferable to mouth breathing. The temperature of the human body is
approximately 98° F., and the air which enters the lungs should not be
far below this temperature. If air reaches the lungs through the nose,
its journey is relatively long and slow, and there is opportunity for
it to be warmed before it reaches the lungs. If, on the other hand,
air passes to the lungs by way of the mouth, the warming process is
brief and insufficient, and the lungs suffer in consequence.
Naturally, the gravest danger is in winter.

41. Cause of Mouth Breathing. Some people find it difficult to
breathe through the nostrils on account of growths, called adenoids,
in the nose. If you have a tendency toward mouth breathing, let a
physician examine your nose and throat.