There has been a lot of talk about radiation exposure levels in Japan, both to workers and the general public.
This chart on radiation exposure as a factor of banana consumption from xkcd has been making the rounds and does help with understanding a lot.
The following is intended to provide a more in-depth and technical primer on the issue of radiation and health effects.
Measuring Radiation Exposure:
Exposure to ionizing radiation is commonly measured in total cumulative dose. Exposure can occur through a number of different paths. It can come from an external source, such as a gamma emitting isotope, from an internal source, such as inhaled or ingested radioisotopes or from a combination of the two. The path of exposure along with the type of radiation (alpha, beta, gamma or neutron) as well as the energy level of the radiation will all affect what biological effect occurs.
In order to quantify the biological effects of radiation exposure, the rem (Roentgen equivalent man) was created. Rems are a weighted to approximate the total biological effect on a human of radiation exposure, which can through either contact, proximity or internal exposure. Rems are also commonly expressed as the unit mrem (for millirem) or one thousandth of a rem.
In the late 1970′s, a new unit the sievert (abbreviated Sv) was created. The conversion from rems to sieverts is very straight forward. One sievert is equal to 100 rem. The sievert has been adopted as the Standard International unit for measurement of radiation dose, and is now considered the preferred unit for scientific settings. However, the rem also continues to be widely used.
Because the sievert is a very large unit of dosage it is very often expressed as milliseiverts or microseiverts.
Conversions (from Wikipedia)
* 1 rem = 0.01 Sv = 10 mSv
* 1 mrem = 0.00001 Sv = 0.01 mSv = 10 Î?Sv
* 1 Sv = 100 rem = 100,000 mrem (or millirem)
* 1 mSv = 100 mrem = 0.1 rem
* 1 Î?Sv = 0.1 mrem
“Dose rate” is another measure, commonly used for ambient radiation levels. It represents the dose a person will receive in a given amount of time. Thus if the dose rate is said to be 1 mSv/hr, then a person exposed for one hour would receive 1 mSv of radiation. However, this does not mean anyone exposed to such an intensity will have this dose. Being exposed to such a level for half an hour will result in only .5 mSv of radiation. If a dose rate this high only exists for a brief period of time, then the total exposure is a factor of how long the high rate occurs for.
For example, a dental X-ray head may produce radiation with the intensity of more than one Sv per hour, but it only is turned on for a fraction of a second. Therefore, knowing the peak dose rate tells you very little if the time is not given.
Personally, I like rems and millirems. Perhaps it’s because I’m American and thus I was brought up to think in screwy units. Thankfully, in this case, it’s easy to convert between the two, unlike pounds, feet and degrees Fahrenheit.
*In actuality, dose measurement is a bit more complex than this, because radiation doses can be measured as being full body dose, skin dose or the dose to a certain organ or region of the body. To keep things simple and because it applies to the circumstances of exposure from a reactor incident, this article is based on full body dose.
Average Annual Dosage:
Lifestyle: A person who happens to live in an area with very low geological radiation, does not live in a masonry building, does not visit masonry structures very often, does not fly or flies very infrequently and spends the majority of their time at a low altitude.
About Average: 3000-5000 microsieverts (300-500 mrem)
Lifestyle: Average. People who live in most parts of the United States, Europe, not very high above sea level,fly occasionally and have other moderate sources of exposure.
Above Average, but not abnormal: 5000-10,000 microsieverts (500-1000 mrem)
Lifestyle: A person who lives in a high altitude city such as Denver Colorado will receive about 400 microsieverts per year right off the bat. If they happen to live in a masonry structure, cook with natural gas or get a few dental x-rays, this will further increase their annual dose. An annual dose o 10,000 microsieverts is above what mot people get, but not unusual.
Moderately High: 10,000-25,0000 microsieverts (1000-2500 mrem)
Lifestyle: It is not typical to get this high an annul dose of radiation exclusively due to natural sources, but it is certainly possible. A few areas of the world have been found to have very high background levels which expose residents to such levels every year. It is also possible that a person’s radiation dose could be pushed up into this level if they required a few medical imaging procedures in a year.
High: >25,0000 microsieverts (>2500 mrem)
Lifestyle: Only on rare occasions would this level be the result of natural sources exclusively, this dose is routinely exceeded in a year by those who need multiple medical imaging procedures. Cancer treatment often results in doses many times higher than this.
Single Event Exposure:
(typical numbers. Actual dose may be slightly more or less)
Extremity X-ray (arm, leg): 10-30 microsieverts (1-3 mrem)
Single Skull X-ray: 80 microsieverts (8 mrem)
Single chest X-ray: 10o microsieverts (10 mrem)
Thyroid scan: 14o microsieverts (14 mrem)
Mammogram Session: 400 microsieverts (40 mrem)
Hip/Pelvis X-ray: 650 microsieverts (65 mrem)
Lumbar Spinal X-ray: 1,200 microsieverts (120 mrem)
Head CT Scan: 2,000 microsieverts (200 mrem)
Upper GI X-ray: 2,250 microsieverts (225 mrem)
Barium Enema (full film series): 5,000 microsieverts (500 mrem)
Chest/Upper Torso CT Scan: 7000 microsieverts (700 mrem)
Abdominal/Pelvis CT Scan: 10,000 microsieverts (1000 mrem)
Full body CT Scan: 12,000 microsieverts (1200 mrem)
CT-Scan Heart Angiogram: 20,000 microsieverts (2000 mrem)
North American Coast to Coast Flight: 30 microsieverts (3 mrem)
Trans-Pacific flight: 50 microsieverts (5 mrem)
Extended Non-Stop flight (eg New York to Hong Kong): 80 microsieverts (8 mrem)
Long Distance Round Trip (eg. London to Sydney and Back): 200 microsieverts (20 mrem)
Radiation levels with acute symptoms (radiation poisoning)
No Acute Symptoms: <.5 Sieverts (<50 rem)
Symptoms: No observable acute symptoms. Possible slight change in blood at very high end of exposure, such as increased white blood cell count.
Time to onset: N/A
Treatment: There is very little active treatment required or possible at such low levels of exposure.
Slight: .5-1 Sievert (50-100 rems)
Symptoms: Mild nausea, possible temporary headache, mild temporary fatigue. Many persons exposed to this level will show no acute symptoms at all. Possible slight increase in risk of some infections.
Time to onset: Several hours to about a day, if symptoms show up at all.
Treatment: Bed rest. Monitoring and observation. Providing fluids.
Mortality: Approximately zero. Otherwise healthy individuals are not in serious risk of death from this level of exposure. Possible complications in individuals with preexisting health conditions.
Slight: 1-2 Sievert (100-200 rem)
Symptoms: Mild to moderate nausea, possible temporary headache, temporary fatigue. Roughly half of individuals exposed to this level will show some signs of . Possible increase in risk of some infections.
Time to onset: A few hours.
Treatment: Bed rest. Monitoring and observation. Providing fluids. In some circumstances antibiotics may be used to reduce the risk of infection.
Mortality: Rare in healthy individuals. Possible complications in individuals with preexisting health conditions. Approximately zero expected deaths with medical intervention.
Moderate: 2-4 Sievert (200-400 rem)
Time to onset: An hour to a few hours.
Symptoms: Moderate to severe nausea, headache, fatigue, diarrhea, fever, possibility of infection. Possible damage to bone marrow ranging from mild to moderate in severity. Possible internal bleeding.
Treatment: Bed rest. Extended period of reduced exertion. Antibiotics. Monitoring for infection. Fever reducers. In some cases, medications are administered to increase the production of white blood cells. Transfusions of red blood cells and platelets may be called. Antibiotics
Mortality: This dose level is life threatening. Even healthy individuals may die within a week without medical intervention. With medical intervention, most otherwise healthy individuals will recover.
Severe: 4-6 Sievert (400-600 rem)
Symptoms: Severe nausea, vomiting, fatigue. Possible loss of consciousness. Significant damage to bone marrow. Bleeding, possibly severe in some cases. Skin ulcers. Organ failure.
Time to onset: Less than an hour.
Treatment: Intensive care. Antibiotics. Surgical or non-surgical intervention to stop bleeding. Blood transfusions. Possibly kidney dialysis or other treatments intended to aid in cases of organ failure. Bone marrow transplant.
Mortality: The prognosis at this level becomes poor. Even healthy individuals have a high probability of death. With medical intervention, the likelihood of survival is less than 50%.
Very Severe: 6-8 Sievert (600-800 rem)
Symptoms: Severe nausea, vomiting, fatigue. Possible loss of consciousness. Significant damage to bone marrow. Bleeding. Skin ulcers and sores. Organ failure.
Time to onset: Less than an hour, possibly as little as minutes.
Treatment: Same as above, but with additional focus on pain management, palliative and hospice care.
Mortality: The prognosis is grim. Although it is possible to survive this high a level of exposure, it’s unlikely. Even with medical intervention, only a small portion of those exposed to this level will survive over the long term.
Unsurvivable: 8+ Sieverts (800+ rem)
Symptoms: Severe nausea, incapacitation, severe internal bleeding, multiple organ failure. Death.
Time to onset: Immediate or nearly immediate.
Treatment: Pain management, palliative and hospice care.
Mortality: 100% Depending on the dose and circumstances, an individual exposed to high doses may survive for a period of a few days to more than a week, but ultimately medical intervention is unlikely to result in longer term survival.
Radiation and Cancer:
There exists considerable debate in the scientific and policy-making sector about the long term effects of low doses of radiation on the probability of developing cancer. There is no doubt that very high doses of ionizing radiation do indeed increase the probability of cancer, but for lower doses it is much harder to measure the effect. Since low levels of exposure would be expected to produce, at most, very low increases in the likelihood of cancer developing, measuring the effect in real world situations is extremely difficult, especially since cancer is already a fairly common condition, with more than one third of humans developing some form of cancer some time in their life.
Early on in the study of radiation and human health, the linear non-threshold hypothesis was formulated. LNT simply means that the impact of radiation on long term health, such as cancer rates, is always directly proportional to the dose and therefore can be extrapolated down to zero. For lack of good data for low dose rates, it was adopted as “worst case scenario” for most policy use.
As scientific data has accumulated, the validity of LNT has come into question. Despite large studies on both humans exposed to radiation and animals, empirical evidence of increases in cancer rates at low doses of radiation continues to be elusive and in some cases the data has been contrary. Studies of populations living in areas with high background radiation levels, such as Ramsar Iran, have found no increase in cancer rates but have found decreases.
Such data supports the concept of radiation hormesis, which holds that increased exposure to radiation up to a point actually decreases cancer rates. The mechanism for this is not entirely understood, but appears to be the stimulation of DNA-repairing enzyme mechanisms in cells. It may also be related to the increased destruction of cells with corrupted genetic material or to increased immune system activity.
Alternately, there is also the threshold model, which holds that radiation exposure has negligible effect, either good or bad, at very low doses.
This is made more complex by the fact that the impact of radiation on health is related to the dose distribution. A single large dose of a radiation having a greater effect than the same cumulative dose if distributed over a longer period of time.
The debate on LNT versus threshold models versus hormesis goes beyond the scope of this post, and the above information is meant only as a very basic primer.
However, if LNT is presumed to be true:
One sievet of exposure (short period of time) = 8% increase in the likelihood of death from cancer
One rem of exposure (cumulative, over about a year) = .04% increase in the likelihood of death from cancer
One sievet of exposure (cumulative, over about a year) = 4% increase in the likelihood of death from cancer
Per individual the risk would be considered higher for younger persons and lower for older, since they are not likely to live as long anyway and cancer does not develop right after radiation exposure occurs but some years later. Thus if a 90 year old person were exposed to a high dose of radiation, there’s a high likelihood that they would die of other natural causes before any radiation-induced cancer had a chance to develop. Because of this the above rates are based on a standard population.
Another way of looking at this is based on the average impact on a standard population per dose, where the number of the probability is expressed as the number of additional cancer cases expected per a given number of persons.
By this measure:
One rem of exposure = One additional cancer-related death per 1,250 individuals
One sievet of exposure = One additional cancer-related death per 12.5 individuals
One rem of exposure (cumulative, over about a year) = One additional cancer-related death per 2500 individuals
One sievet of exposure (cumulative, over about a year) = One additional cancer-related death per 25 individuals
In practice, this would mean that for every rem of exposure to a population of 10,000, there would be an increase in the number of individuals who ultimately die of cancer from about 2,000 to 2,008. (of course the number of 2,000 per 10,000 is very approximate, but it puts this in context.)
Yet another way of quantifying this risk is based on the impact it has on the average life expectancy of an individual in a population exposed to a given dose of radiation.
By this measure:
A person exposed to one additional rem per year for their entire adult life would have a reduced life expectancy of about 50 days.
A person exposed to one additional seivert per year* for their entire adult life would have a reduced life expectancy of 13.65 years.
BUT, IF LNT IS NOT TRUE (which it probably isn’t)
While this does mean that the probability of a small dose of radiation causing cancer is nill, it also means it is a lot more complicated to calculate the actual risk, since it becomes more than just multiplying. To make matters more complicated, there is likely a considerable difference depending on the dose distribution (whether it is all at once or over what period of time the dose occurs).
Current data indicates that the LNT model is not supported by single event dosages of 100 mSv (10 rem) or less or by chronic annual dose levels of 200 mSv (20 REM). Above such dose rates there do appear to be small increases in cancer, although the available data does not indicate that the relationship is linear until much higher doses occurs. At levels o more than half a sievert per event the relationship appears to be more or less linear.
Based on this data, it could be concluded that one time exposure to 100 mSv does not result in an increase in the probability of cancer and that doses of more than 100 mSv but less than 250 mSv may or may not result in a minuscule increase in lifetime cancer risk, depending on the circumstances, and in any event, should not be cause for excessive worry.
Other Health Effects
Aside from cancer, there are a number of other health conditions which are known to be related to exposure to ionizing radiation.
Cataracts – Acute exposure to one to two 1-2 sieverts (100-200 rem) to the eye region is associated with the formation of cataracts. If the exposure is over the course of a longer period of time, the total dose may need to be up to four times higher before cataract formation is detected.
Increased cataracts over the long term, as a result of chronic exposure to lower levels of radiation is less well understood, due in part to the fact that cataracts are common in older individuals to begin with. Current data indicates that only relatively high exposure to radiation is likely to cause cataracts.
Infertility – A dose of 150 mSv (15 rem) to the testies will reduce sperm production and potentially cause temporary infertility in males. This is not enough to cause any measurable long-term effect, however. A similar dose may result in a temporary reduction in egg viability in females. Single doses above one sievert (100 rem) may create longer term infertility. In general, permanent infertility as a result of radiation exposure would require a dose level that would normally be fatal, if the dose is distributed across the entire body. However, if the dose is focused on the gonadal region it is possible for a person to become completely infertile for life without receiving a full body dose sufficient to cause radiation poisoning.
Birth Defects – A single event dose of 250 mSv (25 rem) to a population is believed to result in a small increase in birth defects, although it is more likely to result in non-viable pregnancies than term births of individuals with major birth defects. This topic is so complicated and has so many qualifications that it needs an entire post to itself and really goes beyond the limits of this post.
Chronic gland problems – Radiation is known to cause damage to saliva glands, mucus glands and other glands in the body which can result in insufficient secretions, causing membranes to become dry or irritated. This effect is only seen at very high localized dose levels and is usually found in cancer patients who have received very intense radiation therapy to a local area of the body.
Hair Loss - Most often seen in cancer patients, this is caused by the fact that radiation tends to have a greater effect on fast growing cells, such as hair follicles. It is usually temporary. The effect only happens with very high doses of radiation and is not immediate. Being exposed to radiation will not make your hair suddenly start falling out in clumps.
Radiation and Risk
Radiation Sickness – The Mayo Clinic
NRC Basic References
Typical Patient Exposure
Radiation Exposure: Facts Versus Fiction (University of Iowa)
ANS Dose Chart
An Introduction to Radiation Hormesis
Radiation Hormesis and the Linear-No-Threshold Assumption
This entry was posted on Saturday, March 26th, 2011 at 1:35 pm and is filed under Bad Science, Enviornment, Good Science, Misc, Nuclear. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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