Deposition of Ultrafine Aerosols and Thoron Progeny in Replicas of Nasal Airways of Young Children

The deposition efficiencies of ultrafine aerosols and thoron progeny were measured in youth nasal replicas. Clear polyester-resin casts of the upper airways of 1.5-yr-old (Cast G), 2.5-yr-old (Cast H), and 4-yr-old (Cast I) children were used. These casts were constructed from series of coronal magnetic resonance images of healthy children. The casts extended from the nostril tip to the junction of the nasopharynx and pharynx. These casts were similar in construction to those used in previous studies (Swift et al. 1992; Cheng et al. 1993). Total deposition was measured for monodisperse NaCI or Ag aerosols between 0.0046 and 0.20 !-lm in diameter at inspiratory and expiratory flow rates of 3, 7, and 16 L min -I (covering a near-normal range of breathing rates for children of different ages). Deposition efficiency decreased with increasing particle size and flow rate, indicating that diffusion was the main deposition mechanism. Deposition efficiency also decreased with increasing age at a given flow rate and particle size. At 16 L min -I , the inspiratory deposition efficiencies in Cast G were 33% and 6% for 0.008- and 0.03-!-lm particles, respectively. Nasal deposition of thoron progeny with a mean diameter of 0.0013 !-lm was substantially higher (80%-93%) than those of the ultrafine aerosol particles, but still had a similar flow dependence. Both the aerosol and thoron progeny data were used to establish a theoretical equation relating deposition efficiency to the diffusion coefficient (0 in cm2 S-I) and flow rate (Q in L min-I) based on a turbulent diffusion process. Data from all casts can be expressed in a single equation previously developed from an adult nasal cast: E = 1 - exp( _aDO•5Q -0.125). We further demonstrated that the effect of age, including changes to nasal airway size and breathing flow rate, on nasal deposition can be expressed in the parameter "a" of the fitted equation. Based on this information and information on minute volumes for different age groups, we predicted nasal deposition in age groups ranging from 1.5- to 20-yr-old at resting breathing rates. Our results showed that the nasal deposition increases with decreasing age for a given particle size between 0.001 to 0.2 !-lm. This information will be useful in deriving future population-wide mod· els of respiratory tract dosimetry.