DEPOSITION OF ULTRAFINE AEROSOLS AND THORON PROGENY IN REPLICAS OF NASAL AIRWAYS OF YOUNG-CHILDREN

The deposition efficiencies of ultrafine aerosols and thoron progeny w ere measured in youth nasal replicas. Clear polyester-resin casts of t he 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 se ries of coronal magnetic resonance images of healthy children. The cas ts extended from the nostril tip to the junction of the nasopharynx an d pharynx. These casts were similar in construction to those used in p revious studies (Swift et al. 1992; Cheng et al. 1993). Total depositi on was measured for monodisperse NaCl or Ag aerosols between 0.0046 an d 0.20 mu m in diameter at inspiratory and expiratory flow rates of 3, 7, and 16 L min(-1) (covering a near-normal range of breathing rates for children of different ages). Deposition efficiency decreased with increasing particle size and how rate, indicating that diffusion was t he main deposition mechanism. Deposition efficiency also decreased wit h increasing age at a given flow rate and particle size. At 16 L min(- 1), the inspiratory deposition efficiencies in Cast G were 33% and 6% for 0.008- and 0.03-mu m particles, respectively. Nasal deposition of thoron progeny with a mean diameter of 0.0013 mu m was substantially h igher (80%-93%) than those of the ultrafine aerosol particles, but sti ll 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 (D in cm(2) s(-1)) and flow r ate (Q in L min(-1)) 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(-aD(0.5)Q(-0.125)). We further d emonstrated that the effect of age, including changes to nasal airway size and breathing how rate, on nasal deposition can be expressed in t he parameter ''a'' of the fitted equation. Based on this information a nd information on minute volumes for different age groups, we predicte d nasal deposition in age groups ranging from 1.5- to 20-yr-old at res ting breathing rates. Our results showed that the nasal deposition inc reases with decreasing age for a given particle size between 0.001 to 0.2 mu m. This information will be useful in deriving future populatio n-wide models of respiratory tract dosimetry.