Aerosol Dose - Aerosols: Measurement, Dosimetry, and Health Effects - page 79

According to the Environment Protection Agency of the United States,
In epidemiological studies, an index of exposure from personal or stationary monitors of selected pol-
lutants is analyzed for associations with health outcomes, such as morbidity or mortality. However,
it is a basic tenet of toxicology that the dose delivered to the target site, not the external exposure,
is the proximal cause of a response. Therefore, there is increased emphasis on understanding the
exposure-dose-response relationship. Exposure is what gets measured in the typical study and what
gets regulated; dose is the causative factor.
Despite the fact that the importance of dosimetry is well understood, some uncertainties in inter-
pretations exist even in terms of aerosol dose.
4.4 UNCERTAINTY IN THE DOSE ASSESSMENT
It has been shown (Ruzer et al., 1995; Ruzer, 2001) that in the case of natural radioactive aerosols,
that is, the decay products of radon (in mines), it is possible to measure dose due to alpha radiation
directly by measuring gamma activity from the lungs.
Such opportunities do not exist for nonradioactive aerosols; hence, the main method for nonra-
dioactive aerosol dose assessment is through modeling. It should be noted, however, that calculated
dose is very sensitive to input parameters (Phalen et al., 1990); in other words, the dose assessment
based only on modeling can be unreliable. One of the main sources of uncertainty in lung dosimetry
is in the assessment of particle size distribution in the breathing zone, the location where contact
between aerosols and the organism occurs.
Reliable determination of the dose includes the measurement of aerosol particle size distribution
(concentration) in the breathing zone, a knowledge of the parameters of transformation of particle
size distribution due to humidity, temperature, and other factors inside the respiratory tract, calcu-
lating or measuring directly (e.g., by using radioactive markers) aerosol deposition, and, inally, a
knowledge or direct measurement of parameters of biochemical processes inside the lungs: translo-
cation, clearance, absorption, etc.
Most current information on the clearance of aerosol from the respiratory tract comes from
radioactive aerosol inhalation studies; much less is known about respiratory biokinetics of inhaled
nonradioactive particles.
In terms of biokinetics, the main difference between radioactive and nonradioactive aerosols is
that mass concentrations of radioactive aerosols are typically low in comparison with nonradioac-
tive aerosols, which results in differences in clearance and translocation (EPA, 1996).
4.5 AEROSOL CONCENTRATION STANDARDS
In 1997, the EPA established a new standard for particulate matter as an indicator of mass con-
centration: particle mass with aerodynamic diameters of 10 and 2.5 μm or less (PM10 and PM 2.5,
respectively).
In the light of the theoretical and experimental results of the last 10 years, it has become clear
that quantifying aerosol mass deliveries to the lung is not always adequate for biological effects.
One such very important example was mentioned in Valberg and Watson (1998). According to the
EPA (1996), for a mass concentration of 50 μg/m 3 of ambient particulate aerosol, the daily deposi-
tion in the alveolar region was close to 50 μg.
It is easy to calculate that such a mass will produce, at the alveolar surface, only 1 particle per
day per 1.5 mm 2 and 1 particle per 44 mm 2 for 1 and 3 nm particles, respectively. Such amounts will
cover less than a millionth part of the lung surface. No known chemical constituents of ambient
particulate matter have such a threshold of toxicity at this daily level.