In support of predictions for inhaled particle deposition, morphometric measurements were taken on 20 replica airway casts of people aged 11 days to 21 years. Measurements of right upper lobe airway lengths, diameters, and branching angles were made such that a growth model suitable as input to predictive equations for particle deposition efficiency was obtained. The tracheobronchial airways growth was describable by linear regressions on body length. The length-todiameter ratio of growing airways did not change in any simple way as a function of airway generation. Airflow rates for a given state of physical activity for various ages were found from previously published data to be describable by linear regressions on body mass. Three states of physical exertion-low activity, light exertion, and heavy exertion-were used for modeling purposes. The computed particle deposition efficiencies indicate that under most circumstances smaller (younger) people will have greater tracheobronchial deposition efficiencies than larger (older) people. For example, tracheobronchial dose on a per kilogram body mass basis for 5-pmdiameter particles may be more than 6 times higher in the resting newborn than in the resting adult assuming equivalent deposition efficiencies above the larynx.
Because little is known regarding age-related differ-( 1913) published values for lengths and diameters (sagences in inhaled particle deposition, we have been inter-ittal and frontal) of the trachea and right and left main ested in developing mathematical predictions for particle bronchi for 12 children aged 1 month to 13 years, and a deposition in the tracheobronchial airways correspond-40-year-old adult. Engel's data show relatively rapid but ing to a range of ages. Such predictions are strongly decelerating growth of the measured structures up to dependent on airway anatomy, so measurements of the about age 7 years, but with only one child over age 10 growing airways were taken to develop a model upon years little can be said about growth after age 7. Also, which to base the mathematical predictions for depo-only two generations of airways were well described. sition.
Although there is agreement on the sequence of events Although several investigators have provided quanti-during postnatal lung growth, many fundamental questative descriptions of the dimensions of adult human tions remain. There is general agreement (although perairways, few have described the manner in which these haps a lack of direct proof) that the full number of structures grow. Among the earliest morphometric bronchial airways are present at birth and that alveolarmodels for the complete adult tracheobronchial airways ization of bronchioles continues postnatally in a proxiwere those published by Weibel(1963). To generate these mal direction. Bronchial airway growth generally models, measurements from the trachea downward for parallels changes in stature, but there is confusion reabout 10 generations were made on a plastic replica cast garding details. This state of understanding is described prepared by Liebow from the excised lungs of an adult in a review by Thurlbeck (1977), who summarizes in the male. Formalin-fixed slices from a lung fixed in the following manner. inflated state were measured to obtain values for smaller "Airways. Because airways do not increase in number, airways, and intermediate sizes were estimated by inter-they must increase in dimension. The increase in dimenpolation. Subsequent descriptions of the adult airways sion parallels the increase in stature, as shown by the were published by Horsfield and Cumming (1967) and measurement of anatomical dead space in children (Wood Horsfield et al. (1971), who used a resin cast made from et al., 1971). Tracheal diameter increases directly with the excised lung of a "young man," and by Yeh and the increase in chest circumference (Hieronymi, 1961). Schum (19801, who used silicone rubber casts prepared However, there are discrepant views about the relative in situ in a 50and a 60-year-old man.
rate of growth of central and peripheral airways. This is Limited information on bronchial dimensions in chil-of more than trivial interest-it has been shown that the dren has been published, but it is neither adequately documented nor complete enough to establish growth patterns for the complete tracheobronchial tree. Engel