There is no doubt that large doses of radiation over a short period of time are damaging to health. This has been known for a long time. What is less clear is the harm that might be done by small doses of radiation (see Low Level Effects page).
The health effects of radiation depend on:
Ionising radiation was discovered in 1895 with the discovery of X-rays by Röntgen. A translation of his original paper is available (Nature 53, 274-277 (1896)). Almost immediately their value to the world of medicine was recognized and many people started to experiment with them. Glasgow Royal Infirmary had an X-ray department later that year (John Macintyre and the world’s first x-ray department).
It was not long before the harmful side effects became apparent. A headline in the New York Morning Journal of November 29th 1896, asked “Does danger lurk in the X-rays?” and recounts the unpleasant experiences of Captain Webster after X-rays were used to locate a bullet in his body. In the same year an American inventor Elihu Thomson deliberately exposed his finger to x-rays over several days and reported “pain, swelling, stiffness, erythema and blistering”. In 1901 William Rollins reported that two guinea pigs exposed to X-rays for several hours a day died on the 8th and 11th day of the experiment (The First Fifty Years of Radiation Protection). It was clear that a balance had to be found in which the benefits of radiation outweighed the health costs.
Two main types of radiation harm are known.
Deterministic effects. These have a rapid impact with both the rapidity of the symptom showing and the severity of the effect increasing as the dose increases. These can be compared to sunburn; the stronger the sun the sooner you burn and the longer to stay out in it the worse the burning gets. We reduce these doses by reducing exposure (sun screen, clothing, seeking shade, going indoors, moving to the north) and reducing the exposure time (limiting our time outdoors).
Stochastic effects. These might take months to decades to be noticeable and only some of those exposed will suffer from them. It is the probability of seeing the effect that rises with dose not the severity. These can be compared to the risks of repeatedly crossing a busy road in a careless manner. The more often you cross the more likely you are to be hit by a vehicle. But the number of times you cross does not affect the consequences if you are finally hit by a vehicle. We reduce these effects by reducing the dose to which we are exposed (not crossing the road or crossing it carefully).
The early limits on radiation exposure were designed to reduce the incidence of deterministic effects so aimed to keep the dose rates below the threshold values. More modern limits are designed to keep the risks of stochastic effects within acceptable bounds.
Short term effects (deterministic effects) are the effects that happen quickly during and after large exposures. Deterministic effects include sickness, diarrhoea, and anaemia which can be severe, even fatal, at high doses. They worsen with increasing dose (just as sunburn does) and the time before the symptoms show decreases with increasing dose.
ICRP-103 provides a table of estimated dose thresholds for 1% incidence of morbidity and mortality for adult humans. The Table below is based on this information. It is unlikely that a member of the public would receive these levels of dose in a UK accident, but they are credible for members of staff, particularly for a criticality accident.
| Effect | Organ/tissue | Time to develop | Absorbed dose (Gy) for 1% incidence | |
|---|---|---|---|---|
| Temp Sterility | Testes | 3 - 9 weeks | ~0.1 | |
| Perm Sterility | Testes | 3 weeks | ~6 | |
| Perm Sterility | Ovaries | < 1 week | ~3 | |
| Depression of blood-forming process | Bone marrow | 3 - 7 days | ~0.5 | |
| Main phase of skin reddening | Skin (large areas) | 1 -4 weeks | < 3 - 6 | |
| Skin Burns | Skin (Large areas) | 2-3 weeks | 5 - 10 | |
| Temporary hair loss | Skin | 2 - 3 weeks | ~4 | |
| Cataract | Eye | Several years | ~1.5 | |
| Mortality | ||||
| Bone Marrow syndrome | ||||
| Without medical care | Bone marrow | 30 - 60 days | ~1 | |
| With medical care | Bone marrow | 30 - 60 days | 2 - 3 | |
| Gastro-intestinal syndrome | ||||
| Without medical care | Small intestine | 6 - 9 days | ~6 | |
| With medical care | Small intestine | 6 - 9 days | > 6 | |
| Pneumonitis | Lung | 1 - 7 months | ~6 | |
In the UK the NRPB provided a table of emergency planning threshold doses for serious deterministic injuries Protection of On-site Personnel in the Event of a Radiation Accident . These are for planning purposes and were set cautiously compared to the likely biological thresholds.
| Type of radiation | Dose (Gy) | Integration period |
|---|---|---|
| Low LET | 1 Gy whole body/red marrow 2-3 Gy other radiosensitive organs | Fractions of a second up to a few days |
| Neutron | 0.5 Gy whole body | Fractions of a second up to a few days |
| Alpha | 1 Gy lung | Dose integrated to one year from acute inhalation |
The second type of radiation effect is the longer term damage leading to effects such as cancer sometime after the exposure. Effects of this type are called stochastic effects (sometimes called probabilistic effects). The time delay can be as long as several decades after the exposure. In this case the probability of seeing the effect increases with dose received, but the consequences are not affected by the dose (either you see the effect or you don’t). The likelihood of these effects at low doses is very difficult to assess. The assumption currently used internationally is that the chances of an ill-effect such as a cancer decreases with decreasing dose and that there is no dose below which the chances are zero. This is the linear no-threshold hypothesis and is why scientists and government advisors are reluctant to say whether a particular dose is “safe”.
While saying that "there of no safe level of exposure" it is important to keep the risks associated with radiation in perspective. Not all radiation will cause damage, some radiation damage can be repaired by a cell, many damaged cells will simply die and not all damaged cells which survive will go on to cause a health detriment. It is unlikely that low levels of radiation will give rise to any health issues and it should be borne in mind that all life (including human beings) evolved and continue to be exposed to levels of natural background radiation which are often higher than most planned and accidental exposures.
Someone exposed to a low level of radiation may, or may not, suffer stochastic effects later in their life. It is impossible to say at the individual level. It is also generally impossible to say whether a cancer discovered later in life was due to the exposure or was fated to happen anyway although the increased occurrence of rare cancers in a group exposed to radiation can be seen as a smoking gun.
Many people overestimate the danger of radioactivity and most would be surprised to hear that according to the Japanese Ministry of Health, Labour, and Welfare, 80% of the atomic bomb survivors who were younger than 10 years old in 1945 were still alive in 2015. Their mean age was then more than 80 years old (Ref.), so even if you are unfortunate enough to be exposed to a high dose of radiation you still have a very good chance of a long and healthy life.
A lot of research has been undertaken and the currently accepted model (from ICRP Publication 103, 2007) assigns detriment-adjusted risk coefficients (see note) of 5.5x10-5 per mSv for cancer and 2.0x10-6 per mSv for heritable effects in the whole population (including infants, children and adults). This means that each mSv of dose increases your risk of cancer by 5.5x10-5 which is low compared to your overall risk of cancer (Cancer research UK is claiming that “1 in 2 UK people will be diagnosed with cancer in their lifetime”) but arguably worth avoiding if possible and cost effective.
Note:
Detriment-adjusted risk is defined as “The probability of the occurrence of a stochastic effect, modified to allow for the different components of the detriment in order to express the severity of the consequence(s)”. It is an attempt to summarise in one number the harm that radiation may do to you in the years after exposure.
Acute radiation syndrome, or radiation sickness or radiation poisoning, is the name given to the collection of symptoms and effects that the human body suffers if it is subjected to a large dose of ionising radiation to the whole body, or at least the majority of it, in a short period of time. Estimates of the absorbed dose threshold for symptoms to occur range from about 0.3 - 2.0 Gy.
There are three main phases of acute radiation syndrome; prodromal, latent and manifest. The higher the dose received the faster the stages progress and the more severe the symptoms. By following the stages and severity, doctors can estimate the dose that someone received and from that they can plan treatment.
The prodromal (from the Greek for “running before”) stage lasts for up to two days after exposure. Symptoms include anorexia, nausea, vomiting, diarrhoea, fever, headache and increased heart rate. This stage is mild or absent for doses below 1 Gy and more rapid about 2 Gy. Above 10 Gy onset is almost immediate.
During the latent phase the patient can seem to make a recovery as the initial symptoms subside. However, cell replacement function is inhibited and as mature cells reach the end of their life organ function is disrupted.
The manifest illness phase lasts from 21 to 60 days after exposure and is often considered as three parallel effects and their associated symptoms; bone marrow syndrome, gastrointestinal syndrome and cardiovascular syndrome.
Bone marrow syndrome, or hematopoietic (associated with blood) syndrome (dose between 0.7 and 10 Gy with mild symptoms from 0.3 Gy). The radiation kills the cells in the bone marrow. This reduces the production of the white blood cells that fight infection and the cells that help blood clot at the scene of an injury. The loss of these cells leads to infection and haemorrhage (internal bleeding). The early symptoms are anorexia, nausea and vomiting. Onset occurs 1 hour to 2 days after exposure and lasts for minutes to days. There is then a latent stage where the patient appears to be well but their bone marrow is dying and their blood being depleted of fresh cells. The bone marrow death later shows itself as anorexia, fever, and malaise and a drop in blood cell counts. The rate of loss of these cells can be used to estimate the received dose and to indicate the severity of the response. Recovery is possible at lower doses but for higher doses death follows, usually within a few months, due to infection and haemorrhage. The LD50/60 (the dose that will kill 50% of those exposed in 60 days) is about 2.5 to 5 Gy (250 to 500 rads).
Gastrointestinal (GI) syndrome (dose from 6 to 10 Gy). The radiation affects the ability of the body to generate new cells to line the gut to replace those worn out. This causes infection, dehydration, and electrolyte imbalance. Survival is unlikely. Short term symptoms are anorexia, severe nausea, vomiting, cramps, and diarrhoea. A latent stage, where the patient may appear and feel well of generally less than a week may occur but the stem cells in the bone marrow and the cells lining the gut are dying. Later symptoms are malaise, anorexia, severe diarrhoea, fever, dehydration, and electrolyte imbalance. Death, due to infection, dehydration, and electrolyte imbalance, usually occurs within 2 weeks. LD100 (the dose that will kill 100% of those exposed) is about 10 Gy (1000 rads). Cardiovascular (CV)/Central Nervous System (CNS) syndrome (dose from 20 to 50 Gy). This is thought to be due to the collapse of the circulatory system, oedema (swelling), vasculitis (damage to blood vessels), and meningitis (acute inflammation of the protective membranes covering the brain and spinal cord). The initial symptoms are extreme nervousness and confusion; severe nausea, vomiting, and watery diarrhoea; loss of consciousness; and burning sensations of the skin. Onset occurs within minutes of exposure and lasts for minutes to hours. Later symptoms, possibly after a brief period of partial functionality, are return of watery diarrhoea, convulsions, and coma within 5 to 6 hours after exposure. Death occurs within 3 days of exposure.
In addition cutaneous radiation syndrome, that is damage to the skin and related organs, can be important. This can be caused by external radiation, by contamination of the skin or by a combination of the two and can be complicated by conventional injury to the skin, such as thermal burns received, at the same time. The signs of cutaneous radiation syndrome include erythema (redness caused by dilation and irritation of the superficial capillaries) and skin oedema (swelling caused by a build-up of fluid) which are similar to sunburn, blistering, epidermal denudation, dry desquamation (peeling of the skin), moist desquamation (skin thinning and weeping), ulceration and, especially after high dose skin exposure at high dose rate; hair and nail changes, epilation (loss of hair which may be temporary) and Onycholysis (the nail separating from the underlying skin).
The speed of onset of symptoms and their severity can be used to estimate the whole body dose that somebody was exposed to and a number of crib sheets for doctors who find themselves with irradiated patients are available on the internet. For example the table below (From Radiobiology of the acute radiation syndrome)
| Dose | Time to onset of Vomiting | % of incidence |
|---|---|---|
| Mild (1 -2 Gy) | > 2 hr | 10 - 50 |
| Moderate (2 -4 Gy) | 1 - 2 hr | 70 - 90 |
| Severe (4 - 6 Gy) | < 1 hr | 100 |
| Very Severe (6 -8 Gy) | < 30 minutes | 100 |
| Lethal ( > 8 Gy) | < 10 minutes | 100 |