CHAPTER II

PRESSURE, TEMPERATURE, AND HUMIDITY

PRESSURE

Normal Atmospheric Pressure. The pressure of the atmosphere at sea level is 15 pounds to the square inch, or, as indicated in the barometer, it will maintain a column of mercury 30 inches or 760 millimeters. A man of average size living at sea level is exposed to a total pressure of about 34,000 pounds-more than 15 tons. This great pressure must evidently have physiological importance. All the tissues and fluids of the body are subjected to this pressure and are in equilibrium with it. The interchange of gases on which life depends is largely a phenomenon of atmospheric pressure. The pressure of the air also keeps the heads of the bones in their sockets without muscular action, and doubtless performs other functions less obvious. The small variations in pressure such as occur day by day at sea level have no evident physiological effects.

Diminished Atmospheric Pressure.-A diminution in atmospheric pressure is equivalent to breathing rarefied or diluted air. The most important physiological effects of diminished atmospheric pressure are due to a diminution in the amount of oxygen absorbed, hence the breathing is deeper and the pulse rate quickened. As the altitude increases there is a lowered tension of oxygen in the alveolar air and a diminished tension of carbon dioxid. While the rate of respiration may be variously influenced in different circumstances, the depth of respiration is almost invariably increased. This of itself not only facilitates the oxygen supply, but also increases the elimination of carbon dioxid. Formerly a great compensatory increase in the number of red blood cells was believed to take place as a result of prolonged residence in high altitudes. Thus, assuming the average number of red blood cells per cubic millimeter at sea level to be about 5,000,000, at Davos (elevation 1,560 meters) the number of red blood cells averages 5,500,000 to 6,500,000. At Cordilleras (altitude 4,392 meters) the average number of red corpuscles is 8,000,000. A similar change in the blood has been produced by keeping rabbits and guinea pigs in rarefied air at sea level. According to Bürker, only a comparatively small increase takes place, amounting to 4 or 5 per cent., at altitudes of five or six thousand feet. The same moderate results have likewise been noted lately for much higher

altitudes. The higher figures of earlier workers are now accounted for by the more rapid evaporation of blood samples at higher altitudes, so that with improved technic the belief in the great increase in the oxygencarrying blood constituents disappears.

At a height of 18,000 feet the pressure of the atmosphere is only half the pressure at sea level, thus:

"The highest dwelling place continuously occupied is the Observatory El Mirti, in the Andes, at 5,880 m. The Observatory at Arequipa is at 6,100 m. Thok djalung is a village in the Himalayas at 4,980 m. In Peru, Bolivia, and Northern Chili a very large part of the population live above 3,000 m. Potosi, which has numbered 100,000 inhabitants, is at 4,165 m., Cerro de Pasco at 4,350 m., the mines of Villacota at 5,042 m., the railway from Callao to Oroya culminates in a tunnel at 4,760 m., almost the height of Mont Blanc. An annual fair is held at Gartok, at 4,598 m., in the Himalayas, to which thousands annually come."1

It is evident that man may become adapted to breathing a rarefied air at great heights, which would overcome persons if the change were made suddenly from sea level.

The symptoms produced by a marked diminution in atmospheric pressure vary with circumstances. The effects are increased by cold, active muscular exertion, or improper clothing. The noticeable symptoms are increased rapidity of respiration and acceleration of the circulation, noises in the head and dizziness, impairment of the senses of sight, hearing, and touch, dulness of the intellectual faculties, and a strong desire to sleep. Sudden changes to a rarefied atmosphere cause syncope, weakness, dyspnea, dizziness, and nausea. These threatening

symptoms sometimes go by the name of mountain sickness. Bert and Journet believe this condition is due to lack of oxygen and the symp

toms may, in fact, be relieved by adding oxygen to the air inspired. Bert kept a bird alive in oxygenated air, even though the pressure was reduced to less than 0.1 of an atmosphere. Kronecker concludes that mountain sickness is caused by a congestion of the lungs, impeding the flow of blood through them. Mosso and his followers attribute the physical disturbances of a reduced atmospheric pressure to the fact that the blood loses carbon dioxid more quickly than it loses oxygen, and attributes mountain sickness to this decrease of carbon dioxid in the blood (acapnia). Cohnheim believes there is a concentration of the blood at high altitudes; in fact, insignificant increases have been found. by competent observers. The climate in high altitudes is always dry and evaporation proceeds rapidly. As a result individuals lose water more readily than at lower levels. If this explanation is tenable, an increase in corpuscles and hemoglobin content are in no wise the expression of lack of oxygen, but are rather the outcome of the increased evaporation under the altered conditions of climate.

The limit at which life may be sustained is about 26,000 feet, at which height consciousness is lost. At this height the barometric pressure of the air is 251 mm., which represents a pressure of oxygen of 52, which is the equivalent of 6.8 per cent. oxygen. P. Bert remained 20 minutes in a pneumatic chamber with a pressure of only 248 mm. without serious inconvenience.

Increased Atmospheric Pressure.-While man is often exposed to rarefied air, he is seldom subjected to increased pressure except under artificial conditions, such as in diving bells, diving suits, and caissons. The increase in atmospheric pressure in the deepest mines has little physiological significance. Divers and workers in caissons are not subjected to more than about 42 atmospheres, and work under such pressure for only a few hours at a time. When a diving bell is lowered 10 meters into the water the air contained in it is compressed to one-half its original bulk, and the pressure of the air is accordingly doubled. Each 10 meters' depth means an additional pressure of one atmosphere. At a depth of 30 meters, about 100 feet, a diver is exposed to a pressure of 4 atmospheres or about 60 pounds per square inch. Bert exposed dogs to a pressure of 10 atmospheres, and then slowly released them without harm.

The physiological effects of an increased atmospheric pressure are mainly due to an increase in the amount of atmospheric gases (especially nitrogen) which are taken up by the blood, and also an increase in the chemical absorption of oxygen by the red blood cells. The serious consequences usually result from too rapid decompression.

CAISSON DISEASE.-The effects produced by compressed air in caissons are: (1) those caused when the men are undergoing pressure, and (2) during or after decompression.

The symptoms produced by an increase of atmospheric pressure are a slowing of the respiration, which is evidently compensatory, but on account of compression of intestinal gases the respirations are deeper; the pulse is slower, and evaporation of water-vapor hindered. The voice may be altered; pains in the ear are common, due to pressure upon the drum, and may be obviated by swallowing air and thus passing it up the Eustachian tube into the middle ear. Sometimes the ear drum ruptures; headache and dizziness may also occur. During compression the blood keeps absorbing the gases of the air until the tension of the gases in the blood becomes equal to that in the compressed air. As soon as this equilibrium has been attained relief from immediate troubles is secured.

It is during and after decompression that the greatest danger to health and even risk of life occur. The most frequent symptom is excruciating pains in the muscles and joints, called by the workmen "bends." These pains may continue for a few hours or for two or three days. Occasionally there is bleeding at the nose; also severe abdominal pain, and vomiting, nausea, vertigo, dyspnea, and unconsciousness. Death may result from internal hemorrhage, or paralysis may ensuethe so-called diver's palsy.

The effects of increased atmospheric pressure and too rapid decompression were carefully studied by Paul Bert in 1878, who showed that the lesions are caused by the escape of gases of the atmosphere which have been taken up in excessive amounts, and are released in the blood and tissues when the pressure is diminished. The blood vessels may contain air emboli, which may lodge in vital parts and cause sudden death, or the delicate capillaries may break, leading to hemorrhage with resulting paralysis. Air emboli may be distressing or dangerous if they occur in the labyrinth of the ear, in the spinal cord, in the brain, or in the heart or other vital parts.

The prevention of caisson disease consists in gradual decompression. Sometimes the symptoms come on several hours after the workman has left the caisson. As soon as symptoms come on the workman should at once be hurried back into the compression chamber until equilibrium between the internal and external pressures is restored. He may then be allowed to pass through the decompression chambers, but very gradually. A medical air-lock should be provided at the works, well heated, and furnished with bunks and emergency supplies.

Barometers. The pressure of the air is measured by means of barometers, the principles of construction and use of which are so well known that they do not require special description.

MOVEMENTS OF THE ATMOSPHERE

Moving air is necessary for the maintenance of health and is a prime requisite of good ventilation. The motion of the air serves the twofold purpose of bringing us a fresh supply and taking away the sewage-polluted air from our immediate vicinity. Moving air also favors evaporation and helps to prevent heat stagnation by keeping the surface temperature within normal limits. Paul, Heymann, and Erclentz, in Flügge's laboratory, and also Leonard Hill in England, emphasized the importance of moving air in assisting the heat regulation of our body. They believe that this is a much more important function of moving air than simply the bringing of fresh air or the carrying away of the products of respiration. In still air the body soon becomes surrounded by a warm, moist aerial envelope which causes an overheating of the surface of the body and results in the familiar symptoms of "crowd poisoning.” In a still atmosphere we are soon surrounded by a blanket of stagnant and impure air, whether indoors or outdoors.

Much of the benefit of mountain, seaside, and other health resorts is attributable to the breezes that blow almost continuously at such places. The health of large cities located upon the seacoast or the shores of great lakes is favored by the quantities of moving air with which they are frequently flushed. A healthful climate is always a breezy climate -within reasonable limits. Much of the benefit of driving, of fanning, and of rocking-chairs is due to the motion of the air thus engendered.

If the air in a poorly ventilated room can be kept in motion, say with an electric fain, many of the ill effects of a vitiated atmosphere are avoided, for the products of respiration are diluted, and evaporation and heat interchange are favored. Thus, Leonard Hill placed eight students in a small sealed chamber which held about three cubic meters. He states that "at the end of half an hour they had ceased laughing and joking and their faces were congested. The carbon dioxid had gone up to 4 or 5 per cent. Three electric fans were then turned on, which merely whirled the air about just as it was. The effect was like magic; the students at once felt perfectly comfortable, but immediately the fans were stopped they again felt as bad as before." The relation of moving air to temperature and moisture, with reference to ventilation, is further discussed on page 647.

In nature the atmosphere is kept in almost constant motion as a result of differences in temperature. Thus, the hotter air in the tropics rises and divides into two currents, which flow toward the north and south, while heavier, colder air rushes along a lower level from the north and south to take the place of the lighter currents. The cold