Breaking the myths of nuclear catastrophe into microsieverts
And so, on this March 17th 2011, radiation is continuing to leak from the TEPCO Fukushima Dai-ichi nuclear power plant, following the magnitude 9.0 earthquake and tsunami, but at what rate? I keep hearing conflicting reports about radiation levels monitored, recommended exclusion zone distances, and the possibility of health effects. Will this affect my travel plans to Japan in November 2011? I decided to do some general study into comparatively toxic levels (measured in millisieverts) in different circumstances, and collated some readings from the Japanese Government‘s official data releases (from the MEXT website) of radiation levels that have been detected in various major cities in Japan, and at various outposts within the 80km international exclusion zone. Bear in mind the date of this essay, the potential for further increases in radiation release cannot at this stage be excluded.
It all comes down to millisieverts (mSv). In Australia, the average annual radiation is 1.5mSv (3-6mSv in America). Smoking causes annual exposure to 10-60mSv. The legal limit for nuclear workers in the US is 20mSV annually. Nuclear power workers and airline workers can be exposed from 2-10mSv annually. Radiographers average 3.2mSv annually. Sleeping in bed with a partner every night for a year exposes you to 0.18mSv, whereas living next to an “intact” nuclear power station for a year exposes you to a maximum of only 0.01mSv, the same exposure as you get from eating 20 bananas per year. In terms of single doses, a dental xray is 0.005mSv, a mammogram 3mSv, CT brain up to 5mSv, a CT chest up to 15-20mSv, a plain abdominal series 14mSv, and a chest xray 0.01mSv. A fully body CT would be up to 45mSv. US emergency services personnel are allowed to be exposed to a single dose of 500mSv while preventing a major nuclear escalation, while single exposures of 1000mSv are allowed when lives need to be rescued.
In terms of health effects, it’s conservatively postulated from a linear expression that every 10mSv exposure adds 0.05% to your lifetime risk of cancer, an 0.5% increased risk after continuous exposure at 20mSv/yr has also been reported, although increased cancer rates have only been proven to occur after exposures of more than 100mSv (as a single dose) or over 100mSv per year (ie. 20 head CTs, or 5 chest CTs). By the same linear extrapolation, a single exposure of 1000mSv increases your lifetime risk of cancer by 5%. A dose of well over 50mSv is required to cause a miscarriage in the first 2-3 weeks of pregnancy, doses greater than 300mSv may cause severe developmental problems in the 3rd-8th week of pregnancy, and the risk (of congenital abnormalities, thyroid cancer and leukaemia) decreases over time so that the risk to a foetus over 20 weeks’ gestation is practically identical to the risk presented to an adult exposed to the same amount of radiation. Exposure of over 50mSv/year for 30 years can lead to cataracts. A single dose is comparative to a cumulative dose, in terms of carcinogenicity, although larger doses are safer if staggered over time.
The radiation levels in Japan peaked at 400mSv/hr next to reactor 3 of the Fukushima Da-ichi plant. A person standing next to the plant continuously at these levels would suffer bone marrow toxicity (200mSv) within half an hour, and acute radiation sickness (500-1000mSv) after 2 hours. At these levels, 50% of people would die after 11 hours of exposure (4500mSv), from serious bleeding and infections. After 24 hours of exposure (>10,000mSv) no one would survive. By contrast, at Three Mile Island, the closest resident was exposed to less than 1mSv. The levels have dropped to around 80-700mSV/yr (10-80uSv/h) at the outposts 30km around the reactor (noting that >100mSv/yr is proven to be carcinogenic). Tokyo is experiencing radiation of 0.05uSv/h* (although had a brief wave of up to 10uSv/h following the last reactor explosion), Kyoto 0.04uSv/h, Osaka 0.05uSv/h, Hiroshima 0.05uSv/h*, and Sendai (the area affected by the tsunami just north of the plant) 0.14uSv/h* (or 1.3mSv per year, the same as background leves in Australia). All of these readings are below the level at which health becomes affected. Even if you went to visit Japan for a week and stayed 30km from the nuclear reactor, your total exposure would be up to 13mSv** (the equivalent of an plain abdominal xray series or a CT chest). Living on the cusp of the exclusion zone at the current radiation wouldn’t result in a measurable increase your lifetime risk of cancer until you had been exposed continuously over 6 months**.
A week long visit to the outskirts of the nuclear reactor zone would increase my risk of cancer by 0.07%**, this would be essentially zero in any of the major capital cities. Even camping outside the front gate of the reactor would be unlikely to increase your cancer risk in the first 100 hours. Well, now i’ve got that into perspective, time for me to book those incredibly cheap flights and support the Japanese economic recovery.
Disclaimer: Radiation levels may be subject to change. Watch for updates.
* = all readings current as at 17/3
** = 30km from the Fukushima plant, the peak radiation level detected on 17/3 was 170uSv/h, which would expose you to 30mSv over a week, increasing your lifetime risk of cancer after 3-4 weeks of continuous exposure at this level (however, very safe nearby readings are as low as 1-3uSv/h). A linear extrapolation shows that a week’s exposure at the higher level of radiation could potentially increase your lifetime risk of cancer by0.15% (although this is below the threshold for statistically proven carcinogenesis).
See more on up-to-date radiation monitoring from the Japanese Government (MEXT) at http://eq.wide.ad.jp/index_en.html.