Aerosol Dose - Aerosols: Measurement, Dosimetry, and Health Effects - page 77

Regardless of the environmental conditions, in order to accurately determine the burden of aero-
sol exposure on an individual, dose must be measured or estimated. Typically, surrogates for actual
dose such as aerosol concentration or exposure (time-weighted average concentration* duration of
exposure) of particular aerosol species, or lung tissue dose estimates using inhalation models are
used, since true dose measurements are usually not possible.
The use of any of these quantities in either epidemiological studies or control involves, explicitly
or implicitly, assumptions about their relationships with one another. However, although it is the
dose that is the cause of the biological effect, it is typically the airborne concentration that is mea-
sured, and it is the corresponding estimates of exposure that are used as correlates to health effects
or predictors of risk.
It is the dose or intake to lung tissue that is studied to understand the actual insult leading to lung
cancer or other diseases. If exposure is used as an index of dose, for example, in epidemiological
studies, assumptions are made, typically implicitly, about a comparability or constancy of breathing
rates, and of retention of aerosols in the lung.
It should be noted that, especially in industrial environments, the inhalation rates depend on
the groups involved and on working conditions, speciically the physical load, which can vary sub-
stantially. Retention depends on the properties of the aerosols and also on the load of work and is
typically not known accurately.
In Ruzer (2001) and Ruzer et al. (1995), a correlation was established between measured aerosol
concentration in the breathing zone and gamma activity (dose) in the lungs of miners. The transition
coeficient called iltration ability of lungs (FAL) was deined and measured. FAL is a combination
of a breathing rate and deposition coeficient. It was shown that FAL depends on physical activity
and is different for different groups of miners.
The measured concentration is not complete enough to directly provide accurate exposure char-
acterization, because the concentrations to which individuals are exposed vary substantially in time
and space. For example, a variation of the ventilation rate even for a short period of time can lead
to a substantial change in concentrations. Moreover, the concentrations and especially the particle
size distribution of aerosols in the breathing zone (which is directly responsible for local deposi-
tion) may differ substantially from those measured by standard instruments (Domanski et al., 1989;
Scherwood and Greenhalgh, 1960; Schulte, 1967).
Finally, in the occupation setting, the very concept of “workplace”-associated concentration and
workload are indeinite and job dependent, because workers are typically at a number of places
during their working shift, with variable concentrations and nature of work. A similar situation
exists with indoor exposure. Since the measurements of concentration in many environments may
be performed only infrequently or even constructed, we cannot expect reliable correspondence
between exposures estimated from spatially and temporally spare measurements and actual per-
sonal exposures. Thus, the use of measurements of airborne concentration as a basis for estimating
or comparing dose can lead to substantial errors. In aggregate, these errors may constitute as much
as an order of magnitude or more and, therefore, make risk assessment data unreliable.
Unfortunately, even after the use of measured concentration for radioactive and nonradioactive
aerosols was established as a measure of the dose to the lung more than 40 years ago, little discus-
sion and experimental study were conducted to determine the degree of correlation between the
measured concentration and actual dose for individuals, both workers and for the general popu-
lation. Experimental data on the correlation between the measured concentrations according to
standard procedures (site measurement) and breathing zone measurements, not to mention dose, are
very scarce. The only systematic study for radioactive aerosols provided in Polish mines (Domanski
et al., 1989) showed poor correlation between standard area measurement and breathing zone
sampling.
Using direct measurement of naturally occurring radioactive radon decay products in the lungs
of miners, it was shown that the ratio of the true dose and that based on measured concentration
could be large: 8 (Ruzer et al., 2004).