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The increasing importance of aerosols in industry, in the treatment of pulmonary conditions and more recently in the diagnosis and experimental study of bronchial asthma has intensified the need for a method permitting measurement of the amount of aerosol retained in the human lung. This problem has been approached in various ways. Landahl and Hermann (1) measured the difference in concentration of aerosol in the inspired and expired air. They showed that the relative amount of aerosol removed is markedly influenced by the size and physical properties of the particles and also by the rate and depth of respiration. Using aerosols containing radioactive sodium, Talbot, Quimby, and Barach (2) estimated the quantity of aerosol retained by using a Geiger counter over the axilla and the foot. Wilson and LaMer (3) extended this method to include the use of aerosol droplets of uniform size. They determined the total amount of radioactive sodium retained by the subject by measuring the difference in the amount delivered to the subject and the amount in the expired air. The quantity absorbed was estimated from the concentration of radioactive sodium in the blood. The use of blood concentrations as an index of the amount absorbed from the lungs requires assumptions regarding the distribution of the substance in the body. By assuming that the concentration of radioactive sodium in the interstitial fluid was 25 per cent of that found in the blood 15 minutes after exposure, Wilson and LaMer estimated that between 31 and 63 per cent of an inhaled aerosol composed of particles 0.7 microns in diameter was retained in the alveoli. In these experiments, 71 to 80 per cent of the administered aerosol was removed by the subject, the difference presumably being trapped in the upper respiratory tract. More direct methods for measuring the amount of material retained in the lung have not been described. In the experiments to be described, we determined the pulmonary retention of inhaled aerosols by measuring the difference between the amount in the inspired and expired air and also by measuring the amount absorbed. By using two methods simultaneously, it was hoped that possible errors in measurement would be more easily detected and the validity of both methods could be determined.

THE PULMONARY RETENTION OF AEROSOLS: A QUANTITATIVE METHOD OF MEASUREMENT USING SODIUM PARA-AMINO HIPPURATE