Coughing is produced by irritation in the upper part of the windpipe and larynx.  A deep breath is drawn, the opening of the windpipe is closed, and immediately is burst open with a violent effort which sends a blast of air through the upper air passages.  The object is to dislodge and expel any mucus or foreign matter that is irritating the air passages.

Sneezing is like coughing; the tongue is raised against the soft palate, so the air is forced through the nasal passages.  It is caused by an irritation of the nostrils or eyes.  In the beginning of a cold in the head, for instance, the cold air irritates the inflamed mucous membrane of the nose, and causes repeated attacks of sneezing.

217.  How the Atmosphere is Made Impure.  The air around us is constantly being made impure in a great variety of ways.  The combustion of fuel, the respiration of men and animals, the exhalations from their bodies, the noxious gases and effluvia of the various industries, together with the changes of fermentation and decomposition to which all organized matter is liable,—­all tend to pollute the atmosphere.

The necessity of external ventilation has been foreseen for us.  The forces of nature,—­the winds, sunlight, rain, and growing vegetation,—­all of great power and universal distribution and application, restore the balance, and purify the air.  As to the principal gases, the air of the city does not differ materially from that of rural sections.  There is, however, a vastly greater quantity of dust and smoke in the air of towns.  The breathing of this dust, to a greater or less extent laden with bacteria, fungi, and the germs of disease, is an ever-present and most potent menace to public and personal health.  It is one of the main causes of the excess of mortality in towns and cities over that of country districts.

This is best shown in the overcrowded streets and houses of great cities, which are deprived of the purifying influence of sun and air.  The fatal effect of living in vitiated air is especially marked in the mortality among infants and children living in the squalid and overcrowded sections of our great cities.  The salutary effect of sunshine is shown by the fact that mortality is usually greater on the shady side of the street.

218.  How the Air is Made Impure by Breathing.  It is not the carbon dioxid alone that causes injurious results to health, it is more especially the organic matter thrown off in the expired air.  The carbon dioxid which accompanies the organic matter is only the index.  In testing the purity of air it is not difficult to ascertain the amount of carbon dioxid present, but it is no easy problem to measure the amount of organic matter.  Hence it is the former that is looked for in factories, churches, schoolrooms, and when it is found to exceed .07 per cent it is known that there is a hurtful amount of organic matter present.