Nuclear Radiation and Health Effects
(Updated 16 December 2014)
- Natural sources account for most of the radiation we all receive each year.
- The nuclear fuel cycle does not give rise to significant radiation exposure for members of the public, and even in two major nuclear accidents – Three Mile Island and Fukushima – exposure to radiation has caused no harm to the public.
- Radiation protection standards assume that any dose of radiation, no matter how small, involves a possible risk to human health. This deliberately conservative assumption is increasingly being questioned.
- Fear of radiation causes much harm. Expressed particularly in government edicts following the Fukushima accident (and also Chernobyl), it has caused much suffering and many deaths.
Radiation is energy in the process of being transmitted. It may take such forms as light, or tiny particles much too small to see. Visible light, the ultra-violet light we receive from the sun, and transmission signals for TV and radio communications are all forms of radiation that are common in our daily lives. These are all generally referred to as 'non-ionizing' radiation, though at least some ultra-violet radiation is considered to be ionizing.
Radiation particularly associated with nuclear medicine and the use of nuclear energy, along with X-rays, is 'ionizing' radiation, which means that the radiation has sufficient energy to interact with matter, especially the human body, and produce ions, i.e. it can eject an electron from an atom.
X-rays from a high-voltage discharge were discovered in 1895, and radioactivity from the decay of particular isotopes was discovered in 1896. Many scientists then undertook study of these, and especially their medical applications. This led to the identification of different kinds of radiation from the decay of atomic nuclei, and understanding of the nature of the atom. Neutrons were identified in 1932, and in 1939 atomic fission was discovered by irradiating uranium with neutrons. This led on to harnessing the energy released by fission.
Types of radiation
Nuclear radiation arises from hundreds of different kinds of unstable atoms. While many exist in nature, the majority are created in nuclear reactionsa. Ionizing radiation which can damage living tissue is emitted as the unstable atoms (radionuclides) change ('decay') spontaneously to become different kinds of atoms.
The principal kinds of ionizing radiation are:
These are helium nuclei consisting of two protons and two neutrons and are emitted from naturally-occurring heavy elements such as uranium and radium, as well as from some man-made transuranic elements. They are intensely ionizing but cannot penetrate the skin, so are dangerous only if emitted inside the body.
These are fast-moving electrons emitted by many radioactive elements. They are more penetrating than alpha particles, but easily shielded – the most energetic of them can be stopped by a few millimetres of wood or aluminium. They can penetrate a little way into human flesh but are generally less dangerous to people than gamma radiation. Exposure produces an effect like sunburn, but which is slower to heal. The weakest of them, such as from tritium, are stopped by skin or cellophane. Beta-radioactive substances are also safe if kept in appropriate sealed containers.
These are high-energy beams much the same as X-rays. They are emitted in many radioactive decays and may be very penetrating, so require more substantial shielding. Gamma rays are the main hazard to people dealing with sealed radioactive materials used, for example, in industrial gauges and radiotherapy machines. Radiation dose badges are worn by workers in exposed situations to detect them and hence monitor exposure. All of us receive about 0.5-1 mSv per year of gamma radiation from cosmic rays and from rocks, and in some places, much more. Gamma activity in a substance (e.g. rock) can be measured with a scintillometer or Geiger counter.
X-rays are also ionizing radiation, virtually identical to gamma rays, but not nuclear in origin. (However the effect of this radiation does not depend on its origin but on its energy.)
Cosmic radiation consists of very energetic particles, mostly protons, which bombard the Earth from outer space.
Neutrons are particles mostly released by nuclear fission (the splitting of atoms in a nuclear reactor), and hence are seldom encountered outside the core of a nuclear reactor.* Thus they are not normally a problem outside nuclear plants. Fast neutrons can be very destructive to human tissue. Neutrons are the only type of radiation which can make other, non-radioactive materials, become radioactive.
Units of radiation and radioactivity
In order to quantify how much radiation we are exposed to in our daily lives and to assess potential health impacts as a result, it is necessary to establish a unit of measurement. The basic unit of radiation dose absorbed in tissue is the gray (Gy), where one gray represents the deposition of one joule of energy per kilogram of tissue.
However, since neutrons and alpha particles cause more damage per gray than gamma or beta radiation, another unit, the sievert (Sv) is used in setting radiological protection standards. This unit of measurement takes into account biological effects of different types of radiation. One gray of beta or gamma radiation has one sievert of biological effect, one gray of alpha particles has 20 Sv effect and one gray of neutrons is equivalent to around 10 Sv (depending on their energy). Since the sievert is a relatively large value, dose to humans is normally measured in millisieverts (mSv), one-thousandth of a sievert.
Note that Sv and Gy measurements are accumulated over time, whereas damage (or effect) depends on the actual dose rate, e.g. mSv per day or year, Gy per day in radiotherapy.
The becquerel (Bq) is a unit or measure of actual radioactivity in material (as distinct from the radiation it emits, or the human dose from that), with reference to the number of nuclear disintegrations per second (1 Bq = 1 disintegration/sec). Quantities of radioactive material are commonly estimated by measuring the amount of intrinsic radioactivity in becquerels – one Bq of radioactive material is that amount which has an average of one disintegration per second, i.e. an activity of 1 Bq. This may be spread through a very large mass.
Radioactivity of some natural and other materials
|1 adult human (65 Bq/kg)
|1 kg of coffee
|1 kg of brazil nuts
|The air in a 100 sq metre Australian home (radon)
|The air in many 100 sq metre European homes (radon)
||Up to 30 000 Bq
|1 household smoke detector (with americium)
||30 000 Bq
|Radioisotope for medical diagnosis
||70 million Bq
|Radioisotope source for medical therapy
||100 000 000 million Bq (100 TBq)
|1 kg 50-year old vitrified high-level nuclear waste
||10 000 000 million Bq (10 TBq)
|1 luminous Exit sign (1970s)
||1 000 000 million Bq (1 TBq)
|1 kg uranium
||25 million Bq
|1 kg uranium ore (Canadian, 15%)
||25 million Bq
|1 kg uranium ore (Australian, 0.3%)
||500 000 Bq
|1 kg low level radioactive waste
||1 million Bq
|1 kg of coal ash
|1 kg of granite
|1 kg of superphosphate fertilizer
Older units of radiation measurement continue in use in some literature:
1 gray = 100 rads
1 sievert = 100 rem
1 becquerel = 27 picocuries or 2.7 x 10-11 curies
One curie was originally the activity of one gram of radium-226, and represents 3.7 x 1010 disintegrations per second (Bq).
The Working Level Month (WLM) has been used as a measure of dose for exposure to radon and in particular, radon decay productsb.
Since there is radioactivity in many foodstuffs, there has been a whimsical suggestion that the Banana Equivalent Dose from eating one banana be adopted for popular reference. This is about 0.0001 mSv.
Routine sources of radiation
Radiation can arise from human activities or from natural sources. Most radiation exposure is from natural sources. These include: radioactivity in rocks and soil of the Earth's crust; radon, a radioactive gas given out by many volcanic rocks and uranium ore; and cosmic radiation. The human environment has always been radioactive and accounts for up to 85% of the annual human radiation dose.
Radiation arising from human activities typically accounts for up to 20% of the public's exposure every year as global average. In the USA by 2006 it averaged about half of the total. This radiation is no different from natural radiation except that it can be controlled. X-rays and other medical procedures account for most exposure from this quarter. Less than 1% of exposure is due to the fallout from past testing of nuclear weapons or the generation of electricity in nuclear, as well as coal and geothermal, power plants.
Backscatter X-ray scanners being introduced for airport security will gives exposure of up to 5 microsieverts (μSv), compared with 5 μSv on a short flight and 30 μSv on a long intercontinental flight across the equator, or more at higher latitudes – by a factor of 2 or 3. Aircrew can receive up to about 5 mSv/yr from their hours in the air, while frequent flyers can score a similar incrementc. On average, nuclear power workers receive a lower annual radiation dose than flight crew, and frequent flyers in 250 hours would receive 1 mSv.
The maximum annual dose allowed for radiation workers is 20 mSv/yr, though in practice, doses are usually kept well below this level. In comparison, the average dose received by the public from nuclear power is 0.0002 mSv/yr, which is of the order of 10,000 times smaller than the total yearly dose received by the public from background radiation.
Natural background radiation, radon
Naturally occurring background radiation is the main source of exposure for most people, and provides some perspective on radiation exposure from nuclear energy. The average dose received by all of us from background radiation is around 2.4 mSv/yr, which can vary depending on the geology and altitude where people live – ranging between 1 and 10 mSv/yr, but can be more than 50 mSv/yr. The highest known level of background radiation affecting a substantial population is in Kerala and Madras states in India where some 140,000 people receive doses which average over 15 millisievert per year from gamma radiation, in addition to a similar dose from radon. Comparable levels occur in Brazil and Sudan, with average exposures up to about 40 mSv/yr to many people. (The highest level of natural background radiation recorded is on a Brazilian beach: 800 mSv/yr, but people don’t live there.)
Several places are known in Iran, India and Europe where natural background radiation gives an annual dose of more than 100 mSv to people and up to 260 mSv (at Ramsar in Iran, where some 200,000 people are exposed to more than 10 mSv/yr). Lifetime doses from natural radiation range up to several thousand millisievert. However, there is no evidence of increased cancers or other health problems arising from these high natural levels. The millions of nuclear workers that have been monitored closely for 50 years have no higher cancer mortality than the general population but have had up to ten times the average dose. People living in Colorado and Wyoming have twice the annual dose as those in Los Angeles, but have lower cancer rates. Misasa hot springs in Japan attract people due to having high levels of radium (up to 550 Bq/L), with health effects long claimed, and in a 1992 study the local residents’ cancer death rate was half the Japan average. (Japan J.Cancer Res. 83,1-5, Jan 1992) A study on 3000 residents living in an area with 60 Bq/m3 radon (about ten times normal average) showed no health difference.
Radon is a naturally occurring radioactive gas, which concentrates in enclosed spaces such as buildings and underground mines, particularly in early uranium mines where it sometimes became a significant hazard before the problem was understood and controlled by increased ventilation. Radon has decay products that are short-lived alpha emitters and deposit on surfaces in the respiratory tract during the passage of breathing air. At high radon levels, this can cause an increased risk of lung cancer, particularly for smokers. (Smoking itself has a very much greater lung cancer effect than radon.) People everywhere are typically exposed to around 0.2 mSv/yr, and often up to 3 mSv/yr, due to radon (mainly from inhalation in their homes) without apparent ill-effectd. Where deemed necessary, radon levels in buildings and mines can be controlled by ventilation, and measures can be taken in new constructions to prevent radon from entering buildings.
However, radon levels of up to 3700 Bq/m3 in some dwellings at Ramsar in Iran have no evident ill-effect. Here, a study (Mortazavi et al, 2005) showed that the highest lung cancer mortality rate was where radon levels were normal, and the lowest rate was where radon concentrations in dwellings were highest.
Public exposure to natural radiatione
|Source of exposure
||Annual effective dose (mSv)
||Directly ionizing and photon component
|Total cosmic and cosmogenic
|External terrestrial radiation
|Total external terrestrial radiation
||Uranium and thorium series
|Total inhalation exposure
|Uranium and thorium series
|Total ingestion exposure
Crews of nuclear submarines have possibly the lowest radiation exposure of anyone, despite living within a few metres of a nuclear reactor, since they are exposed to less natural background radiation than the rest of us, and the reactor compartment is well shielded.1 US Naval Reactors’ average annual occupational exposure was 0.06 mSv per person in 2013, and no personnel have exceeded 20 mSv in any year in the 34 years to then. The average occupational exposure of each person monitored at Naval Reactors' facilities since 1958 is 1.03 mSv per year.
Effects of ionizing radiation
Some of the ultraviolet (UV) radiation from the sun is considered ionizing radiation, and provides a starting point in considering its effects. Sunlight UV is important in producing vitamin D in humans, but too much exposure produces sunburn and, potentially, skin cancer. Skin tissue is damaged, and that damage to DNA may not be repaired properly, so that over time, cancer develops and may be fatal. Adaptation from repeated low exposure can decrease vulnerability. But exposure to sunlight is quite properly sought after in moderation, and not widely feared.
Our knowledge of the effects of shorter-wavelength ionizing radiation from atomic nuclei derives primarily from groups of people who have received high doses. The main difference from UV radiation is that gamma and X-rays can penetrate the skin. The risk associated with large doses of this ionizing radiation is relatively well established. However, the effects, and any risks associated with doses under about 200 mSv, are less obvious because of the large underlying incidence of cancer caused by other factors. Benefits of lower doses have long been recognised, though radiation protection standards assume that any dose of radiation, no matter how small, involves a possible risk to human health. However, available scientific evidence does not indicate any cancer risk or immediate effects at doses below 100 mSv per year. At low levels of exposure, the body's natural mechanisms repair radiation damage to cells soon after it occurs (see following section on low-level radiation). However, high-level irradiation impairs those systems and is harmful. Dose rate is as important as overall dose.
|Some comparative whole-body radiation doses and their effects
||Typical background radiation experienced by everyone (average 1.5 mSv in Australia, 3 mSv in North America).
|1.5 to 2.5 mSv/yr
||Average dose to Australian uranium miners and US nuclear industry workers, above background and medical.
|Up to 5 mSv/yr
||Typical incremental dose for aircrew in middle latitudes.
||Exposure by airline crew flying the New York – Tokyo polar route.
||Maximum actual dose to Australian uranium miners.
||Effective dose from abdomen & pelvis CT scan.
||Current limit (averaged) for nuclear industry employees and uranium miners.
||Former routine limit for nuclear industry employees, now maximum allowable for a single year (average to be 20 mSv/yr max). It is also the dose rate which arises from natural background levels in several places in Iran, India and Europe.
||Allowable short-term dose for emergency workers (IAEA).
||Lowest annual level at which increase in cancer risk is evident (UNSCEAR). Above this, the probability of cancer occurrence (rather than the severity) is assumed to increase with dose. No harm has been demonstrated below this dose.
Allowable short-term dose for emergency workers taking vital remedial actions (IAEA).
||Long-term safe level for public after radiological incident, measured 1 m above contaminated ground, calculated from published hourly rate x 0.6. Risk too low to justify any action below this (IAEA).
||7-day provisionally safe level for public after radiological incident, measured 1 m above contaminated ground (IAEA).
||Allowable short-term dose for workers controlling the 2011 Fukushima accident, proposed set emergency limit in Japan, set as emergency limit elsewhere.
||Natural background level at Ramsar in Iran, with no identified health effects. (Some exposures reach 700 mSv/yr.)
||Criterion for relocating people after Chernobyl accident.
||Allowable short-term dose for emergency workers taking life-saving actions (IAEA).
||Tolerance dose level allowable to 1955 (assuming gamma, X-ray and beta radiation).
||Suggested threshold for maintaining evacuation after nuclear accident.
(IAEA has 880 mSv/yr over one month as provisionally safe.
||Highest level of natural background radiation recorded, on a Brazilian beach.
|1,000 mSv short-term
||Assumed to be likely to cause a fatal cancer many years later in about 5 of every 100 persons exposed to it (i.e. if the normal incidence of fatal cancer were 25%, this dose would increase it to 30%).
|1,000 mSv short-term
||Threshold for causing (temporary) radiation sickness (Acute Radiation Syndrome) such as nausea and decreased white blood cell count, but not death. Above this, severity of illness increases with dose.
|5,000 mSv short-term
||Would kill about half those receiving it within a month. (However, this is only twice a typical daily therapeutic dose applied to a very small area of the body over 4 to 6 weeks or so.)
|10,000 mSv short-term
||Fatal within a few weeks.
The main expert body on radiation effects is the UN Scientific Commission on the Effects of Atomic Radiation (UNSCEAR), set up in 1955 and reporting to the UN General Assembly. It involves scientists from over 20 countries and publishes its findings in major reports. The UNSCEAR 2006 report dealt broadly with the Effects of Ionizing Radiation. Another valuable report, titled Low-level Radiation and its Implications for Fukushima Recovery, was published in June 2012 by the American Nuclear Society.
In 2012 UNSCEAR reported to the UN General Assembly on radiation effects. It had been asked in 2007 "to clarify further the assessment of potential harm owing to chronic low-level exposures among large populations and also the attributability of health effects" to radiation exposure. It said that while some effects from high acute doses were clear, others including hereditary effects in human populations were not, and could not be attributed to exposure, and that this was especially true at low levels. "In general, increases in the incidence of health effects in populations cannot be attributed reliably to chronic exposure to radiation at levels that are typical of the global average background levels of radiation." Furthermore, multiplying very low doses by large numbers of individuals does not give a meaningful result regarding health effects. UNSCEAR also addressed uncertainties in risk estimation relating to cancer, particularly the extrapolations from high-dose to low-dose exposures and from acute to chronic and fractionated exposures. Earlier (1958) UNSCEAR data for leukaemia incidence among Hiroshima survivors suggested a threshold of about 400 mSv for harmful effects.
Epidemiological studies continue on the survivors of the atomic bombing of Hiroshima and Nagasaki, involving some 76,000 people exposed at levels ranging up to more than 5,000 mSv. These have shown that radiation is the likely cause of several hundred deaths from cancer, in addition to the normal incidence found in any populationg. From this data the International Commission on Radiological Protection (ICRP) and others estimate the fatal cancer risk as 5% per sievert exposure for a population of all ages – so one person in 100 exposed to 200 mSv could be expected to develop a fatal cancer some years later. In Western countries, about a quarter of people die from cancers, with smoking, dietary factors, genetic factors and strong sunlight being among the main causes. Radiation is a weak carcinogen, but undue exposure can certainly increase health risks.
In 1990, the US National Cancer Institute (NCI) found no evidence of any increase in cancer mortality among people living near to 62 major nuclear facilities. The NCI study was the broadest of its kind ever conducted and supported similar studies conducted elsewhere in the USA as well as in Canada and Europe.h
About 60 years ago it was discovered that ionizing radiation could induce genetic mutations in fruit flies. Intensive study since then has shown that radiation can similarly induce mutations in plants and test animals. However there is no evidence of inherited genetic damage to humans from radiation, even as a result of the large doses received by atomic bomb survivors in Japan.
In a plant or animal cell the material (DNA) which carries genetic information necessary to cell development, maintenance and division is the critical target for radiation. Much of the damage to DNA is repairable, but in a small proportion of cells the DNA is permanently altered. This may result in death of the cell or development of a cancer, or in the case of cells forming gonad tissue, alterations which continue as genetic changes in subsequent generations. Most such mutational changes are deleterious; very few can be expected to result in improvements.
The relatively low levels of radiation allowed for members of the public and for workers in the nuclear industry are such that any increase in genetic effects due to nuclear power will be imperceptible and almost certainly non-existent. Radiation exposure levels are set so as to prevent tissue damage and minimize the risk of cancer. Experimental evidence indicates that cancers are more likely than inherited genetic damage.
Some 75,000 children born of parents who survived high radiation doses at Hiroshima and Nagasaki in 1945 have been the subject of intensive examination. This study confirms that no increase in genetic abnormalities in human populations is likely as a result of even quite high doses of radiation. Similarly, no genetic effects are evident as a result of the Chernobyl accident.
Life on Earth commenced and developed when the environment was certainly subject to several times as much radioactivity as it is now, so radiation is not a new phenomenon. If there is no dramatic increase in people's general radiation exposure, there is no evidence that health or genetic effects from radiation could ever become significant.
Low-level radiation effects
A lot of research has been undertaken on the effects of low-level radiation. The findings have failed to support the so-called linear no-threshold (LNT) hypothesis. This theory assumes that the demonstrated relationships between radiation dose and adverse effects at high levels of exposure also applies to low levels and provides the (deliberately conservative) basis of occupational health and other radiation protection standards.
Increasing evidence suggests that there may be a threshold between 100 and 700 mSv below which no harmful effects of radiation occur. However, this is not yet accepted by national or international radiation protection bodies as sufficiently well-proven to be taken into official standards. However, at low levels of exposure, the body's natural mechanisms do repair radiation and other damage to cells soon after it occurs, and some adaptive response is stimulated which protects cells and tissues, as with exposure to other external agents at low levels. The ICRP recommends that the LNT model should be assumed for the purpose of optimising radiation protection practices, but that it should not be used for estimating the health effects of exposures to small radiation doses received by large numbers of people over long periods of time.
A November 2009 technical report from the Electric Power Research Institute in USA drew upon more than 200 peer-reviewed publications on effects of low-level radiation and concluded that the effects of low dose-rate radiation are different and that "the risks due to [those effects] may be over-estimated" by the linear hypothesis3. "From an epidemiological perspective, individual radiation doses of less than 100 mSv in a single exposure are too small to allow detection of any statistically significant excess cancers in the presence of naturally occurring cancers. The doses received by nuclear power plant workers fall into this category because exposure is accumulated over many years, with an average annual dose about 100 times less than 100 mSv". It quoted the US Nuclear Regulatory Commission that "since 1983, the US nuclear industry has monitored more than 100,000 radiation workers each year, and no workers have been exposed to more than 50 mSv in a year since 1989." A 2012 Massachusetts Institute of Technology study4 exposing mice to low-dose rate radiation for an extended period showed no signs of DNA damage, though a control group receiving the same dose acutely did show damage. This test on live animals confirms other work and epidemiological studies suggesting that people exposed to as much as 1000 mSv/yr at low dose rate will not suffer adverse health effects.
In addition, there is evidence of beneficial effect from low-level radiation (up to about 10 mSv/yr). This 'radiation hormesis' may be due to an adaptive response by the body's cells, the same as that with other toxins at low doses. In the case of carcinogens such as ionizing radiation, the beneficial effect is seen both in lower incidence of cancer and in resistance to the effects of higher doses. This potential hormetic effect is most clearly evident in the data (see Appendix) for over 50,000 survivors of the Hiroshima bomb 1.5 to 3 km from the hypocenter, with dose range 1 to 100 mSv, compared with a large control group.
Further research is under way and a debate continues. Meanwhile standards for radiation exposure continue to be deliberately conservative.
Public dose limits for exposure from uranium mining or nuclear plants are usually set at 1 mSv/yr above background.
In most countries the current maximum permissible dose to radiation workers is 20 mSv per year averaged over five years, with a maximum of 50 mSv in any one year. This is over and above background exposure, and excludes medical exposure. The value originates from the International Commission on Radiological Protection (ICRP), and is coupled with the requirement to keep exposure as low as reasonably achievable (ALARA) – taking into account social and economic factors.
Radiation protection at uranium mining operations and in the rest of the nuclear fuel cycle is tightly regulated, and levels of exposure are monitored.
There are four ways in which people are protected from identified radiation sources:
- Limiting time. In occupational situations, dose is reduced by limiting exposure time.
- Distance. The intensity of radiation decreases with distance from its source.
- Shielding. Barriers of lead, concrete or water give good protection from high levels of penetrating radiation such as gamma rays. Intensely radioactive materials are therefore often stored or handled under water, or by remote control in rooms constructed of thick concrete or lined with lead.
- Containment. Highly radioactive materials are confined and kept out of the workplace and environment. Nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained.
Standards and regulation of radiation exposure
Radiation protection standards are based on the conservative assumption that the risk is directly proportional to the dose, even at the lowest levels, though there is no actual evidence of harm at low levels, below about 100 mSv as short-term dose. To the extent that cell damage is made good within a month (say), chronic dose rates up to 100 mSv per month could also be safe, but the standard assumption, called the 'linear no-threshold (LNT) hypothesis', discounts the contribution of any such thresholds and is recommended for practical radiation protection purposes only, such as setting allowable levels of radiation exposure of individuals.
LNT was first accepted by the International Commission on Radiological Protection (ICRP) in 1955, when scientific knowledge of radiation effects was less, and then in 1959 by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) as a philosophical basis for radiological protection at low doses, stating outright that “Linearity has been assumed primarily for purposes of simplicity, and there may or may not be a threshold dose”. (Above 100 mSv acute dose there is some scientific evidence for linearity in dose-effect.) From 1934 to 1955 a tolerance dose limit of 680 mSv/yr was recommended by the ICRP, and no evidence of harm from this – either cancer or genetic – had been documented.
The LNT hypothesis cannot properly be used for predicting the consequences of an actual exposure to low levels of radiation and it has no proper role in low-dose risk assessment. For example, LNT suggests that, if the dose is halved from a high level where effects have been observed, there will be half the effect, and so on. This would be very misleading if applied to a large group of people exposed to trivial levels of radiation and even at levels higher than trivial it could lead to inappropriate actions to avert the doses. At Fukushima following the March 2011 accident, maintaining the evacuation beyond a few days did in fact lead to about 1100 deaths, according to the Japan Reconstruction Agencyi.
Much of the evidence which has led to today's standards derives from the atomic bomb survivors in 1945, who were exposed to high doses incurred in a very short time. In setting occupational risk estimates, some allowance has been made for the body's ability to repair damage from small exposures, but for low-level radiation exposure the degree of protection from applying LNT may be misleading. At low levels of radiation exposure the dose-response relationship is unclear due to background radiation levels and natural incidence of cancer. However, the Hiroshima survivor data published in 1958 by UNSCEAR for leukaemia (see Appendix) actually shows a reduction in incidence by a factor of three in the dose range 1 to 100 mSv. The threshold for increased risk here is about 400 mSv. This is very significant in relation to concerns about radiation exposure from contaminated areas after the Chernobyl and Fukushima accidents.
The International Commission on Radiological Protection (ICRP), set up in 1928, is a body of scientific experts and a respected source of guidance on radiation protection, though it is independent and not accountable to governments or the UN. Its recommendations are widely followed by national health authorities, the EU and the IAEA. It retains the LNT hypothesis as a guiding principle.
The International Atomic Energy Agency (IAEA) has published international radiation protection standards since 1962. It is the only UN body with specific statutory responsibilities for radiation protection and safety. Its Safety Fundamentals are applied in basic safety standards and consequent Regulations. However, the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) set up in 1955 is the most authoritative source of information on ionizing radiation and its effects.
In any country, radiation protection standards are set by government authorities, generally in line with recommendations by the ICRP, and coupled with the requirement to keep exposure as low as reasonably achievable (ALARA) – taking into account social and economic factors. The authority of the ICRP comes from the scientific standing of its members and the merit of its recommendations.
The three key points of the ICRP's recommendations are:
- Justification. No practice should be adopted unless its introduction produces a positive net benefit.
- Optimisation. All exposures should be kept as low as reasonably achievable, economic and social factors being taken into account.
- Limitation. The exposure of individuals should not exceed the limits recommended for the appropriate circumstances.
National radiation protection standards are framed for both Occupational and Public exposure categories.
The ICRP recommends that the maximum permissible dose for occupational exposure should be 20 millisievert per year averaged over five years (i.e. 100 millisievert in 5 years) with a maximum of 50 millisievert in any one year. For public exposure, 1 millisievert per year averaged over five years is the limit. In both categories, the figures are over and above background levels, and exclude medical exposure.j
These low exposure levels are achievable for normal nuclear power and medical activities, but where an accident has resulted in radioactive contamination their application has no net health benefit. There is a big difference between what is desirable in the normal planned operation of any plant, and what is tolerable for dealing with the effects of an accident. Here, restrictive dose limits will limit flexibility in managing the situation and thus their application may increase other health risks, or even result in major adverse health effects, as near Fukushima since March 2011 (see earlier endnote). The objective needs to be to minimize the risks and harm to the individual and population overall, rather than focusing on radiation in isolation.
This is recognised to some extent in the occupational health limits set for cleaning up such situations: the IAEA sets 100 mSv as the allowable short-term dose for emergency workers taking vital remedial actions, and 500 mSv as allowable short-term dose for emergency workers taking life-saving actions. At Fukushima, 250 mSv was set as the allowable short-term dose for workers controlling the disabled reactors. But even these are low, and there has been no corresponding allowance for neighbouring members of the public – ALARA was the only reference criterion regardless of its collateral effects due to prolonging the evacuation beyond a few days. The death toll and trauma from prolonged evacuation at Fukushima were clearly very much greater than the risks of elevated radiation exposure after the first few days.
This led to the IAEA in May 2013 publishing allowable dose rates for members of the public living normally in affected areas, measured 1m above the contaminated ground. A level of 220 mSv/yr over a full year is "safe for everyone" providing any ingested radioactivity is safe. Shorter term, at 40 times this level, 170 mSv for one week is provisionally safe, and at four times the yearly level – 880 mSv – is provisionally safe for one month.
When making decisions on evacuations, optimization of all the health risks (not just radiation exposure) is required. Rather than ALARA, a concept of As High As Relatively Safe – AHARS – would be more appropriate in dealing with emergency or existing high exposure situations, based on scientific evidence. This would be similar to the 680 mSv Tolerance Dose allowable to 1950s (assuming gamma & beta radiation), and it would take into account the dose rate. This AHARS approach is supported by Allison (2011) and Cuttler (2012 & 2013) among others, and an AHARS level of 1000 mSv/yr or 100 mSv per month is suggested. This would also mean that most or all of the displaced residents from near the Fukushima plant could return home, without any elevated cancer risk.
Another proposal is for As High as Naturally Existent – AHANE. That is, the highest naturally occurring radiation exposures in the world experienced by a large number of people without discernable negative health effects. At Ramsar in Iran, about 2000 people are exposed to at least 250 mSv/yr with no adverse effect. But in Guarapari, Brazil (pop. 73,000), Kerala, India (pop. 100,000), and Yangjiang, China (pop. 80,000) the average exposures are about 50 mSv/year, 38 mSv/year and 35 mSv/year respectively. In all cases, the residents have life expectancies at least as long as their national peers, and cancer rates slightly lower than fellow countrymen. US Navy shipyard worker data confirm the safety of 50 mSv/yr.
Nuclear fuel cycle radiation exposures
The average annual radiation dose to employees at uranium mines (in addition to natural background) is around 2 mSv (ranging up to 10 mSv). Natural background radiation is about 2 mSv. In most mines, keeping doses to such low levels is achieved with straightforward ventilation techniques coupled with rigorously enforced procedures for hygiene. In some Canadian mines, with very high-grade ore, sophisticated means are employed to limit exposure. (See also information page on Occupational Safety in Uranium Mining.) Occupational doses in the US nuclear energy industry – conversion, enrichment, fuel fabrication and reactor operation – average less than 3 mSv/yr.
Reprocessing plants in Europe and Russia treat used fuel to recover useable uranium and plutonium and separate the highly radioactive wastes. These facilities employ massive shielding to screen gamma radiation in particular. Manual operations are carried by operators behind lead glass using remote handling equipment.
In mixed oxide (MOX) fuel fabrication, little shielding is required, but the whole process is enclosed with access via gloveboxes to eliminate the possibility of alpha contamination from the plutonium. Where people are likely to be working alongside the production line, a 25mm layer of perspex shields neutron radiation from the Pu-240. (In uranium oxide fuel fabrication, no shielding is required.)
Interestingly, due to the substantial amounts of granite in their construction, many public buildings including Australia's Parliament House and New York Grand Central Station, would have some difficulty in getting a licence to operate if they were nuclear power stations.
Accidental radiation exposure (nuclear and other)
(ARS = Acute radiation Syndrome)
Three Mile Island – nuclear power reactor
The March 1979 accident at Three Mile Island nuclear power plant in the USA caused some people near the plant to receive very minor doses of radiation, well under the internationally recommended level. Subsequent scientific studies found no evidence of any harm resulting from that exposure. In 1996, some 2,100 lawsuits claiming adverse health effects from the accident were dismissed for lack of evidence. INES rating 5.
Chernobyl – nuclear power reactor
Immediately after the Chernobyl nuclear power plant disaster in 1986, much larger doses were experienced. Apart from the residents of nearby Pripyat, who were evacuated within two days, some 24,000 people living within 15 km of the plant received an average of 450 mSv before they were evacuated. A total of 5200 PBq of radioactivity (iodine-131 equivalent) was released.
In June 1989, a group of experts from the World Health Organization agreed that an incremental long-term dose of 350 mSv should be the criterion for relocating people affected by the 1986 Chernobyl accident. This was considered a "conservative value which ensured that the risk to health from this exposure was very small compared with other risks over a lifetime". (For comparison, background radiation averages about 150-200 mSv over a lifetime in most places.)
Out of the 134 severely exposed workers and firemen, 28 of the most heavily exposed died as a result of acute radiation syndrome (ARS) within three months of the accident. Of these, 20 were from the group of 21 that had received over 6.5 Gy, seven (out of 22) had received between 4.2 and 6.4 Gy, and one (out of 50) from the group that had received 2.2-4.1 Gy.5 A further 19 died in 1987-2004 from different causes (see information page on Chernobyl Accident Appendix 2: Health Impacts).
Regarding the emergency workers with doses lower than those causing ARS symptoms, a 2006 World Health Organization report6 referred to studies carried out on 61,000 emergency Russian workers where a total of 4995 deaths from this group were recorded during 1991-1998. "The number of deaths in Russian emergency workers attributable to radiation caused by solid neoplasms and circulatory system diseases can be estimated to be about 116 and 100 cases respectively." Furthermore, although no increase in leukaemia is discernible yet, "the number of leukaemia cases attributable to radiation in this cohort can be estimated to be about 30." Thus, 4.6% of the number of deaths in this group are attributable to radiation-induced diseases. (The estimated average external dose for this group was 107 mSv.)
The report also links the accident to an increase in thyroid cancer in children: "During 1992-2000, in Belarus, Russia and Ukraine, about 4000 cases of thyroid cancer were diagnosed in children and adolescents (0-18 years), of which about 3000 occurred in the age group of 0-14 years. For 1152 thyroid cancer patient cases diagnosed among Chernobyl children in Belarus during 1986-2002, the survival rate is 98.8%. Eight patients died due to progression of their thyroid cancer and six children died from other causes. One patient with thyroid cancer died in Russia."
There has been no increase attributable to Chernobyl in congenital abnormalities, adverse pregnancy outcomes or any other radiation-induced disease in the general population either in the contaminated areas or further afield.
Reports two decades after the accident make it clear that the main health effects from the accident are due to the evacuation of many people coupled with fear engendered, and thousands have died from suicide, depression and alcoholism. The 2006 Chernobyl Forum report said that people in the area suffered a paralysing fatalism due to myths and misperceptions about the threat of radiation, which contributed to a culture of chronic dependency. Some "took on the role of invalids." Mental health coupled with smoking and alcohol abuse is a very much greater problem than radiation, but worst of all at the time was the underlying level of health and nutrition. Psycho-social effects among those affected by the accident are similar to those arising from other major disasters such as earthquakes, floods and fires.
After the shelterf was built over the destroyed reactor at Chernobyl, a team of about 15 engineers and scientists was set up to investigate the situation inside it. Over several years they repeatedly entered the ruin, accumulating individual doses of up to 15,000 mSv. Daily dose was mostly restricted to 50 mSv, though occasionally it was many times this. None of the men developed any symptoms of radiation sickness, but they must be considered to have a considerably increased cancer risk. INES rating 7.
Fukushima – nuclear power reactors
The March 2011 accident at Fukushima Daiichi nuclear power plant in Japan released about 940 PBq (iodine-131 equivalent) of radioactive material, mostly on days 4 to 6 after the tsunami. In May 2013 UNSCEAR reported that "Radiation exposure following the nuclear accident at Fukushima Daiichi did not cause any immediate health effects. It is unlikely to be able to attribute any health effects in the future among the general public and the vast majority of workers." The only exception are the 146 emergency workers that received radiation doses of over 100 mSv during the crisis.7 Thyroid doses in children were significantly lower than from the Chernobyl accident. Some 160,000 people were evacuated as a precautionary measure, and prolonging the evacuation resulted in the deaths of about 1100 of them due to stress, and some due to disruption of medical and social welfare facilities. The highest internal radioactivity from ingestion was 12 kBq, some 1000 times less than the level causing adverse health effects at Goiania (see below).
Certainly the main radiation exposure was to workers on site, and the 146 with doses over 100 mSv will be monitored closely for "potential late radiation-related health effects at an individual level." Six of them had received over 250 mSv – the limit set for emergency workers there, apparently due to inhaling iodine-131 fume early on. There were around 250 workers on site each day. INES rating 7.
Kyshtym, Techa River – military nuclear reprocessing plant
There was a major chemical accident at Mayak Chemical Combine (then known as Chelyabinsk-40) near Kyshtym in Russia in 1957. This plant had been built in haste in the late 1940s for military purposes. The failure of the cooling system for a tank storing many tonnes of dissolved nuclear waste resulted in an ammonium nitrate explosion with a force estimated at about 75 tonnes of TNT (310 GJ). Most of the 740-800 PBq of radioactive contamination settled out nearby and contributed to the pollution of the Techa River, but a plume containing 80 PBq of radionuclides spread hundreds of kilometres northeast. The affected area was already very polluted – the Techa River had previously received about 100 PBq of deliberately dumped waste, and Lake Karachay had received some 4000 PBq. This ‘Kyshtym accident’ killed perhaps 200 people and the radioactive plume affected thousands more as it deposited particularly Cs-127 and Sr-90.
Many people received doses up to 400 mSv at relatively low dose rates from liquid wastes released into the river. This population has shown an increase in cancer rates at levels above 200 mSv. But below this level, cancer incidence falls below the LNT expectations. INES rating 6.
Goiania – orphan source
In 1987 at Goiania8 in Brazil, a discarded radiotherapy source stolen from an abandoned hospital and broken open caused four deaths, 20 cases of radiation sickness and significant contamination of many more. The teletherapy source contained 93 grams of caesium-137 (51 TBq) encased in a shielding canister 51 mm diameter and 48 mm long made of lead and steel, with an iridium window. Various people came in contact with the source over two weeks as it was relayed to a scrapyard, and some were seriously affected. The four deaths (4-5 Sv dose) were family and employees of the scrapyard owner, and 16 others received more than 500 mSv dose. Overall 249 people were found to have significant levels of radioactive material in their bodies. In the 25 years since 1987 there have been zero cancers from radiation among those 249 people affected at Goiania, in spite of the ingestion of up to 100 MBq giving doses as high as 625 mSv/month (8 individuals had higher activity than 100 MBq internally of whom 4 died of Acute Radiation Syndrome but none of cancer). Two healthy babies were born, one to a mother among the most highly contaminated. However fear of the contamination has been the cause of severe stress and depression. In March 2012 Yukiya Amano, the Director General of IAEA, described Goiania as the best illustration of the effect of a terrorist dirty bomb – a few deaths but widespread fear and stress. INES rating 5.
Fleurus – commercial irradiation
In March 2006 at the Institute for Radioelements (IRE) in Fleurus9 in Belgium a worker at a commercial irradiation facility received a high radiation dose of about 4.6 Sv from cobalt-60, resulting in severe health effects. INES rating 5.
In August 2008 about 45 GBq of iodine-131 was released through the stack of the Institute for Radioelements (IRE) in Fleurus, Belgium. The release occurred following the transfer of liquid waste from one tank to another. INES rating 3.
Stamboliysky – commercial irradiation
In June 2011 in Bulgaria, preparations for the recharging of a gamma-irradiation facility with cobalt-60 sources were being undertaken at Stamboliysky. A device already recharged with sources had been taken out, instead of an empty one due to personnel error. As a result, four workers were exposed to a powerful gamma radiation for approximately five minutes, giving them effective doses of over 1 Sv. INES rating 3.
Mexico City – orphan source
In 1962 a boy took home an unshielded cobalt-60 radiography source. Nine people suffered ARS and four of them died from it.
Therac-25 – radiotherapy
In 1985, due to software failure and a fundamental design flaw in the Therac-25 medical irradiation device produced by Atomic Energy of Canada Limited (AECL), delivering 100 times the beta radiation dose intended, there were 3 fatalities from acute radiation syndrome, and three survivors of ARS.
Morocco – orphan source
In March 1984 an iridium-192 source used for industrial radiography was removed from its shielded container and taken home by a worker. Some 11 people suffered from ARS, of whom eight died of ARS.
Zaragoza – radiotherapy
In December 1990 at a clinic in Spain, at least 27 cancer patients were exposed to very high doses from an incorrectly repaired GE electron accelerator. Eleven of them died of ARS.
Costa Rica – radiotherapy
In 1996 at a hospital in San Jose 114 people received an overdose of radiation from cobalt-60 in therapy, about half suffering from ARS, and 13 died of ARS.
Samut Prakan – orphan source
In Thailand in 2000 a 15.7 TBq orphan cobalt-60 source in a scrap metal yard exposed many people. Ten people were hospitalized with ARS, of whom three died.
Mayapuri – orphan source
In India in April 2010 a cobalt-60 source from university equipment in a scrap metal yard exposed many people. Eight people were hospitalized with ARS, of whom one died.
The health effects of exposure both to radiation and to chemical cancer-inducing agents or toxins must be considered in relation to time. There is cause for concern not only about the effects on people presently living, but also about the cumulative effects of actions today over many generations.
Some radioactive materials which reach the environment decay to safe levels within days, weeks or a few years, while others continue their effect for a long time, as do most chemical cancer-inducing agents and toxins. Certainly this is true of the chemical toxicity of heavy metals such as mercury, cadmium and lead. These of course are a natural part of the human environment anyway, like radiation, but maintain their toxicity forever. The essential task for those in government and industry is to prevent excessive amounts of such toxins harming people, now or in the future. Standards are set in the light of research on environmental pathways by which people might ultimately be affected.
Data points from the left:
i) A control group of 32,963 people over 3 km from the hypocenter. 273 people per million developed leukaemia.
ii) 32,692 people between 2 and 3 km of the hypocenter, with estimated average exposure of about 20 mSv. 92 people per million developed leukaemia.
iii) and iv) 20,113 people between 1.5 and 2 km of the hypocenter, where average doses “were greater than” 500mSv. The left data point (iii) represents calculated radiation exposure for that zone; the right (iv) represents what is thought to be more a more accurate dose, given the cohort's other radiation-induced symptoms. 398 people per million developed leukaemia.
v) 8810 people between 1 and 1.5 km of the hypocenter, with estimated average exposure of about 5000 mSv. 3746 people per million developed leukaemia.
vi) 1241 survivors less than 1 km from the hypocenter, where over 50,000 were killed. 12,087 people per million developed leukaemia.
The latency period for leukaemia is less than six months. NB this is a log-log graph, and the green line would otherwise be straight.
a. Three of the main radioactive decay series relevant to nuclear energy are those of uranium and thorium. These series are shown in the Figure at www.world-nuclear.org/uploadedImages/org/info/radioactive_decay_series.png [Back]
b. The concentration of radon decay progeny (RnDP) is measured in Working Levels or in microjoules of ultimately-delivered alpha energy per cubic metre of air. One 'Working Level' (WL) is approximately equivalent to 3700 Bq/m3 of Rn-222 in equilibrium with its decay progeny (the main two of which are the very short-lived alpha-emitters), or to 20.7 μJ/m3. The former assumes still air, not proper ventilation. One working level month (WLM) is the dose from breathing one WL for 170 hours, and the former occupational exposure limit was 4 WLM/yr. Today the ICRP recommended limit is 3.5 μJ/m3, which is a measure of the actual RnDP situation in whatever conditions of ventilation prevail. It is generally equivalent to about 2000 hours per year exposure to 3000 Bq/m3 of radon in a ventilated mine where the radon is removed and so is not in equilibrium with its decay progeny. [Back]
c. At an altitude of 30,000 feet, the dose rate is 3-4 μSv per hour at the latitudes of North America and Western Europe. At 40,000 feet, the dose rates are about 6.5-8 μSv per hour. Other measured rates were 6.6 μSv per hour during a Paris-Tokyo (polar) flight and 9.7 μSv per hour on the Concorde, while a study on Danish flight crew showed that they received up to 9 mSv/yr. [Back]
d. A background radon level of 40 Bq/m3 indoors and 6 Bq/m3 outdoors, assuming an indoor occupancy of 80%, is equivalent to a dose rate of 1 mSv/yr and is the average for most of the world's inhabitants. Exposure levels of less than 200 Bq/m3 (and arguably much more) are not considered hazardous unless public health concerns are based on LNT, contrary to ICRP recommendations. [Back]
Range for cosmic and cosmogenic dose for sea level to high ground elevation.
Range for external terrestrial radiation depends on radionuclide composition of soil and building material.
Range for inhalation exposure depends on indoor accumulation of radon gas.
Range for ingestion exposure depends on radionuclide composition of foods and drinking water.
Source: Table 12 from Exposures of the Public and Workers from Various Sources of Radiation, Annex B to Volume I of the 2008 United Nations Scientific Committee on the Effects of Atomic Radiation Report to the General Assembly, Sources and Effects of Ionizing Radiation, available on the UNSCEAR 2008 Report Vol. I webpage (www.unscear.org/unscear/en/publications/2008_1.html) [Back]
f. A reinforced concrete casing was built around the ruined reactor building over the seven months following the accident. This shelter – often referred to as the sarcophagus – was intended to contain the remaining fuel and act as a radiation shield. As it was designed for a lifetime of around 20 to 30 years, as well as being hastily constructed, a second shelter – known as the New Safe Confinement – with a 100-year design lifetime is planned to be placed over the existing structure. [Back]
g. The actual doses received by atomic bomb survivors are uncertain. Also much of the radiation then was from neutrons, though gamma radiation is the prime concern for radiation protection. Some 65 years after the acute exposure it can be seen that cancer rate in the irradiated survivors is lower than the controls, and lower than in the Japanese population as a whole2. [Back]
h. In the UK there are significantly elevated childhood leukaemia levels near Sellafield as well as elsewhere in the country. The reasons for these increases, or clusters, are unclear, but a major study of those near Sellafield has ruled out any contribution from nuclear sources. Apart from anything else, the levels of radiation at these sites are orders of magnitude too low to account for the excess incidences reported. However, studies are continuing in order to provide more conclusive answers. [Back]
i. As of October 2012, over 1000 disaster-related deaths that were not due to radiation-induced damage or to the earthquake or to the tsunami had been identified by the Reconstruction Agency based on data for areas evacuated for no other reason than the nuclear accident. About 90% of deaths were for persons above 66 years of age. Of these, about 70% occurred within the first three months of the evacuations. (A similar number of deaths occurred among evacuees from tsunami- and earthquake-affected prefectures. These figures are additional to the 19,000 that died in the actual tsunami.) [Back]
j. The most recent revision of the ICRP's recommendations were issued in 2007 (Publication 103) which replaced the 1990 recommendations (Publication 60) without making any changes to the dose limits for occupational or public exposure. These values are also implemented by the IAEA in its Basic Safety Standard. [Back]
1. U.S. Environmental Protection Agency, Fact Sheet on Nuclear submarines [Back]
2. T. D. Luckey, Nuclear law stands on thin ice, International Journal of Nuclear Law, Vol 2, No 1, P 33-65 (2008) [Back]
3. Program on Technology Innovation: Evaluation of Updated Research on the Health Effects and Risks Associated with Low-Dose Ionizing Radiation, Electric Power Research Institute (EPRI), Palo Alto, California, USA, 1019227 (November 2009). A much-quoted 2005 study of low-dose exposure of nuclear workers – Cardis et al, Risk of cancer after low doses of ionising radiation: retrospective cohort study in 15 countries, British Medical Journal (BMJ 2005; 331:77) – depended heavily on Canadian data which was subsequently withdrawn by CNSC in 2011. Without that flawed data, the study showed no increased risk from low-dose radiation.[Back]
4. Werner Olipitz et al, Integrated Molecular Analysis Indicates Undetectable Change in DNA Damage in Mice after Continuous Irradiation at ~ 400-fold Natural Background Radiation, Environmental Health Perspectives (2012 August 2012), 120(8), 1130-1136. See also MIT news article, A new look at prolonged radiation exposure (15 May 2012) [Back]
5. Table 11 from Exposures and effects of the Chernobyl accident, Annex J to Volume II of the 2000 United Nations Scientific Committee on the Effects of Atomic Radiation Report to the General Assembly, available on the UNSCEAR 2000 Report Vol. II webpage (www.unscear.org/unscear/en/publications/2000_2.html) [Back]
6. Health Effects of the Chernobyl Accident and Special Health Care Programmes, Report of the UN Chernobyl Forum, Expert Group "Health", World Health Organization, 2006 (ISBN: 9789241594172). [Back]
7. Chapter II Part B Section 1 Paragraph 9(a), Report of the United Nations Scientific Committee on the Effects of Atomic Radiation, 59th session (21-25 May 2012), available on the UNSCEAR General Assembly reports and resolutions webpage, also Press Release 31 May 2013 at conclusion of 60th session of UNSCEAR. [Back]
8. International Atomic Energy Agency, The Radiological Accident in Goiania (1988) [Back]
9. Wm. Robert Johnston, Database of radiological incidents and related events, Fleurus irradiator accident, 2006 [Back]
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