Twenty five years ago, during the Chernobyl disaster, I was a physics undergraduate in the UK. I clearly remember our lecturers at the time scrambling as they realized that, falling into complacency since earlier nuclear facility disasters such as Winscale in the UK and Three Mile Island in the US, they had neglected to give us a sound grounding in the health implications of exposure to radioisotopes and ionizing radiation.
Twenty five years on, watching the events unfolding around the tragic earthquake in Japan and the failure of the Fukishima nuclear power plant, I’m experiencing a strong sense of déjà vu. Except this time round I’m the lecturer, and it’s my students that are asking the tough questions.
As the situation develops in Japan, I am having to brush off stuff I haven’t thought about in over two decades as I try to make sense of what is happening, and its potential implications to the health of those in the vicinity and further abroad. And it isn’t helping that there seems to be a dearth of concise and accessible information on the web that might help interpret the news reports on radiation leaks and exposures.
Realizing that others are probably struggling with the same issues, here’s a really quick run-down of some of the top questions I’m currently grappling with:
What are current radiation exposure levels around the Fukushima reactor site?
On Saturday March 12, the International Herald Tribune was reporting radiation levels outside the plant as 1.2 micro Sieverts per hour(µSv/h). According to a March 13 news release from the Japanese Nuclear and Industrial Safety Agency boundary measurements on March 13 recorded 40 µSv/h at the boundary of reactor unit 1, and between 0.07 µSv/h and 4.5 µSv/h elsewhere around the plant’s boundary. At 15:29 (local time) on March 12 a measurement exceeding 500 µSv/h was monitored at the site boundary. And Geoff Brumfiel at Nature News reported this morning measurements of 21 µSv/h at the plant
And what is a micro Sievert anyway?
Rather confusingly, radiation measurements are a mess of non-standard units, measures of radiation emission, measures of how much your body will absorb, and measures of how much potential damage exposure might be caused. So here’s a quick run-down of what’s what:
Radioactive material activity. The activity of radioactive materials is characterized by the number of times per second atoms within them undergo radioactive decay, releasing alpha, beta and/or gamma radiation. The old unit here – which is still widely used – is the Curie (Ci). One Ci is equivalent to 37 billion decays per second.
In SI units, activity is measured in Becquerels (Bq) – one Bq is equivalent to one disintegration per second, so one Ci is equivalent to 37 giga Becquerels – or GBq.
Radiation Absorbed Dose. Radioactivity itself doesn’t indicate how harmful something is – this depends as much on the energy and the type of radiation. To get a better idea of how damaging a particular source of ionizing radiation might be, Radiation Absorbed Dose is used. The non-SI unit for radiation absorbed dose is the rad, and is equivalent to the absorption of 100 Ergs of energy per gram of absorbing material. An Erg by the way is an archaic (in my books) measure of energy!
In more “sensible” units, radiation absorbed dose is measured in Grays (Gy). One Gy is equivalent to the absorption of 1 Joule of ionizing radiation by kilogram of absorbing material. Named the Gray after the British physicist Louis Harold Gray.
Biological effect of ionizing radiation. Unfortunately, even radiation absorbed dose doesn’t give a clear indication of how damaging exposure to ionizing radiation is likely to be. For this, an equivalent dose is needed that takes into account the different levels of harm that can be caused by different forms of ionizing radiation. The non-SI measure of equivalent dose is the Röntgen Equivalent in Man, or rem. Rems are the product of radiation absorbed dose in rads, and a weighting factor, which depends on the type of radiation and tissue exposed.
In SI units, equivalent dose is measured in Sieverts (Sv). Sieverts are the product of radiation absorbed dose in Grays, and a weighting factor which depends on radiation type and type of tissue that is exposed. Fortunately, the conversion between Sv and rems is relatively simple – 1 Sv is the equivalent of 100 rem
For both rem and Sv, the weighting factor depends on the type of radiation, and the type of biological tissue that is exposed. Wikipedia has a useful list of weighting factor components.
Because the effects of radiation are cumulative, biologically-relevant exposure is usually measured with respect to time – usually Sv per hour (Sv/h) or Sv per year (Sv/year). And because a whole Sv is a massive dose – certainly above the level of concern – exposures are frequently measured in micro-Sieverts, or one millionth of a Sievert.
So a micro Sievert per hour – or µSv/h – is a measure of how much biologically relevant radiation exposure someone is receiving each hour.
Are these levels of radiation dangerous?
Exposure to ionizing radiation is part and parcel of living on Earth – not only are we constantly bombarded by radiation from space, but the the Earth itself is significantly radioactive – this is what keeps the planet’s core hot and powers volcanoes, plate tectonics and other geological phenomena. As a result, we are exposed to an average background radiation level of around 2.4 mSv/year -or roughly 0.27 µSv/h. According to the National Council on Radiation Protection and Measurement (NCRP 93), we are typically exposed to a further 0.63 mSv/year (0.07 µSv/h) from artificial sources. These figures are nicely presented by the University of Idaho Radiation Information Network, along with a list of radiation dose levels from various sources.
These background levels aren’t necessarily safe – but they are considered acceptable. The US Nuclear Regulation Commission (NRC) estimate the risk of death associated with exposure to 30 µSv as one in a million – the same as spending two days exposed to the air pollution in New York.
At higher levels, things get more serious – but the levels at which substantial harm occurs are much higher than background – or even those being experienced in Japan. According to the Centers for Disease Control and Prevention (CDC), Acute Radiation Syndrome (ARS) occurs at exposures above 0.7 Gy (although mild symptoms can be experienced as low as 0.3 Gy). Unfortunately, this isn’t that helpful if exposure is measured in terms of Sv. But assuming a weighting factor of 0.24 (assuming beta radiation and target organs/systems such as bone marrow, colon, lungs, breasts, stomach), 0.7 Gy is the equivalent of being exposed to 233 µSv/h constantly for one month, or 1000 µSv/h for one week, or 7000 µSv/h for one day.
In other words, current levels of radiation being experienced around the Fukushima reactor are generally well below those that raise concerns for acute effects.
Chronic health impacts are harder to gauge. The US EPA has qualitative information on the health impacts of ionizing radiation, but very little quantitative information. Likewise, the US Nuclear Regulatory Commission has recently released a fact sheet on the biological impacts of radiation, which is similarly non-specific. What is know is that elevated levels of radiation are associated with increased incidence of cancer, but the relationship is a complex one – and dependent amongst other things on the type of radioactive material, the route of exposure, and the body’s natural ability to repair damage to cells and their internal structure caused by radiation.
Surely the type of material released matters, as well has how radioactive it is?
Although measures of equivalent radiation dose account for radiation types and tissue susceptibility, they don’t account for the specific risks represented by specific materials. With radioactive material leaks from nuclear power plants, the three materials of particular concern are radioactive Iodine, Caesium and Strontium.
Iodine-129 and Iodine-131 are radioactive by-products of fission. As with any source of Iodine, if ingested they are rapidly taken up by the thyroid gland – especially in children. Because of this potential for a highly localized and concentrated source of radiation if exposed to radioactive Iodine isotopes, it is common to administer non-radioactive Potassium Iodide to people if exposure is possible – blocking the uptake of the radioactive material. According to the New York Times, the International Atomic Agency is preparing to distribute Iodine to people living near the Fukushima power plant.
Caesium-137 is a highly radioactive by-product of fission with a relatively long half life of 30 years. Current exposures are almost entirely due to artificial generation in nuclear events. Clearance from the body following exposure is relatively rapid according to the US EPA.
Strontium-90 – also a fission by-product – behaves very similarly to calcium if ingested, with the result that it accumulates in bones. This accumulation leads to long term exposure to sensitive tissue, leading to it being associated with bone cancer, cancer of the soft tissue near the bone, and leukemia
Should residents on the west coast of the US be concerned?
Over the weekend there were false reports of a plume of radioactive material being carried from Japan, via the gulf stream, to the west coast of the US. Current indications are that the release of radioactive material from the plant is low, and remains localized. However, this is still a developing situation, and there is considerable uncertainty over whether the cores of all reactors on the site will be successfully contained.
There’s lots of stuff being written on the unfolding events in Fukushima on the web, but these are some of the resources I found to be more helpful:
Nuclear Experts Explain Worst-Case Scenario at Fukushima Power Plant (Scientific American)
Idaho State University’s Radiation Information Network
Canadian Center for Occupational Health and Safety Information on Ionizing Radiation
US Nuclear Regulatory Commission Fact Sheet on the Biological Effects of Radiation
Wikipedia on Radiation Poisoning
Added 3/13/11, 1:04 PM: A long but useful description of the Fukushima reactor vessel, the processes currently playing out, and the chances of large scale radioactive material release (very low)
Added 3/13/11, 1:12 PM: This BBC News analysis piece contains useful information on potential impacts, given the nature of the crisis.
Added 3/13/11, 4:19 PM: I forgot to include this link from the International Atomic Energy Agency earlier.
Added 3/15/11 10:02 AM: This piece from Geoff Brumfiel and Nature News provides a useful overview of the exposure numbers
Added 3/16/11 6:53 AM: World Health Organization resources on the Japan nuclear crisis.
Added 3/19/11 8:23 AM: A useful piece on radiation exposure and potential impacts from MITNucreal Science & Engineering Nuclear Information Hub.
Added 3/20/11 10:52 AM: A useful chart of comparative radiation dose
There are bound to be many other useful resources out there that people will benefit from as the crisis unfolds – if you know of any, please do add them to the comments below.