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 (O_{3}) 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 (NH_{3}) 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