Case Study Challenges in Human Health Hazard and Risk Assessment of Nanoscale Silver - Safety of Nanomaterials along Their Lifecycle - page 427

there is no direct evidence that such a mechanism is also causing adverse effects in
humans, the hypothesis itself was translated to humans for theoretical purposes by
considering their larger macrophage volumes (Oberdörster 1995). In the NanoGEM
project, the level at which 6% of the macrophage volume is occupied was considered
as a theoretical lowest observed effect dose (t-LOEL). This LOEL was converted
into a theoretical no observed adverse effect level (t-NOAEL) using two factors: a
factor of 10 for the intraspecies variability and another factor of 10 for extrapolation
from LOEL to NOAEL (which is considered very conservative). This calculation
leads to a theoretical NOAEL rat = 0.6 µm 3 loading with inert biopersistent material
per alveolar macrophage. Using data on the diameter of the particles and the density
of the material, the number of particles occupying the t-NOAEL rat volume can be
calculated. The density can be adjusted to account for spaces between individual
particles. In a worst case scenario that would have those spaces with no density, the
adjusted density equals the difference in volume between a solid particle and the
same weight of any size of smaller particles r Vcube / Vsphere = 1 / 6 π r material . Assuming a
particle diameter of 18 nm, 3.3 pg of silver nanoparticles (corresponding to 1 × 10 5
particles) would occupy a volume of t-NOAEL rat = 0.6 µm 3 using the adjusted den-
sity. Hence, disregarding any specific properties of silver nanoparticles an exposure
to 1 × 10 5 18 nm-nanosilver particles per rat alveolar macrophage should remain
without adverse effects according to this concept. Data on the loading of individual
macrophages can be modeled using the algorithms included in the multiple-path
particle dosimetry software (MPPD), which includes nanoparticle specific modeling
in version 2.11. This software may therefore be used to compare the t-NOAEL of
0.6 µm 3 (or 1 × 10 5 ) nanosilver particles per rat macrophage to the results of inha-
lation toxicity studies: In a subchronic inhalation toxicity study on rats, a dose of
0.133 mg/m 3 18 nm silver nanoparticle aerosol with a geometric standard deviation
of 1.5 remained without adverse effect in the lung and was identified as no observed
adverse effect concentration (NOAEC), whereas indications for alveolar inflamma-
tory and lung function changes were observed at 0.515 mg/m³, identified as lowest
observed adverse effect concentration (LOAEC) (Sung et al. 2009). For these doses,
MPPD calculates a mass deposition of such particles per macrophage per breath of
2.7 and 7.8 attograms, corresponding to a number of particles per macrophage of
0.084 and 0.21 per breath, or 3102 and 7601 particles per macrophage per 6-hour
exposure day, respectively. Thus, the cumulative doses at the experimental NOAEC
and LOAEC would reach the theoretical NOAEL (t-NOAEL) of 1 × 10 5 particles
per macrophage on day 3 of the seventh of 13 weeks or already on day 4 of the third
week of exposure, respectively. However, this calculation did not adjust for particle
clearance from the lung and macrophage cell turnover. When clearance is taken into
account by using the default parameters for clearance of MPPD software, the cumu-
lative dose at the experimental NOAEC would reach the t-NOAEL just before termi-
nation of the study on day 4 of week 12. In contrast, the same loading is predicted for
day 2 of week 4 at the experimental LOAEC. Overall, there was a surprising level
of agreement between the theoretical approach based on macrophage loading and
the experimental data even though aspects like dissolution and specific toxicity of
(nano)silver in the lung were not accounted for, warranting further development and
validation of this model.