Distribution and deposition of inhaled particles in respiratory tract.

Spread of viral and bacterial diseases by way of the atmosphere requires, among other things, that infectious particles be inhaled by susceptible individuals and deposited at effective sites within the respiratory system for the initiation of disease. The critical nature of the site undoubtedly varies from one disease to another. In some cases it may be enough for particles to be deposited on surfaces of the upper respiratory tract; for others the essential point of deposition may be in the deep pulmonary spaces, which are reached only by very small particles. It is known that the depth to which inhaled particles penetrate before deposition and the percentage of particles removed from the air at various sites within the respiratory system vary with aerodynamic particle size and with breathing pattern. Hence, systematic knowledge of the respiratory retention of particulate substances, in relation to size and other factors, is necessary to a full understanding of the problems of airborne infection. This point was given emphasis in the American Association for the Advancement of Science Symposium on Aerobiology in 1941 (4) but much of the discussion was necessarily speculative. Since then, respiratory deposition of particulate matter has been studied extensively using human subjects as well as animals. Measured values in man and animals have agreed very well with theoretical predictions based on the physical laws that govern the removal of fine particles from still and moving air. Agreement has been shown also between mineral particles and living organisms when these are described according to a common scale of aerodynamic particle size (which recognizes density and shape as well as actual dimension of the particle) and, in the case of bacterial and other hygroscopic particles, proper allowance is made for the growth of the particle by water absorption from the moist air in the respiratory system. The findings are briefly summarized in the following paragraphs with useful distinction being made between total, upper respiratory, and deep-lung deposition. A more extensive review of the subject recently prepared by the author (5) includes references to some major studies in this area. Excellent reviews on both the initial deposition of inhaled particles and their subsequent clearance from the lungs are found in references (2) and (6). Total retention is essentially 100% for particles 10 lO' in diameter and larger. Retention remains high down to 5 js and then drops off (see Fig. 1), reaching a minimum of 20 to 30% at y to Y2 tt, below which it increases again, returning to 60% or better for submicroscopic particles ( <0.1 it). The reasons for these changes with size will be made clear from more detailed examination of regional dust-trapping within the respiratory system. The total respiratory deposition increases as breathing frequency goes down for two reasons: the transit time of air (and particles) into and out of the system is increased, thus providing a longer time for particles to settle out; and a larger fraction of inspired air reaches the lung depths (where efficiency of particle removal is highest) as the tidal volume increases with decreasing breathing frequency. Upper respiratory retention. Total respiratory retention is the sum of the separate fractions of particles deposited at different sites within the system. As a minimum, we may distinguish between two significantly different zones of deposition: along the upper respiratory tract, down to and including the terminal bronchioles; and within the pulmonary lobule. The dust-trapping characteristics of these must be examined separately. Upper respiratory removal is essentially 100% for particles 10 ,u and larger and approximately 80% at 5 A. It drops progressively with further reduction in size and approaches zero at 1 to 2 1