Aerosol Dose - Aerosols: Measurement, Dosimetry, and Health Effects - page 78

4.3 EXPOSURE AND DOSE DEFINITION
The term “exposure” came to the aerosol ield from the study of the effect of gases. Aerosol science,
including epidemiological studies of the health effect of aerosol exposure, was developed mainly in
the second half of the twentieth century.
In 1924, German chemist Fritz Haber proposed the following deinition of exposure:
A simple and practical measure for toxicity can be obtained that sufices for all practical purposes. For
each war gas, the amount (c) present in one cubic meter of air expressed in milligrams and multiplied by
the time (t) in minutes necessary for the experimental animal inhaling this air to obtain a lethal effect.
The bigger this products (c*t), the greater is the toxicity of the war gas.
This was the original formulation of “Haber's law.” The product of concentration and time
was simply a measure of acute lethality in cats, nothing else. To expect a constant response to the
same (c*t) product had been postulated in 1921 by experimental toxicologist and biochemist Flury.
Haber's law did not take into account the difference between acute and chronic exposure.
The deinition of exposure in the case of aerosols is much more complicated. The main differ-
ence in dosimetry between gases and aerosols is that aerosol distribution, spatially and temporally,
and deposited aerosols in the lungs are very nonuniform. The reason for this is that respirable
fractions of aerosols consist of particles with diameters ranging from nanometers to 5-10 μm.
Therefore, their particle size distribution and their deposition inside the lungs are very uneven.
According to the 1991 National Academy of Science report, the deinition of exposure is as follows:
An event that occurs when there is contact at a boundary between humans and the environment
with a contaminant of a speciic concentration for an interval of time; the units are concentration
multiplied by time.
A statistical deinition of exposure has been proposed (Ott, 1966):
An exposure at some instant of time is a joint occurrence of two events:
1. The pollutant of concentration C is present at a particular location in space at a particular time.
2. The person is present at the same time and location in space.
A later deinition (Duan and Ott, 1990) addresses the notion that the target remains important, and
also that different parts of the target can receive different exposures at the same time.
The last deinition is more adequate for aerosols, because it takes into account the speciic aero-
sol problem of nonuniformity.
In all these deinitions, the key word is contact, which means that in the case of aerosols only
breathing zone measurement should be used for the exposure and particle size measurement. If con-
centration (and particle size characterization) is provided at a distance from the breathing zone, the
correlation should be established between breathing zone and sampling site measurement.
Based upon this discussion, we can formulate the deinition of dose as a physical value respon-
sible for a biological effect:
Dose is the speciic quantity of aerosols delivered to a target site that is directly responsible for
a biological effect.
The term “quantity” is deined as follows:
1. In the case of radioactive aerosols, deposited energy per unit mass for alpha, beta, or
gamma radiation is expressed in units of J/kg (Gray) or rads (100 erg/g) or the equivalent.
2. In the case of nonradioactive aerosols, quantity is the deposited number of particles,
surface area, or mass of a discrete particle size.
3. The term “directly” means that dose is a quantity of the deposited amount of aerosol par-
ticles after the completion of all biokinetic processes.