million particles of dust to the cubic meter. It is little wonder that bronchitis and phthisis are common.1

Dust consisting of inorganic particles is more harmful than dust consisting of organic particles, because the former are sharper and more irritating. House dust is more harmful than outside dust, not only because there is more of it, especially in badly ventilated and illkept rooms, but because it is more apt to contain living pathogenic bacteria. House dust may be kept down by cleanliness and avoidance of dry dusting and sweeping; the use of vacuum cleaning; and by a free system of ventilation. Much house dust is blown in from the outside, and some of it comes in on dirty shoes. In buildings ventilated with a mechanical system the air may be filtered through bags or passed through a water curtain, which will eliminate much dust. Oiling floors with a wax or paraffin mixture helps to keep down indoor dust. Carpets tacked down are sanitary abominations and should be replaced with rugs that permit outdoor cleaning and sunning.

Street dust contains coal dust, metallic dust from the operation of trolley cars, material swept from houses and from shaking rugs from windows, the grinding up of roadbeds by vehicles, ashes, and other materials blown from barrels and teams; the bacteria are derived from dried fecal matter from horses and other animals, dried sputum, the soil, and a variety of other sources. Street dust may contain pathogenic organisms, such as the tubercle bacillus, many varieties of cocci, the colon bacillus, Bacillus aerogenes capsulatus, and possibly, under special conditions, tetanus, malignant edema, and occasionally other pathogenic microorganisms. Street dust, therefore, becomes more than a nuisance, for it is not only irritating, but may be a source of infection.

To keep down street dust requires, first of all, a well-constructed road with a good surface, oiled or properly cared for; the control of animals; the covering of ash barrels and carts hauling dusty loads; the use of automobile vacuum cleaners to replace the old or the present-day methods of dry sweeping. Attention must also be given to spitting on sidewalks and streets, the enforcement of smoke ordinances, the more extensive flushing of streets, and general attention to cleanliness.

The pollen of certain plants flying in the air as dust leads to hay fever in susceptible individuals (see Anaphylaxis, page 407).

Methods for Examining Dust.-PETRI DISH METHOD. The simplest and one of the most useful methods of determining the amount of dust and its composition is by means of suitable receptacles, such as Petri dishes, upon which the dust is allowed to settle for a sufficient period of time to enable a considerable quantity to accumulate.

For a discussion of the dusty trades, see chapter on Industrial Hygiene.

Particles are then examined under the microscope, or, if desired, they can be gathered upon a watch glass and weighed.

Weighing. The air may be passed through cotton or filters of other material, the quantity of air being measured either by means of a gas meter or other device and the increase in weight of the filter determined. Whatever the filtering medium the quantity of air should be large, in order that the quantity of dust may be appreciable in amount and be fairly representative. By weighing the filtering material before and after passing the air through it, the aggregate weight of dust in the quantity of air taken for examination can be determined. It is necessary to guard against increase in weight of the filtering material through the absorption of water. This can be done by placing the filtering material in a desiccator before and after filtration and just before weighing in each case.

THE KONISCOPE.-The koniscope, invented by Professor John Aitken, consists of two brass tubes connected at right angles and suitably fitted with stopcocks and a small air pump. By exhausting the air from one of the tubes, allowing the space to become saturated with water vapor by evaporation from wet blotting paper within, and then allowing this moisture to condense upon the dusty atmosphere under examination, clouds of different degrees of density will form inside the tube. The approximate density of the clouds can be measured by looking through the tubes, windows being provided for this purpose. A table is supplied with the instrument to give the approximate number of dust particles corresponding to clouds of different degrees of density.

CHAPTER IV

BACTERIA AND POISONOUS GASES IN THE AIR

BACTERIA IN THE AIR

The number of bacteria in the air ordinarily has a direct relation to the amount of dust; in fact, many of the bacteria in the air are attached to dust particles. Bacteria in the air are commonly considered as one kind of dust, but on account of their significance they are given separate consideration.

Bacteria are not found everywhere in the air; uninhabited places are quite free and the number diminishes as we ascend.

Bacteria do not multiply in the air; in fact, most of them soon die, especially when exposed in dry air to sunshine. For the most part, the bacteria in the air belong to the harmless varieties, although the number and kind vary greatly with circumstances. They come chiefly from the soil and are carried into the air by the wind and traffic movements; that is, bacteria in the air are derived from practically the same sources as dust. The dangerous bacteria in the air, however, come directly or indirectly from man and some of the lower animals.

The number of bacteria differs greatly with the local conditions. There are more in the air of towns than in the open country; few in high mountains, desert places, or at sea; more in windy weather than calm air; more indoors than in outside air; more in dry air than in moist air; more before than after rain. The air of badly ventilated rooms, especially if not kept clean, contains very many bacteria, and more when occupied, as the movements of the occupants stir up the dust.

Miquel of the Observatory of Montsouris studied the number of bacteria in the air of various localities. He found about 150 per cubic foot in the air of Paris, but only 6 after rain; on the top of the Pantheon he found 12; in the streets about 12 per cubic foot; in a neglected hospital 3,170; in a gram of laboratory dust 75,000, and in a gram of house dust 2,100,000.

Flügge considers that on the average there are about one hundred

microorganisms to a cubic meter of city air-an average evidently below that of Paris.

Dr. Jean Binot did not find a single bacterium in 100 liters of outside air taken at the summit of Mont Blanc; and he found a progressive decrease in the number as the height increased. Thus, he found:

Again, Graham Smith found at the top of the Clock Tower of the Houses of Parliament in London only one-third of the number at ground level.

Pasteur, in experiments that will ever remain classic, exposed organic infusions in flasks to the air of various places, and used the results thus obtained to prove the presence or absence of bacteria in the air and to dispel the illusion of spontaneous generation. Of 20 such flasks exposed to the air of the Mer de Glace 19 showed no contamination. About the same time (1875) Tyndall exposed 27 flasks containing an infusion to the air of the Aletsch glacier (8,000 feet); none showed putrefaction, while 90 per cent. of the flasks opened in a hayloft were "smitten."

It is estimated that a person living in London breathes about 300,000 microbes in the inspired air each day.

The expired air, during normal respirations, is practically bacteriafree, no matter how many may be contained in the inspired air. The moist mucous membranes of the upper respiratory passages act as a bacterial trap. When the expired air contains bacteria it is only as a result of coughing, sneezing, talking, or other forced expiratory efforts (see Droplet Infection).

The harmful bacteria in the air and the danger of contracting disease through air-borne infection are considered in the next section.

Method for Determining Bacteria in the Air.-A rough idea of the bacterial population of the air may be obtained by exposing suitable culture media in Petri plates for various periods of time, and counting the colonies which develop from the germs falling upon them.

A large number of different devices have been described for a more accurate determination of the number of bacteria in the air. These are all adaptations of three general methods: (1) filtration of air; (2) bubbling air through some liquid medium; (3) precipitating the bacteria from a given volume of air. Each of these methods can be made to give fairly satisfactory results in the hands of competent workers,

but the Committee of the American Public Health Association recommend the following method of Petri on account of its simplicity and general applicability.

FILTRATION METHOD OF PETRI.-The filter tubes are glass tubes 12 cm. in diameter and 10 cm. long. In the end of each is placed a perforated cork stopper, through which a glass tube 6 mm. in diameter is passed. The filtering material consists of sand which has been passed through a 100-mesh sieve. The sand in the filter tube is 1 cm. deep and supported by a layer of bolting cloth covering the cork. Two filter tubes are connected in tandem, and a measured volume of air, 10 liters or more, is drawn through at a constant rate by suction. The suction is applied by means of an aspirator of known volume, preferably one of the double or continuous type. Either the Magnus aspirator (Fig. 87) or the double aspirator (Fig. 88) are suitable for this purpose. Before using a pair of filter tubes a test for possible leakage is made by placing the thumb over the cotton stopper and applying the aspirator; if the suction is weak or absent the corks must be tightened or the tubes discarded. All corks should be tightened and connections wired and the apparatus sterilized before using the filters. The collection of the sample should take from 1 to 2 minutes per liter.

After filtering a definite volume through the tubes the sand is poured into 10 c. c. of sterile water, thoroughly shaken, and aliquot portions plated in ordinary nutrient agar, all plates being made in duplicate. The plates are incubated at room temperature for five days, when final counts are made.

RETTGER'S METHOD.-A new and improved method of enumerating air bacteria has just been described by Rettger, which commends itself as the best method yet devised. The method consists of bubbling a given quantity of air through salt solution. The bacteria in the FIG. 88.-DOUBLE air are trapped in the salt solution, which may then be planted in the usual way and the number of colonies counted.

ASPIRATOR.

Air and Infection. The air was long regarded as the vehicle and even the source of the communicable diseases. Theories, such as noxious effluvia, poisonous emanations, and infectious miasmata, gave way with the advent of bacteriology. When the early classical researches 'Jour. of Med. Res., June, 1910, XXII, 3, pp. 461-468,