Environmental Engineering Reference
In-Depth Information
Similar studies (Ruzer and Harley, 2004) were conducted with miners who during their normal
work activities were exposed to much lower concentrations of radon, much higher gamma-
background, and simple instrumentation that quantiied post-exposure gamma emissions from the
chest of the subjects.
16.2.2 c
Haracteristics
oF
r
adon
P
rogeny
The presence of radon and its decay products in the air is due to the abundance of unstable heavy
metals (radioactive elements at the end of the periodic table) in the earth. One of them, uranium,
undergoes a long series of transformations to yield radium. The chain of radioactive decay contin-
ues beyond radium to generate radon, a radioactive noble gas. Due to its inert chemical properties,
radon does not bind completely to suricial soils in the earth or stay in water but enters the atmo-
sphere as a gas.
The elements following radon in the radioactive decay chain—isotopes of polonium, bismuth
and lead, atom-sized radionuclides—may attach to aerosol particles to become radioactive aerosols
or exist in unattached forms in the air. Eventually, they may be deposited in the lung and cause irra-
diation of the lung tissue. The speciic biological consequences depend upon the dose of radioactive
aerosols which in turn depends on physiological characteristics including human breathing rates,
especially changes with physical activity as well as the amount of the radium in the soil and radon
and its decay products in the air, and atmospheric conditions both in the open air, dwellings and the
underground environment.
The decay products represent a very complicated system consisting of a series of radioac-
tive elements and various types of decay (alpha, beta, and gamma). In terms of the radiation
safety, the most important radionuclides are alpha-emitters because the alpha particles have the
greatest ionization density (Linear Energy Transfer [LET]). Given identical absorbed energy,
the biological effect of alpha-particles is thought to be 20 times greater than the corresponding
effect of beta-particles and gamma-radiation (i.e., the “quality coeficient” for alpha particles is
20). However, due to low particle penetration through human tissue, it is impossible to externally
measure the alpha-activity of aerosols deposited in the lung of a living subject. As a result, this
alpha-radioactivity is typically measured in the air and the absorbed dose to the lungs is then
calculated according to the known concentration, breathing rate, and coeficient of deposition in
the lungs (which is not accurate).
16.2.3 a
ssessMent
oF
P
article
d
ePosition
in
l
ungs
Ruzer (1964a,b) and Ruzer and Harley (2004) presented another, more precise, opportunity for
assessment of the alpha-dose to the lung from radon progeny. It was based on the derived correla-
tion between the alpha-dose and gamma-activity of radon progeny measured directly from the lung.
This possibility was studied irst on animals, then in model experiments, and inally, after certiica-
tion from the Soviet Ministry of Health, on hundreds of miners in the former Soviet Republics of
Tajikistan, Uzbekistan, and Kazakhstan. It was shown that for concentrations in the range of maxi-
mum permissible in mines, gamma-activity of radon progeny in the lung can be measured directly
by means of a simple technique, such as the use of NaI (Tl) crystal detectors with standard lead
shielding.
This approach of direct measurement of the natural marker such as radon progeny can also be
used for the assessment of deposition of nonradioactive aerosols, particularly nanometer aerosols,
in the lung.
The formula for gamma-activity Aγ is
A
γ =
a v k q
(
+
q
)
(16.1)
b
c
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