Basic Information on Radiation Exposure from Radioactive Contamination
1. Radiation, radioactivity, and radioactive contamination
Radiation exposure can cause disease. Radiation damages organs and tissue in humans,
sometimes resulting in disease. If radiation is emitted by particular substances (radioactivesubstances), they are referred to as being radioactive, and the situation in which radioactive
substances are not managed securely with potential human exposure is called radioactive
contamination.
2. Radiation exposure pathways from radioactive contamination
a. External exposure and internal exposure
There are two main pathways for human radiation exposure: external exposure and internalexposure. External exposure happens when radiation is emitted by radioactive substances(referred to as radiation sources) outside the body, and internal exposure occurs whenradiation is emitted from radiation sources taken into the body.
b. Radiation exposure pathways from nuclear power plant accidents
There are two or more pathways for radiation exposure in the case of radioactive
contamination caused by nuclear power plant accidents, as follows:(i) External exposure to radiation emitted by radioactive substances from nuclear powerplants
The level of this type of radiation exposure decreases as the distance between a person
and the radiation source increases (gamma-ray doses, for example, decrease inverselywith square of distance) and as the sojourn time decreases (doses decrease proportionallywith sojourn time). Exposure level is further reduced if shielding is present between theperson and the radioactive substance.
(ii) External exposure to radiation emitted by radioactive substances that are released
from nuclear power plants, carried by wind and rain, and affixed to skin, clothes, andsoil
The level of radiation exposure from radioactive substances carried far from nuclear
power plants is determined by the distance between a person and the radioactive
substance, the time the person spends close to the radioactive substance, and shielding
between the person and the radioactive substance (how the person is shielded from thesubstance).
(iii) Internal exposure to radiation emitted by radioactive substances that are released
from nuclear power plants and taken into the body through breathing, food and drink,
and wounds
This type of exposure continues for the duration of time such radioactive substances
remain in the body.

13. Radioisotope and radiation
An atom, the basic unit of matter, consists of an atomic nucleus and electrons. The atomic
weight is determined almost exclusively by the numbers of protons and neutrons, and the total
number of protons and neutrons combined is known as the atomic mass. The atomic nucleus
contains protons and neutrons. The same atoms (or elements) have the same number ofprotons (e.g., hydrogen has one, and helium two). However, there are numerous elements innature with different weights (or masses) that still are the same element. For example, themost common atomic mass of a carbon atom is 12, which is expressed as carbon-12 with themass number indicated after the element name, but the isotope used for archeological dating
estimates is a carbon isotope with the atomic mass of 14 (carbon-14). The difference
between the two isotopes is the number of neutrons in the atomic nucleus (the number ofprotons is six for both isotopes). Carbon-12 has six protons and six neutrons in its nucleus
and does not emit radiation, while carbon-14 has two extra neutrons and does. Strangely
enough, having too few or too many neutrons in a nucleus destabilizes the atom. Variants ofatoms of a particular element with differing mass numbers are called “isotopes” andradioactive isotopes are called “radioisotopes.”a. Nuclear decay
Radioisotopes have unstable nuclei, and with a certain probability, emit excess energy as
radiation, thus transforming to different elements. This process is called nuclear decay(disintegration). Three types of radiation are emitted from this process of nuclear decay -alpha, beta, and gamma rays -- and the type of radioisotope (radionuclide) determines what
type of radiation is emitted.
b. Half life of radionuclide
The time for half of a radionuclide’s atoms to decay is called the half-life (physical half-life)of a radioactive substance, and this timeframe differs among radionuclides. Another type ofhalf-life called biological half-life is the time required for a radionuclide taken into the bodyto lose half of its radiologic activity through such biological processes as metabolism andexcretion. The time it takes for a radioactive substance taken into the body to halve its
radioactivity as a result of the combined action of nuclear decay and biological elimination is
known as effective half-life.
c. Radionuclides released in nuclear power plant accidents
Whether from nuclear power generation or an atomic bomb, energy is generated from atomic
fission caused by the collision of neutrons in uranium or plutonium. Atomic fission (ornuclear fission) is, just as the term indicates, a phenomenon marked by an atomic nucleus
splitting into two or more parts. When uranium-235 undergoes nuclear fission in a nuclear
reactor, it produces such radioactive substances as iodine-131, cesium-137, strontium-90, andcobalt-60. Release of such radioactive substances into the environment in large quantities
due to a nuclear power plant accident could cause serious health effects from radiation
exposure to these substances.

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Iodine-131: Physical half-life of 8.02 days, meaning that it takes about eight days for 100
radioactive iodine-131 nuclei to be reduced to half that number (50 nuclei), and another eightdays to be further reduced to 25 nuclei. Iodine tends to bind to the thyroid. The
relationship between childhood exposure to radioactive iodine and increased risk of thyroid
cancer has been confirmed as a result of the Chernobyl nuclear power plant accident.
Cesium-137: Half-life of 30.07 years.

It is readily absorbed into muscle tissue.

Strontium-90: Half-life of 28.78 years.

It is readily absorbed into bone tissue.

Cobalt-60: Half-life of 5.27 years.
4. Measurement of radiation
Radiation is measured by many methods and expressed in different units, depending on thepurpose, as follows:a. Measurement of contamination: unit = counts per minute (cpm)
The number of ionizing events per minute (cpm) resulting from radiation emission ismeasured at the sensor screen of radiation measurement devices (e.g., Geiger-Mueller[GMsurvey] meters). This method, usually employed to evaluate contamination levels, isthe type of radiation measurement (contamination test) being conducted for residents living
near the Fukushima Daiichi nuclear power plants after the accident. Cpm readings by GM
survey meters mainly reflect beta rays, which do not penetrate deeply inside the body (tracesof gamma rays are also measured). Therefore, cpm readings themselves cannot be converted
to exposure doses. In measuring levels of radiation contamination in foodstuffs, instruments
much more complex than survey meters are used to count nuclear decay per kg (unit: decay
per minute, dpm, or decay per second, dps). Dps is equivalent to the unit becquerel (Bq).
With information on types of radionuclides, energy of radiation emitted can be determined,making it possible to convert readings to exposure doses.
b. Measurement of radiation energy absorbed by substances: unit = gray (Gy)The energy absorbed is called “absorbed dose” and 1 Gy means absorption of 1 joule ofenergy by a substance with a mass of 1 kg. Joule is a unit of energy, and 4.2 joules is
equivalent to 1 calorie, the amount of energy needed to increase the temperature of 1 gram ofwater by 1 °C. This means that absorption of (exposure to) a dose of 1 Gy is a minusculeamount of thermal energy barely enough to heat 1 kg of water by 0.0002 °C. However,radiation energy has the unique characteristic of damaging DNA and other substances at amolecular level imperceptible to humans.
c. Risks from radiation exposure (risk of developing cancer in the future): unit = sievert (Sv)There are two types of doses: equivalent dose and effective dose.
by a specific factor(s) for estimation of such risks.

Each dose is multiplied3

(i) Equivalent dose = absorbed dose  radiation weighting factor
Radiation weighting factors are used, in expressing exposure doses, to take into accountdifferent radiation effects and risks for the same energy absorption, depending on radiation
type, such as alpha, beta, and gamma rays. The factors for gamma ray, neutron, andalpha ray are 1, 10, and 20, respectively. This means that absorption of energy equivalent
to 1 joule is expressed differently for gamma rays and neutrons: 1 Sv and 10 Sv,respectively.
(ii) Effective dose = absorbed dose  radiation weighting factor  tissue weighting factor
Tissue weighting factors are used in expressing exposure doses to reflect different
radiation effects depending on the organ and tissue.
5. Radiation risks and radiation protection standards
a. Radiation risks as scientific findings
Radiation risks are scientific estimates based on epidemiological studies and related basic
research of radiation-exposed populations. Results of epidemiological studies and basic
research throughout the world are collected, assessed, and assembled as radiation risks by theUnited Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). This
organization’s 2000 report concluded that “The results of that research (i.e., the studies of theJapanese survivors) provide the primary basis for estimating the risk of radiation-inducedcancer” (para. 76), and that “Statistically significant risks for cancer in the Life Span Study
are seen at organ doses above about 100 mSv” (para. 78). The report also refers to studies ofthe victims of the Chernobyl nuclear power plant accident in terms of studies of internal
exposure and states that “an elevated risk of thyroid cancer ... as a result of the Chernobyl
accident shows a link with radioactive iodine exposure during childhood” (para. 81).
b. Radiation protection standards based on radiation risks
The International Commission on Radiological Protection (ICRP) recommends specificregulatory standards for radiation protection, such as maximum permissible dose, based onradiation risks scientifically assessed with UNSCEAR’s and ICRP’s standards, which take
into consideration societal demands, ethics, and past experience in application of standards.
ICRP’s recommendations are incorporated in the international Basic Safety Standards (BSS)of the International Atomic Energy Agency (IAEA) and in national regulations of various
countries.

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