A text-book on hygiene and pediatrics from a chiropractic standpoint

AIR

CONSTITUENTS OF AIR

Air is a mixture of gases, not a chemical compound, which envelops the earth and it is estimated to be at least one hundred miles high. This gaseous envelope is known as the atmosphere. The water upon the earth’s surface is called the aquasphere. The solid substance composing the earth is known as the petrosphere.

Air performs a very important function in the interchanging of gases in respiration and in the regulation of bodily temperature. The gases contained in the atmosphere are given by most authorities in the following proportions, per volume:

It is also found that there is a varying amount of water in the air, as well as dust and other substances. It is worthy of note that the air maintains a wonderful uniformity of composition over the entire surface of the earth. This is undoubtedly due to the fact that the atmosphere is in constant motion which maintains its proper mixture. It must also be noted that the air is not a chemical compound but is a mixture of gases.

Oxygen (O)

Probably the most important element in air is the oxygen which represents about one-fifth of its composition. This percentage is maintained with very slight variation; for instance, the percentage of oxygen in towns will be slightly less than 20.93%, about 20.87%.

When the atmosphere contains only 11% or 12% of oxygen it becomes dangerous, and death results when the percentage gets as low as 7.2%. A candle will not burn with the oxygen at 16%. Slight variation in the amount of oxygen is of no special importance.

The amount of oxygen in the air has little to do with the amount absorbed in respiration, as this is governed more by the need of the body than by the amount taken into the lungs.

Nature has made a wonderful provision in the protection of the body from extreme changes in the amount of oxygen in the air. This provision increases the degree of intellectual adaptation which is possible in the body. This is shown by the fact that there is a larger amount of oxygen contained at all times in the lungs than is required to supply the red blood cells. It is maintained that the alveolar air contains normally 16% of oxygen. The red blood cells are practically saturated with oxygen as they leave the lungs; however, this amount of oxygen may not be needed by the tissue cells which the red blood cells supply and the amount they absorb upon their return will depend upon what they have given off to the tissue cells.

It can readily be seen that the air in the lungs at no time contains the full percentage of oxygen, since one at no time completely exhales the entire amount of air. In this way the residual air loses some of its oxygen and collects carbon dioxide.

Animal life is sustained by the oxygen in the air while the carbon dioxide is essential to plant life. The oxygen is carried into the lungs during the inhalation which is produced by the expression of Innate Intelligence through the organs of respiration. The oxygen passes into the blood and is combined loosely with the hemoglobin of the red blood corpuscles; then under the direction of Innate it is carried to all the tissue cells of the body. Here the oxygen leaves the blood and is used in the oxidation which is necessary in the metabolism of the body.

The amount of required oxygen varies with different conditions that obtain in the body and is dependent upon age, the activity of the individual and his condition of health. Some authors assert that the average person will inhale about thirty-four pounds of air in twenty-four hours. This would mean a little over seven pounds of oxygen. Only about one-fourth of the oxygen inhaled is absorbed; therefore, according to these figures the individual would absorb on an average of about two pounds of oxygen in twenty-four hours.

Nitrogen (N)

The nitrogen in the air is of more importance to plant life than to animal life. It is of little significance from a hygienic standpoint, yet it is an important constituent of all matter containing protein. Nitrogen tends to regulate or influence the rate of combustion by diluting the oxygen in the air. Respiration does not seem to affect the amount of nitrogen in the air as there is no noticeable difference between the amount of nitrogen in the inspired and expired air.

Argon (A)

In 1894 Lord Rayleigh and Prof. William Ramsay discovered a gaseous substance in the atmosphere which had no chemical affinity. They gave this element the name argon which means in the Greek, “inactive.” So far as is known argon will not combine with any other element.

The atmosphere contains about 0.94% argon, but so far, according to Rosenau, it has not been demonstrated in the body. Argon has no hygienic significance. Helium, krypton, xenon, neon and argon form what is known as the argon group, since they will not unite with other elements to form compounds. These elements are all found in very small quantities in the atmosphere.

Carbon Dioxide (CO₂)

Carbon dioxide is a gas, a very small amount of which is found in the atmosphere. About 0.03% is ordinarily expressed. This means there are three parts of carbon dioxide in 10,000 parts of atmosphere. This is a very small percentage, but when we consider the enormous bulk of atmosphere we appreciate the fact that the total amount is beyond our comprehension. It is claimed that there is more carbon in the atmosphere in the form of carbon dioxide than there is in all other forms on the earth.

In densely populated areas there is a slight increase in the amount of carbon dioxide. It may reach 0.04% or even 0.05%. There is also likely to be more carbon dioxide in the air close to the soil than there is a few feet above. This is because the processes going on in the soil tend to produce this gas and hence the soil air contains a larger percentage. The air receives its carbon dioxide from such sources as respiration, fermentation, from chemical action in the soil, from mineral springs, and from oxidation of organic matter.

Carbon dioxide is a transparent, odorless and colorless gas. It is a very important constituent in the atmosphere and a very slight variation either in its increase or decrease will vitally interfere with both animal and vegetable life. Green plants in the sunlight absorb great quantities of carbon dioxide and give off oxygen. The body obtains its carbon from the food that is taken in while the oxygen is obtained from the air that is breathed. It is maintained that one acre of ordinary tree land will withdraw as much as four and one-half tons of carbon dioxide from the air in one season. Carbon dioxide is also absorbed by water; hence the oceans and other large bodies of water perform a very important function in this respect. In certain localities, such as industrial centers, a very large amount of carbon dioxide is given off into the air; but this is quickly taken up and thoroughly mixed with the atmosphere by the constant motion which is maintained by the varying temperature, air pressure and other involved factors. It is not allowed to accumulate in any one place in larger amounts than normal. In this way the proportion is kept the same at all times.

Hygienists maintain that the regulation of breathing is largely dependent on the concentration of carbon dioxide in the air cells of the lungs. This, however, is not true since the process of respiration is not a question of chemical action nor the influence of any chemical element in the lungs or in any part of the respiratory system. The breathing, like all other functions of the body, is under the direct control of Innate Intelligence within the body.

Innate Intelligence, recognizing the need for more oxygen in the tissue cells, will bring about an intellectual adaptation by increasing the respiration. This does not take place, however, until the need has exceeded the already adaptative possibility when the reserve amount of oxygen contained in the residual air in the lungs has been exhausted.

In inspiration the air is taken into the lungs and the oxygen is taken up by the hemoglobin of the blood as before stated; then the oxygen is given off to the tissue cells. The hemoglobin absorbs the carbon dioxide from the tissues and it is then carried back to the lungs where the process is reversed. That is, the carbon dioxide is given off and the oxygen is absorbed by the hemoglobin.

The carbon dioxide which is carried to the lungs by the venous blood escapes into the air cells. The resistance which it meets here depends upon the percentage of carbon dioxide present in the alveolar air. Normally the percentage of the carbon dioxide in the alveolar air is maintained at a pressure of about 5% of an atmosphere. If the pressure becomes less than this Innate Intelligence will adapt herself to this condition by decreasing the respiration until the normal percentage is again attained; or if the pressure goes above normal Innate will increase the rate of respiration until the amount of carbon dioxide has been decreased in the alveolar air, for this poisonous gas must be eliminated. This shows how Innate at all times is working in an adaptative way to maintain equilibrium in the body.

Ozone (O₃)

Ozone is not a constant element in air, although of such potency that a very small amount may be of great importance. In centers of population it is usually absent. In thickly wooded sections and at sea it will be found in large quantities, but even here only traces will be found. It is thought to be formed by the friction of the sea water against the air in connection with the respiration of vegetation. It is also formed in nature by electric discharges during storms.

Ammonia (NH₃)

Normally there is only a trace of ammonia in air. It is produced by the decomposition of organic matter. This accounts for the larger amount being found in the air near the ground. Ground air may contain ammonia in sufficient quantity to be perceptible to the senses.

Other Constituents

Hydrogen peroxide is a very active oxidizing agent. Rain water and snow contain very small traces of it. Hydrogen peroxide is not constant in air, but may be found in very small quantities.

Air also contains small traces of helium, krypton, neon, xenon, which have no important bearing from a hygienic standpoint. Nitric, sulphuric and other acids will at times be found in the atmosphere. These acids are formed principally from industrial processes and will, therefore, be found near industrial centers, rather than in the open country or at sea.

AIR PRESSURE

Normal Air Pressure

Ordinary air pressure on the surface of the earth at sea level is, in round numbers, fifteen pounds to the square inch. It has been found that the average sized man is exposed to about 34,000 pounds of pressure. This pressure may be increased or decreased, and if the change takes place gradually so that Innate Intelligence will have time to adapt the body to the change, no harm will result. There is a limit, however, either in a decrease or in an increase of pressure to which the body may be adapted.

This intellectual adaptation obtains in every tissue cell of the body as well as in all of its secretions and chemicals so it can be seen that any sudden change in the atmospheric pressure will disturb the equilibrium of the entire body.

Reduced Air Pressure

When the atmospheric pressure is decreased it has the same effect on the individual that the breathing of rarefied or diluted air has. In this event the adaptation consists in an increased rate of breathing. It is also asserted that the number of red blood corpuscles is increased, which in turn increases the capacity of the blood to carry oxygen. In the diminished air pressure there is a marked decrease in the amount of oxygen absorption and this accounts for the increase in the respiration and quickened pulse rate.

As previously stated, if the change from a normal air pressure to a greatly decreased pressure takes place gradually, thus giving Innate sufficient time to bring about an adaptation, man will be enabled to live in a healthy state in a rarefied air. As the altitude is increased the air pressure is decreased. This is because there is less air above to exert pressure. Naturally the air near the earth has the greatest weight above it to exert compression. The air pressure decreases gradually until, it is assumed, it is gradually thinned into a perfect vacuum. If oxygen is added to the air it is possible to maintain life at a remarkably reduced air pressure. A bird was kept alive by P. Bert in oxygenated air at a pressure of 0.1 of an atmosphere.

The symptoms produced in diminished air pressure will vary with the general conditions of the individual. People suffering from cardiac disturbances will be affected most by high altitudes. This is possibly the first symptom to be manifested in rarefied air. If there is any inclination toward heart weakness it will be observed when the air pressure is decreased.

The effects of reduced air pressure will vary with circumstances. There will be increased and deep breathing, with an increase in the pulse rate. There is likely to be dizziness and ringing in the ears. The sense of hearing, of sight and possibly of sensations, become impaired. There may be drowsiness and the patient may have a strong desire to sleep. The intellectual faculties become dull. Where the change has taken place very suddenly to the decreased air pressure there will be syncope, dyspnea, dizziness, nausea and weakness. These symptoms are commonly known as mountain sickness.

Increased Air Pressure

The greatest air pressure in nature is at sea level. Even the increased pressure in the deepest mines is so slight that it has no physiological significance; hence man is exposed to increased air pressure only under artificial conditions. This subject will be referred to again under the head of Industrial Hygiene.

Some of the conditions under which man is exposed to increased air pressure are, for example, divers in diving bells and diving suits, and caisson workers. At a depth of ten meters of water the air in a diving bell is compressed to one-half its original bulk, and as a result the pressure of air is doubled. At thirty meters, or about 100 feet, the pressure is quadrupled, exposing the diver to four atmospheres or about sixty pounds per square inch.

The danger in increased air pressure is not in going from a normal pressure to a high pressure, nor does the danger come from the high pressure. The danger is not from the compression, but rather from the decompression. The individual must be brought from the increased air pressure to the normal air pressure very gradually, allowing Innate sufficient time to adapt the body to the normal pressure of fifteen pounds to the square inch. If this is done no great amount of harm will result, although there are some men who are not able to withstand these changes. This shows a lack of intellectual adaptation through the body on the part of its Innate. In an experiment P. Bert exposed dogs to an air pressure of ten atmospheres, or about 150 pounds to the square inch, and then released them very gradually, producing no ill effects.

When the air pressure is increased it results in an increase in the absorption of the gases of the atmosphere. There is an increased amount of oxygen taken up by the blood and the tissues absorb large amounts of nitrogen. When the pressure is released slowly these gases gradually escape from the lungs and no bubbles will be formed.

During compression the symptoms are not so severe as those during and after decompression. During compression the symptoms are a deepening but slowing of the respiration, a slowing of the pulse rate, and an interference in evaporation in the water vapor. The increased pressure on the ear drums will cause pain in the ears and sometimes the ear drums rupture. There may be an alteration in the voice, headache and dizziness. However, in a short time, intellectual adaptation takes place and equilibrium is established by the internal change and there is at least temporary relief from these symptoms.

Caisson Dis-ease

The greatest risk to health and life occurs during the time the individual is coming from the high to normal pressure after the decompression has entirely taken place. This may produce a condition known as caisson dis-ease. It may be several hours after the workman has been taken from the decompressing chambers before the symptoms appear. Gradual decompression is the only thing that will prevent the manifestation of these symptoms. When the workman shows any symptoms of caisson dis-ease he should be rushed back into the compression chambers and kept there a short time until Innate has a chance to establish equilibrium in the pressure of the body, and he should then be taken very gradually through the decompression chambers.

The symptoms that appear after decompression are vertigo, nose bleeding, nausea and vomiting. The most common symptom is the severe pains in the muscles and joints which is known by the layman as bends. There may also be temporary or permanent paralysis called diver’s palsy. In the more severe cases there will be unconsciousness, and even death.

When the workman is taken too quickly from the high pressure there is formed gas and air emboli. These may form in the labyrinth of the ear, in the spinal cord, in the brain, in the heart, or in any other vital part of the body and not only be distressing but may even prove fatal.

It must be remembered that the conditions produced by changes in the air pressure may be classed as traumatic, immunity from which is not entirely a question of uninterrupted transmission. If there are subluxations in the spine at the time the workman goes into the caisson, or if subluxations are produced at the time of decompression, the interference thus produced will interrupt the process of intellectual adaptation. Such subluxations should be adjusted, but it must be remembered that if emboli are formed or if bends occur the result or the relief is a question of the natural processes of Innate in the body, and the results will depend entirely upon the ability of Innate to cope with the traumatic condition. Innate Intelligence operates through the body according to law, and it is possible for the body to become so affected and deteriorated that it becomes a physical impossibility for her then to repair it and restore it to normal.

We have seen that the normal air pressure at sea level is fifteen pounds to the square inch. In order that the body will not be crushed by this weight it is necessary to have an internal resistance to equal this weight. This internal resistance is maintained in the body by the tone of all of its parts; it is maintained by the expression of mental impulses in the tissue cells.

We have also observed that the combining of chemicals is influenced by the pressure exerted; therefore, in order that the chemical combinations of the body be constant there must be equilibrium established between the external pressure and the internal resistance. This is maintained through the adaptability of the expression of Innate in the body. As the change takes place externally there is a corresponding adaptative change taking place internally and this all requires time. Therefore, in passing the body from one air pressure to that of another degree, there must be a sufficient amount of time intervening to allow Innate Intelligence to bring about the necessary adaptation.

The changes that take place under differing degrees of air pressure are not purely the result of chemical changes in the body as is maintained by some, but in all these processes we see the evidence of intelligent action which we believe is the result of the reasoning of the intelligence in the body. One evidence of this is that when the workman is brought gradually from an increased air pressure to the normal air pressure the gases that have been absorbed by the tissues under the abnormal condition will be given off through the normal channels, and less injury will be done. It is interesting to note that in Nature, man is not exposed to a greater air pressure than that at sea level; it is only under artificial, man-invented environments that the body is called upon to withstand a greater pressure than the normal. It may also be noted that without the inventions of man it requires quite a little time for man to be transported from the air pressure at sea level to that of the higher altitudes as on the mountain tops. This gives Innate Intelligence an opportunity to bring about an adaptation to this change, since the change in the pressure takes place so gradually. But with man-made inventions, such as the automobile and aeroplane, one may transfer himself from sea level to great heights where the pressure is less in a very short time. This does not allow sufficient time for adaptation to take place and is much different from the slow process of climbing the mountains.

So we see that, after all, most of the necessity for immediate adaptation of the body to changed environmental conditions is the result of the work of the educated mind of man, and not the result of the laws of Nature. In Nature we seldom see the necessity for sudden or extreme adaptation. But under our present artificial means of living there are such cases, and these necessitate a study of the artificial conditions as well as a study of the Innate laws of adaptation that there may be as far as possible an educated adaptation to the environment.

HUMIDITY AND TEMPERATURE OF AIR

Humidity

Water vapor is present at all times in the atmosphere. It is the least constant of all of the air constituents, varying greatly under different conditions. The temperature produces a greater change in the amount of aqueous vapor in the air than any other factor. There may be so much water vapor in the air that the air is spoken of as being completely saturated. This is known as absolute humidity. If there is any excess over and above this complete saturation, it is given off as dew; it is spoken of then as having reached the dew point. It must be remembered that this absolute humidity does not represent a constant amount of water vapor, for the amount of moisture necessary to produce complete saturation varies with the degree of temperature. It is erroneous to speak of the air holding water.

“As a matter of fact, the air has nothing to do with it, for it has always been clearly observed that the presence of water vapor in any given space is independent of the presence or absence of air in the same space. The amount of aqueous vapor which a space contains depends entirely upon the temperature and not upon the presence of the air.”—Rosenau in Preventive Medicine and Hygiene.

The higher the temperature the greater the amount of water vapor in one cubic foot of air at a temperature of 10° F., while at 100° F. there would be 19.1 grains at complete saturation. Since increased temperature increases the amount of aqueous vapor and this aqueous vapor in turn absorbs heat, we thus see a reciprocal action of the aqueous vapor upon the temperature.

Absolute humidity is all of the water vapor that may be contained in the air at a given temperature. Relative humidity is the difference between the amount of water vapor that must be contained in the air at a given temperature to reach absolute saturation and the amount actually contained in the air at that same temperature.

If the relative humidity of the air in a room becomes as high as 85% the moisture will begin to condense and form on the walls and objects. This makes the room damp and interferes with the ventilating and heating of the room.

There is less water vapor contained in the air at high altitudes, the air being cooler. A large amount of rainfall does not necessarily produce an increase in the relative humidity. That is to say, a country with a very high average of rainfall is not necessarily a damp country so far as the atmosphere is concerned.

Cold Dry Air

Cold dry air is exhilarating and tends to quicken metabolism in the body, while warm damp air is depressing and tends to retard metabolism.

The body possesses great possibilities of adaptation to the varying degrees of temperature and humidity through the action of Innate Intelligence. With the aid of the educated mind in bringing about adaptation in the way of clothing, for example, it is possible to increase the range of temperature and humidity to which the body may be adapted.

Due to the fact that heat is being constantly formed in the body by the different processes that are carried on within, it naturally follows that this heat must be carried out of the body or it will accumulate and result in harm to the tissues, producing what is known as heat stroke. This heat dissipation is greatly influenced by the humidity, or in other words, the amount of water vapor in the air. The temperature of the air also has some influence on heat dissipation.

Cold air is made to feel colder by an increase in the amount of moisture while warm or hot air is made hotter by increasing the moisture. The reason for this is that the moisture in the cold air favors heat conduction, hence draws the heat from the body at a more rapid rate than is normal, while the moisture in the hot air hinders evaporation.

Innate Intelligence is able, through the specially devised machinery of the body, to maintain a perfect balance between heat production and heat dissipation or heat loss. Even though the temperature of the air may rise, yet if the body is normal it will not produce an increase in the bodily temperature. Indeed, it is asserted that when the temperature of the air goes above 70° F. the bodily temperature would rise if it were not for the perspiration which Innate will produce through the sweat glands. As long as the perspiration is produced and is evaporated from the surface of the body the heat production and heat loss will be kept in perfect balance. But when something interferes with this adaptative process of Innate and the individual can not perspire, there will soon be symptoms of overheating and the temperature of the body will begin to rise.

Evaporation is decreased in an atmosphere in which the humidity is high. The reason for this is obvious; the atmosphere already filled with water vapor is slow to take up more. This is because molecules of vapor given off from the body collide with those in the air and are returned to the surface of the body as moisture. When this condition obtains there is an adaptation produced by increasing the amount of blood to the skin; this increases the temperature of the surface of the body, but allows for an increase in the heat loss by radiation, conduction and convection.

The conductivity of the atmosphere for heat is increased by an increase in the humidity; hence a cool damp air will chill the body for the reason that the conductivity is increased and bodily heat is lost more rapidly through conduction. Increased humidity interferes with the evaporation of perspiration; hence a hot, moist air is heating to the body and deprives the body of force, making the patient feel sluggish and fatigued.

There is much moisture given off from the body each day. It is estimated by Pettenkofer, Voit, Rosenau and others that the average individual under ordinary circumstances will give off through the lungs about 290 grams, and from the skin from 500 to 1800 grams daily. If this fact is kept in mind some idea of the necessity of proper ventilation will be appreciated.

Warm Moist Air

Workers fatigue much more easily when in warm moist atmosphere. Work is done much more rapidly in cool dry air and the efficiency of the worker is noticeably raised; in warm damp air the bodily temperature rises and the pulse rate increases.

Mental and physical activities are reduced in an atmosphere of high humidity and increased temperature. This is due mainly to the reluctance on the part of the individual to put forth an effort sufficient to perform any great amount of work. There is a general feeling of languor because of the enervating effect of the air.

There is no serious injury resulting from working in such an atmosphere, unless there is an increase in the bodily temperature, and then there may be serious results to the health unless relief is obtained. When the humidity has reached the point of complete saturation and the temperature is above 88° F., compensation can no longer obtain through evaporation and heat stroke may result. The most noticeable effect of warm moist air under ordinary circumstances is reluctance to put forth any mental or physical exertion, and a loss of appetite. With a temperature at 75° F. and the relative humidity 80%, an individual not accustomed to such will require complete rest.

It will be noticed that under such conditions Innate Intelligence is constantly working to bring about intellectual adaptation and that she is able to do so to a remarkable degree. The glands of the body are used to produce secretions which constantly bathe the tissues and keep them cool in the high temperatures, and at a proper degree of warmth in the low temperatures. The fact that the individual is indisposed to mental and physical activity in such an atmosphere is adaptative on the part of Innate. This inactivity is suggested by means of the languid feeling in order that the body will not be over-exercised and thus will not increase the amount of heat in the body; because under these circumstances the process of evaporation is interfered with and this is one of Innate’s principal means of regulating the temperature of the body.

Perspiration is an adaptation on the part of Innate, for in this way the surface of the body is kept moist and as this moisture evaporates the body is cooled; otherwise the temperature of the body would increase with every rise in the temperature of the atmosphere. Not only does this help to regulate the bodily temperature, but it keeps the surface tissue of the body soft. If there was no perspiration, the surface of the body would soon become dry, parched and hard. The skin would become scaly and would crack and become chafed.

Because of these adaptative processes it is possible for the body to be adjusted to great extremes in temperature. The body may become accustomed to extremely high temperatures even with high relative humidity, providing the change takes place gradually and sufficient time is allowed for the processes of adaptation to take place.

It is very important that the kidneys be able to perform their normal function in order that these processes of adaptation may take place. The kidneys are important not only because of the function which they perform in the excretion of poisons, but because of the function which they perform in relation to the serous circulation. This is important because of the secretions that are involved. This subject will be treated more fully under the subject of Water.

Cold Damp Air

The body quickly becomes chilled in a cold damp air because the increase in the water vapor increases the conductivity of the air for heat. It can thus be seen that the heat producing processes of the body must be increased in order to maintain the normal temperature of the body. As we know, all activity of the body requires an expenditure of energy, so if the body is exposed to a cold damp air for an abnormal length of time it will necessitate an increased expenditure of internal energy to meet the increased demands for heat and thus dissipate forces that should and would, under normal conditions, be utilized in the metabolism of the body. If the heat production is at a minimum in the body, which it may be, due to several factors such as old age, infancy, or dis-ease, this exposure to lowered temperature and high humidity will result in injury to the body.

An interference with the transmission of mental impulses to the kidneys resulting in a decrease in their functional activity may mean a retention of poisons in the body and produce a condition diagnosed as rheumatism, for example; or an interference with the serous circulation may result in the tissue cells being under-nourished. Both of these conditions will interfere with the processes of intellectual adaptation. Educationally, man may help in the adaptation by proper clothing and by giving attention to proper exercise.

Warm Dry Air

By far the most desirable air is the warm, relatively dry air; but as has already been noted, it is possible to have an atmosphere with a relative humidity that is too low. If the air is abnormally dry, and at the same time warm, there will be a great loss of body moisture due to the increased evaporation. When the loss of water from the body reaches 21% death ensues. In an experiment performed by Rubner and Lewaschew it was found that a man weighing about 127 pounds gave off about 54.1 grams of water in an hour in a temperature of 68° F. with a relative humidity of 82%; the same individual in a temperature of 68° F., but with a relative humidity of 82%, gave off only 15.3 grams.

Proper Temperature and Humidity

The most desirable atmospheric conditions are obtained at a temperature of 68° F. to 70° F., with the relative humidity from 40% to 60%. This is given only as a general average and will necessarily vary with many conditions, such as seasons of the year and occupation.

Moderately cool and relatively dry air increases the activities of the body, makes breathing easy and more frequent, and hence increases the circulation of the blood. Innate is thus enabled, in such an atmosphere, to bring about more perfect metabolism.

Proper temperature and humidity of the air in houses, impurities found therein, source of impurities and their effect upon health, will all be considered thoroughly under Hygienic Housing.