There's been a lot of concern about the events taking place in Japan with the nuclear reactors. The media is really mixed in it's reporting, but much of it is designed to be sensational rather than educational or factual. I am a medical imaging program director with all sort of initials behind my name that don't mean a lot to any of you. I teach radiobiology and radiation physics. I'm responsible for ensuring 50 students a year understand radiation safety for themselves, their patients and the general public. All that is to provide professional background. Most people are afraid of radiation, yet most really don't have any understanding of what it means. Just this week one of my students had a patient get very upset when she left her glasses in the room after a chest x-ray. She was convinced they were now radioactive. So, I'm going to post some basic information about radiation dose and what that means from a biologic perspective. Feel free to post questions and I'll try and answer. I will leave answers regarding the physical aspects of the nuclear plant to a couple of poster who actually work at such facilities, but feel free to post those questions as well.
1. All ionizing radiation has the potential to cause biological harm. That includes UV and infrared light, x-ray, particulate radiation and gamma rays (what's coming from the reactors). Potential is the key. The probability of any harm actually occurring is dependent on how much radiation (dose) is received, to what body part (whole body vs. selected organs), and the quality of the radiation. I could take a piece of Polonium-218 and put it on my desk, cover it with a piece of paper, and receive no radiation. However, if I swallowed that piece of Polonium, I'd probably die because of the severe damage to my GI tract. This is what killed the Russian spy several years ago. That's because the radiation produced by Polonium has a very high rate of transferring it's energy to tissue, but it does it within a couple of millimeters. Gamma radiation, OTOH, has a relatively low rate of energy transfer, but it has a significant range (distance) in which to do so.
2. How far can it travel? It depends. The gamma ray will interact with the air in which it is traveling. It ionizes or causes an electron to be removed from atoms it encounters. The greater the distance it travels, the less energy it has when it encounters something else. It's the transfer of energy along with the removal of an electron that can cause biological harm. So, the farther away you are, the less exposure you will receive. In fact, we call the Cardinal Principles of Radiation Safety : Time, Distance, and Shielding. Minimize the time of exposure, maximize the distance and shielding. Shielding is putting something between you and the radiation.
3. What does it mean when they say levels are 8 x normal? I think everybody knows what 8 x means, so the real question is, is that bad? What's normal?
The other basic principal of radiation safety is called ALARA which stands for As Low As Reasonably Achievable. This is the underlying principal for medical radiography as well as industrial radiation production. We want to keep any dose level as low as possible. Minimum dose levels are set by the International Commission on Radiation Protection and the National Commission on Radiation Protection (US). They base these lower than what research has taught us, sadly much from the Hiroshima and Nagasaki data along with other populations, are the dose thresholds for different biological effects.
4. These minimum levels take into consideration radiation from multiple sources: cosmic, cosmogenic (within the Earth's atmosphere), terrestrial (radon is a prime example), naturally occurring radioisotopes etc. I'll give some sample amounts below. The real key here is that the dose limits are set well below levels of known biological effects. It is undeniable that even a very low dose, one photon, could cause a biological effect, but the statistical odds are really small. My students are always a little annoyed after I've presented all the things that could happen when they do a single chest x-ray when I conclude with the greater probability that they won't. It's always a weighing of risk vs. benefit.
5. Some example radiation doses: if you live in France, you receive approximately 0.00071 Sieverts annually, but if you live in Austria you only get 0.00037. If you smoke you get about 0.08 Sievert to your lungs. Dose limits vary but the goal is to keep public exposure below 1milliSv/year for a total body exposure (individual organs and the lens of the eye are somewhat higher). That's less then what your lungs are getting all ye smokers. This would be based on medical x-ray exposure or anything above the natural background exposures. The exposure coming out of the plant was approximately 70 microSv on the last report I saw. Meaning somebody exposed would be well below the accepted minimum annual limit. We would not expect them to have any biological ramifications.
I don't use Wikipedia for references, but I'll be happy to supply a list of technical journal or book references if anybody so desires. I know how some want their links.