Aerosol Dose - Aerosols: Measurement, Dosimetry, and Health Effects - page 80

However, the corresponding numbers of deposited particles per mm 2 and part of the covered
surface of alveoli will be orders of magnitude higher for particles at the level of tens of nanome-
ters. For the same mass, the number of particles is inversely proportional to the cube of the diam-
eter. Therefore, in case of substantial nanometer particle concentration, even exposure to nontoxic
chemicals can result, as experiments on animals have shown, in acute biological effect. Thus,
in terms of dosimetry, the concentration of nanometer particles should be measured correctly.
It should be mentioned that the measurement of particles in this size presents some dificulties.
In Banse et al. (2001), it was shown that detection eficiency in this range of sizes decreases
dramatically.
4.6 NONUNIFORMITY OF DEPOSITION AND ULTRAFINE/
NANOMETER-SIZED PARTICLES
Many reasons can explain the discrepancy between dosimetric factors and health effects in the case
of aerosols, the irst being biological factors. However, from the point of view of dosimetry, two
main factors can be used for the explanation:
1. Nonuniformity in the distribution of aerosols both in breathing space and inside the lungs,
making the use of average numbers inappropriate
2. The special role of ultraine/nanometer aerosols in health effects
Ultraine particles contribute little mass to the ine fraction; however, they dominate particle surface
area and particle number concentration.
The special role of nanometer-sized, ultraine particles, and nonuniformity in aerosol dosimetry
was demonstrated by Báláshazy and Hoffman (2000). Present lung dosimetry models for radon
decay products are based on deposition eficiencies for straight cylindrical airways, which are
equivalent to the commonly accepted assumption that inhaled particles are uniformity deposited
in these airways.
Because aerosol deposition depends only on particle size and not on radioactivity, we can assume
that the same approach will be correct for nonactive aerosols. In the present models, depth-dose
distributions on bronchial epithelium are obtained by integrating the surface activities over the
surface of the cylindrical airways within the range of alpha particles, thereby assuming again that
alpha particle sources are uniformly distributed on airway surfaces. The assumption of uniformity
is further supported by the theory that Brownian motion in straight cylindrical tubes a priori
produces uniform deposition patterns.
In contrast, experimental studies about molecular-sized, ultraine, and submicron particle deposi-
tion in single-pathway tracheobronchial models, in airway casts of the human tracheobronchial tree,
and in single bronchial bifurcation models have demonstrated that diffusion-dominated particle
deposition patterns are highly nonuniform.
Experimental evidence exists to show that the main features of the deposition patterns within
airways exist at bifurcation points in the lung.
Research showed that
1. Deposition is enhanced at airway branching zones relative to cylindrical airway portions.
2. Deposition within a bifurcation is highest at the dividing spur.
3. Deposition is also enhanced at the inner sides of the daughter airways.
Computed enhancement factors indicate that the cells located at carinal ridges may receive local-
ized doses that are 20-40 times (1 nm) and 50-115 times higher (10-200 nm), respectively, than the
corresponding average doses.