Assuming that the measurements are accurate and the radiation releases are steady, which they probably aren't, the radiation levels for 30km out translate to potential elevated cancer levels (100 mSv) after 24.5 days, and for 60km out, elevated rates after 347 days. Of course, it's much more complicated than that, but I know I'd be very concerned at that rate of radiation release if I lived anywhere within those zones.
But there is another issue that was not really discussed, and that's biological concentration. Depending on the type of radiation, merely showering off the particles only addresses the immediate threat. If it gets into the water and vegetation, then the process of biological concentration begins, where plants filter it out of the water/soil, concentrating it, then the plant material gets eaten, concentrating it in the milk/flesh of the animal, to be further concentrated by the animal eating that animal. Persistent radioactivity in the environment can be a pretty serious issue when viewed this way, and there has already been concerns about both vegetation and milk showing trace radioactivity in the area, plus radioactive runoff into the ocean from attempts to pour water into the cooling pools of the reactor.
What would be very useful for folks to know is the nature of the radioactive particles being released and its half life. From what I can tell, it sounds like some of the forms are fairly persistent, hence the concern.
Finally, a minor correction is in order: the scale of the Fukushima accident was retroactively raised to a 5 last week, and hopefully won't go up any more!
Corrected on 21 March to reflect updated alert level.
Over a week after the magnitude 9 earthquake and the massive tsunamis it triggered, information continues to coalesce on the health risks from the Fukushima Daiichi nuclear power station explosion. Here’s a roundup of what various people are saying about radiation and other threats.
First, here is the latest info from the Japanese government on radioactivity measurements. And here’s today’s update on the location of the radiation plumes.
1. On millisieverts and cancer
Radiation fears may be greatly exaggerated. Everyone is exposed to small amounts of radiation every day just from living on earth or flying in an airplane.
That all adds up to about 2.4 units, known as millisieverts (mSv), a year – though it varies from 1 to 10 mSv depending on where you live. The average American absorbs 6.2 mSv a year. US guidelines recommend evacuation with 10 to 50 mSv.
In Tokyo, 150 miles from the nuclear plant, readings rose about 10 times above the norm. That’s 0.809 microsieverts (not milli) per hour – or about 10 times less than a chest X-ray.
A Japanese broadcast reported 0.17 mSv per hour 30 km northwest of the reactor. There’re also reports of 0.012 mSv/hr in Fukushima City, 60 km away from the plant.
In terms of long term health effects, it’s generally acceptable to make a worst-case estimate by multiplying the dose by hours in the day and days in the year. That’s roughly 1,500 mSv/yr around 30 km away and 100 mSv/yr in Fukushima City. An exposure rate of 100 mSv/yr is considered the threshold at which cancer rates begin to increase, and 1,500 mSv/yr is certainly dangerous.
To be lethal, the blast of radiation would have to top 5,000 mSv delivered within minutes. Measurements at the damaged plants are at 400 mSv. Unprotected workers may have been exposed to about 4 times the level for elevated cancer risk (or 20 times the annual exposure for uranium miners).
Fukushima was rated a 4 on a scale of nuclear incidents, but that level has been raised to a 5. Three Mile Island rated a 5 and had no impact on cancer incidence in the region. Chernobyl was a 7, and people exposed to radioactive fallout showed higher rates of thyroid cancer because the gland sequesters radioactive iodine.
2. And iodine
Potassium iodide, which there are ample supplies of, prevents radiation poisoning of the thyroid gland. The drug blocks the uptake of radioactive iodine by filling the gland with a safer form of iodine.
But potassium iodide can interfere with the body’s normal production of thyroid hormone, leading to hypothyroidism, or can provoke an already diseased thyroid gland to make too much of the hormone.
3. Shower off the plume
Outside the immediate vicinity of the nuclear site, the primary danger is airborne radioactive material released into an atmospheric plume. That material can pose additional hazards if inhaled, swallowed or absorbed through the skin to emit radiation from inside the body.
“If there’s a plume that passes overhead and some of the material precipitates down, you may be externally contaminated, but it’s nothing that a change of clothes and a shower can’t take care of,” according to a medical physicist from the University of California, Davis.
Finally, the World Health Organization says that the public health risk from the radiation leak appears to be quite low.
4. Food contamination?
Many countries have started to test food imported from Japan for radiation. But radiation experts say there was virtually no chance of major contamination of industrial products, even if the leakage were to worsen. Particles like the ones containing escaped radioactive iodine or cesium can be deposited on products, but given the nature of the manufacturing industries in Japan, there is little danger of contamination reaching harmful levels.
5. Other (ignored) health threats
Radiation fears divert attention from other (and worse) health threats. These include disrupted supplies of safe drinking water and the disposal of sewage to prevent outbreaks of diseases like typhoid and cholera.
Image of the ocean floor by Japan from NOAA
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