IntroductionRadiation is energy that comes from a source and travels through some material or through space. Ionising radiation is produced by unstable atoms. Unstable atoms differ from stable atoms because they have an excess of energy or mass or both. Unstable atoms are said to be radioactive. In order to reach stability, these atoms give off, or emit, the excess energy or mass. These emissions are called radiation. When the radiation interacts with other atoms, it ionises the atoms altering their chemical properties, hence ionising radiation.
The kinds of radiation are electromagnetic (like light) and particulate (mass given off with the energy of motion). Gamma radiation and X-rays are examples of electromagnetic radiation. Beta and alpha radiation are examples of particulate radiation. Ionising radiation can also be produced by devices such as X-ray machines. There is also natural background radiation exposure. It comes from cosmic rays and from naturally occurring radioactive materials contained in the earth and in living things.
Ionising radiation comprises four basic types:
- Gamma rays and x-rays
- Beta particles
- Alpha particles
Gamma radiation and X-rays are electromagnetic radiation like visible light, radio waves, and ultraviolet light. These electromagnetic radiations differ only in the amount of energy they have. Gamma radiation is able to travel many metres in air and many centimetres in human tissue. It readily penetrates most materials and is sometimes called "penetrating radiation." Radioactive materials that emit gamma radiation and X-rays constitute both an external and internal hazard to humans. Dense materials are needed for shielding from gamma radiation. Clothing and turnout gear provide little shielding from penetrating radiation but will prevent contamination of the skin by radioactive materials.
Beta radiation consists of sub atomic particles (electrons) ejected from a radioactive atom. Beta radiation may travel metres in air and is moderately penetrating. Beta radiation can penetrate human skin to the "germinal layer," where new skin cells are produced. If beta-emitting contaminants are allowed to remain on the skin for a prolonged period of time, they may cause skin injury. Beta-emitting contaminants may be harmful if deposited internally and clothing and turnout gear provide some protection against most beta radiation. Turnout gear and dry clothing can keep beta emitters off of the skin.
Alpha radiation consists of specific particles ejected from some radioactive atom. Alpha particles are essentially helium nuclei. They have low penetrating power and short range. Alpha radiation is not able to penetrate skin, but they can be harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds. Alpha radiation is not able to penetrate turnout gear, clothing, or a cover on a probe.
Neutrons are uncharged sub atomic particles produced by the fission of radioactive atoms. Within tissue, neutrons predominantly lose energy in collisions with protons in the nuclei of hydrogen atoms, in body water. The interaction results in ionisation within the tissue atoms so irradiated. Except at lethal levels the neutron flux is not sufficiently high to cause the tissue to become radioactive.
Table 1.1 Summary of types of ionising radiation
||External and internal|
Radiation QuantitiesRadioactivity (and contamination by radioactive material) is measured in Becquerels (1 Bq = 1 disintegration per second). The absorbed dose of radiation (the amount of energy absorbed by per unit mass of tissue) is measured in gray (Gy), where 1 Gy = 1 joule/kg of tissue.
Different types of radiation have different effects on human tissue (gray for gray, alpha particles and neutrons are more damaging than beta particles, gamma rays or X-rays in terms of the risks of cancer or of heritable genetic defects), so the absorbed dose in tissue is multiplied by a radiation weighting factor to account for this. This gives the equivalent dose (to an organ or tissue), measured in sievert (Sv). For X-rays, gamma rays, and beta particles, the weighting factor = 1.
The amount of damage caused by exposure to radiation depends on the efficiency with which it transfers energy into body tissues. Radiation comprised of particles with relatively high mass delivers a greater proportion of their energy into tissues than do electromagnetic radiation, such as x-rays and gamma-rays, which may pass through the body. Doses of different types of radiation are, therefore, converted into ‘equivalent doses’ using a weighting factor for each kind of radiation.
Table 1.2 Weighting factors for ionising radiation
|Radiation||Energy transfer||Weighting factor|
|Neutron||High||5 - 20|
|Beta particle, electrons||Low||1|
|Gamma ray, x-ray||Low||1|
Reference: Recommendations for limiting exposure to ionizing radiation (1995) and national standard for limiting occupational exposure to ionizing radiation (republished 2002); Radiation Protection Series No. 1; ARPANSA.Equivalent doses are measured in Sieverts (Sv), which is equal to the absorbed dose in Grays multiplied by the weighting factor. A dose of 1/100 Gray delivered entirely as alpha particles would, for example, equal 20/100 Sieverts.
Tissues differ in their susceptibility to radiation for a given absorbed dose. Some organs are more radiosensitive than others (e.g. bone marrow is more sensitive than thyroid), and exposures are rarely uniform. Weighting the equivalent doses received by different organs and tissues during an exposure to allow for each organ’s radiosensitivity, and then summing the results, gives the effective dose. The “effective dose” is calculated by multiplying the absorbed dose by a tissue weighting factor which represents the sensitivity of each tissue to radiation.
Table 1.3 Tissue weighting factors by organ
|Organ||Tissue weighting factor T|
|Bone marrow (red)||0.12|
|Adrenals, brain, small intestine, kidney, muscle, pancreas, spleen, thymus, uterus||the weighting factor 0.05 is applied to the average dose of these organs|
Reference: Recommendations for limiting exposure to ionizing radiation (1995) and national standard for limiting occupational exposure to ionizing radiation (republished 2002); Radiation Protection Series No. 1; ARPANSA.
Radiation ExposureRegardless of where or how an incident involving radiation happens, three types of radiation-induced injury can occur: external irradiation, contamination with radioactive materials, and incorporation of radioactive material into body cells, tissues, or organs.
External irradiation is exposure to penetrating radiation from a radiation source. People exposed to a source of radiation can suffer radiation illness if their dose is high enough, but they do not become radioactive. For example, an x-ray machine is a source of radiation exposure. A person does not become radioactive or pose a risk to others following a chest x-ray. Irradiation occurs when all or part of the body is exposed to radiation from an unshielded source. External irradiation does not make a person radioactive.
Radioactive contamination occurs when material that contains radioactive atoms is deposited on skin, clothing, or any place where it is not desired. If is important to remember that radiation does not spread or get "on" or "in" people; rather it is radioactive contamination that can spread. A person contaminated with radioactive materials will be irradiated until the source of radiation (the radioactive material) is removed.
- A person is externally contaminated if radioactive material is on skin or clothing
- A person is internally contaminated if radioactive material is breathed in, swallowed, or absorbed through wounds
- The environment is contaminated if radioactive material is spread about or uncontained
Table 1.4 Radiation exposure from common radionuclides
|Dose rate at 1m||Time at 1m|
to exceed 1mSv
|137Cs||30 y|| (662)|
|Sterilization and food preservation||0.1–400 PBq||24,000 Sv/h||< 1 s|
|Whole blood irradiation||2-100 TBq||6 Sv/h||1 s|
|Moisture/density detector||400 MBq||20 uSv/h||2 d|
|60Co||5.3 y|| (1173; 1333)|
|Sterilization and food preservation||0.1–400 PBq||120,000 Sv/h||< 1 s|
|192Ir||74 d|| (317)|
|Industrial Radiography||0.1-5 TBq||0.4 Sv/h||9 s|
|241Am||432.2 y|| (60)|
|Well logging||1-800 GBq||2 mSv/h||20 m|
|Moisture/density d (Am-241/Be)||0.1-2 GBq||6 uSv/h||7 d|
|Smoke detectors||0.02-3 MBq||9 nSv/h||10 y|
Radiation Health EffectsAdverse health effects from exposure to radiation may be deterministic, occurring soon after exposure, or stochastic, occurring some time, often many years, after exposure.
Deterministic effects are dose-related, acute health effects caused by exposure to high levels of radiation that cause large numbers of cells to die or lose their ability to replicate. Organs containing these cells then fail to function correctly. Such effects include nausea (radiation syndrome), reddening of the skin, cataracts, sterility and bone marrow failure. Each effect becomes apparent only above a threshold level and the severity of the effect depends on the level of exposure above its threshold. Below the threshold, the body can cope with the level of cell death by repair and replacement, when no explicit damage is seen.
Extreme doses of radiation to the whole body (around 10 sievert and above), received in a short period, cause so much damage to internal organs and tissues of the body that vital systems cease to function and death may result within days or weeks. Very high doses (between about 1 sievert and 10 sievert), received in a short period, kill large numbers of cells, which can impair the function of vital organs and systems. Acute health effects, such as nausea, vomiting, skin and deep tissue burns, and impairment of the body’s ability to fight infection may result within hours, days or weeks. The extent of the damage increases with dose. However, ‘deterministic’ effects such as these are not observed at doses below certain thresholds. By limiting doses to levels below the thresholds, deterministic effects can be prevented entirely.
Table 1.5 Biological effects of acute, total body irradiation
|Amount of Exposure||Effect|
|50 mSv||No detectable injury or symptoms|
|1 Sv||May cause nausea and vomiting for 1-2 days and temporary drop in production of new blood cells|
|3.5 Sv||Nausea and vomiting initially, followed by a period of apparent wellness. At 3-4 weeks, there is a potential for deficiency of white blood cells and platelets. Medical care is required.|
|Higher levels of exposure can be fatal. Medical care is required.|
Reference: ARPANSA Technical Report Series No. 131; Medical management of individuals involved in radiation accident. 2000Doses below the thresholds for deterministic effects may cause cellular damage, but this does not necessarily lead to harm to the individual: the effects are probabilistic (occurring by chance) or ‘stochastic’ in nature. Stochastic effects are believed to result from damaged cells not dying but surviving in a modified form. These modified cells may, after a prolonged process, develop into a cancer. These stochastic effects usually appear many years after the exposure and, although they do not occur in every exposed individual, for radiation protection purposes it is assumed that there is no threshold below which they will not occur. Rather, the likelihood of a cancer or hereditary effect occurring after exposure is assumed to be proportional to the level of exposure.
If the modified cell is a germ cell, then the damage may be passed on to that person’s future descendants. Then, hereditary effects may be observed in these descendants. However, as the risk of serious stochastic effects to the individual is higher than that of hereditary effects to the individual descendants, if the individual is suitably protected the risk to the descendants will be minimised.
It is known that doses above about 100 millisievert, received in a short period, lead to an increased risk of developing cancer later in life. There is good epidemiological evidence – especially from studies of the survivors of the atomic bombings - that, for several types of cancer, the risk increases roughly linearly with dose, and that the risk factor averaged over all ages and cancer types is about 1 in 100 for every 100 millisievert of dose (i.e. 1 in 10,000 per millisievert).
At doses below about 100 millisievert, the evidence of harm is not clear-cut. While some studies indicate evidence of radiation-induced effects, epidemiological research has been unable to establish unequivocally that there are effects of statistical significance at doses below a few tens of millisieverts. Nevertheless, given that no threshold for stochastic effects has been demonstrated, and in order to be cautious in establishing health standards, the proportionality between risk and dose observed at higher doses is presumed to continue through all lower levels of dose to zero. This is called the linear, no-threshold (LNT) hypothesis and it is made for radiation protection purposes only.
There is evidence that a dose accumulated over a long period carries less risk than the same dose received over a short period. Except for accidents and medical exposures, doses are not normally received over short periods, so that it is appropriate in determining standards for the control of exposure to use a risk factor that takes this into account. While not well quantified, a reduction of the high-dose risk factor by a factor of two has been adopted internationally, so that for radiation protection purposes the risk of radiation-induced fatal cancer (the risk factor) is taken to be about 1 in 20,000 per millisievert of dose for the population as a whole.