Page 361 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

328 SECTION | III Nanoparticles, Radiation and Carcinogens

VetBooks.ir MEASURING IONIZING RADIATION transportation accidents involving associated materials
can result in harmful impacts on humans, animals, and
The number of radioactive disintegrations per second
the environment.
(radioactive decay) is measured in becquerels (Bq) or
curies (Ci). Curies compares the number of disintegrations
per second to that of roughly 1 g of Radium-226 (Rella, Natural Sources
2015). One Bq is equal to one atomic disintegration per
Among the human population of the United States,
second.
approximately 52% of natural background radiation expo-
1Ci 5 3:7 3 10 10 Bq sures are due to radon, 16% from cosmic radiation, 20%
from terrestrial sources, and 12% from internal exposures
Radiation that simply passes through the body causes (National Research Council, 2006). These natural expo-
no harm. Biological injury results from the deposition of sures account for 82% of annual human radiation expo-
radioactive energy in a tissue or organ, quantified as the sure. Radon (Rn) is a natural decay product of uranium
absorbed dose. Linear energy transfer (LET) for a charged and thorium. Daughter isotopes of 222 Rn decay to emit
particle refers to its average energy lost due to interac- alpha particles and are associated with increased inci-
tions per unit length of its trajectory (Hoel, 2013). X-rays, dences of lung cancer in humans (Rella, 2015).
gamma rays, and beta particles are classified as low-LET
radiation because they produce sparse ionization tracks.
Neutrons and alpha particles dissipate large amounts of Nuclear Weapons
energy over very short distances. Two atomic bomb blasts at the end of World War II in
Calculating an effective dose, using a relative biologic Japan were estimated to have killed or injured more the
effectiveness (RBE) multiplying factor, makes it possible to 200,000 people. Most people died as a result of the bomb
compare biologic effects from different types of radiation blast, though many others eventually succumbed to either
exposures. RBE is defined as the biological potency of one
acute radiation syndrome (ARS) or radiation-induced
radiation exposure compared with another to produce the
cancers.
same biological endpoint (National Research Council,
In the United States, radioactivity released into the
1990). RBE varies with the type and energy of the radia-
Columbia River by the nuclear weapons plant in Hanford,
tion, the selected biologic endpoint, and with dose and dose
Washington resulted in radioisotope contamination of
rate (Nussbaum and Ko ¨hnlein, 2003; Hoel, 2013).
fish and shellfish hundreds of miles downriver to levels
Effective dose 5 RBE 3 Absorbed dose greater than 100,000 times what was detectable in the
river water (Stenehjem, 1990). Radioactive waste is still
The units for effective dose are Sieverts (Sv) or rems.
stored at Hanford. Global fallout from atmospheric
1Sv 5 100 rem nuclear weapons testing carried out primarily in the 1950s
and early 1960s is the main source of anthropogenic
Absorbed dose is expressed as grays (Gy) or radiation radionuclides, including Cesium 137 ( 137 Cs), in the
absorbed dose (rads). environment (Povinec et al., 2013).
1Gy 5 100 rad
Nuclear Power
One gray is defined as one joule of energy released in
one kilogram of tissue. Uranium (U) ore as mined is approximately 99%
Gas-filled tubes are used in ionization survey meters and 238 U and 1% 235 U(Rella, 2015). Uranium enrichment
Geiger-Mueller survey meters to accurately detect gamma separates these so the 235 U can be used as nuclear fuel.
radiation associated with nuclear incidents (Berger et al., The leftover 238 U, also known as depleted uranium (DU),
1987). A typical Geiger-Mueller survey meter reads dose is approximately 40% less radioactive than natural ura-
rate as mrem per hour (mR/h) with a range switch that indi- nium ore and can be used in munitions. A nuclear power
cates how to multiply the dial reading. For example, when plant (NPP) generating 1000 MW of electricity requires
the needle is pointing to 10 and the range switch is set at about 30 tons of enriched uranium fuel rods each year
X100, the dose rate is 1000 mrem (or 10 Sv) per hour. (Nussbaum and Ko ¨hnlein, 2003). The resulting waste
must be isolated from people and water supplies. Due to
high levels of heat output, these spent fuel rods are stored,
SOURCES OF RADIATION
at least temporarily, in on-site storage pools, potentially
While radiation is ubiquitous, some natural exposures and vulnerable to natural disasters such as earthquakes.
accidental or intentional exposures related to incidents The two most notable NPP incidents in recent history
involving nuclear or medical uses of radiation and are the 1986 Chernobyl NPP accident that released large