Environmental Engineering Reference
In-Depth Information
The term “exposure” as “the amount (c) present in one cubic meter of air expressed in milligrams
and multiplied by the time (t) in minutes” came to the aerosol ield from the study of the effect of
gases as a “Haber Law.” (Ruzer, Apte, Sextro; in Chapter 7 of Ruzer and Harley [2004].)
Strictly speaking, it was incorrect to simply transfer the term “exposure” from gases to aerosols.
The deinition of exposure in the case of aerosols is much more complicated than that for gases. The
main difference in dosimetry between gases and aerosols is that aerosol distribution is uneven both
spatially and temporally, and deposition of aerosol in the lungs is not uniform.
According to the National Academy of Science report (NAS, 1991), 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.
From our point of view this deinition is not free from some contradiction. If exposure is an
event, it cannot be expressed in physical units.
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,
and
2. the person is present at the same time and location in space.
A later deinition (Duan and Ott, 1989) 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 the Guidelines for Exposure Assessment (Zartarian et al., 2005) a slightly different deinition
was proposed:
Exposure - Contact of a chemical, physical, or biological agent with outer boundary of an organism.
Exposure is quantiied as the concentration of the agent in the medium in contact integrated over the
time duration of that contact.
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
concentration (and particle size distribution) is provided at a distance from the breathing zone, the
correlation should be established between breathing zone and sampling site measurement.
The risk has to be deined in quantitative terms as a ratio of the number of cases of mortality,
morbidity, attendance of hospitals, etc., divided by the cause of the effect, that is, dose.
Based upon this discussion, we can formulate the deinition of dose as a physical value respon-
sible for a biological effect (Ruzer and Harley, 2004):
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, quantity is the deposited energy per unit mass for alpha,
beta, or gamma radiation which 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, sur-
face area, or mass of a discrete particle size.
3. The term “directly” means that dose is a quantity of the deposited amount of aerosol
particles after the completion of all biokinetic processes.
Search WWH ::
Custom Search