It’s too bad that it’s this difficult. For the environmental data, one must go from counts per second (becquerels) to energy absorbed (grays) to biologically effective energy absorbed (sieverts) to cancer risk via BEIR VII. That last step has been most of what I’ve done in this series. So it’s not surprising that this aspect, which is what most citizens are most interested in, is dealt with poorly by the media, which present multiples of various radiation standards, not enough to understand health risks.
From TEPCO, via the International Atomic Energy Agency, as of the end of March, twenty-one of the emergency workers had received combined internal and external doses of more than 100 mSv. Two workers had effective doses of 200-250 mSv; eight workers had effective doses of 150-200 mSv; and eleven workers had effective doses of 100-150 mSv. These range from twice the yearly allowable dose for workers to five times.
What does that mean in cancer risk? In the post before the last, I pulled some numbers from the BEIR VII tables:
The lifetime attributable risks of cancer incidence and mortality of the one-time doses, 100 mSv for protection of valuable property and 250 mSv for protection of human life, are found in Tables 12D-1 and 12D-2. If we assume that the workers are male and their age is 40, then 100 mSv gives 648 cancer cases and 337 cancer deaths per 100,000 people exposed; 250 mSv gives 1620 cancer cases and 843 cancer deaths per 100,000 people exposed.If we compare that with the numbers of cancer deaths normally observed in the general population (42 cases per 100 people, or 42,000 per 100,000 people), that’s an increase in the probability of developing cancer of 1.5% for those exposed to 100 mSv and 3.9% for those exposed to 250 mSv.
As reported by the Federation of Electric Power Companies of Japan in their Weekly Update for May 25, 2011, the radiation level at the west gate (approximately 3,609 feet from Unit 2 reactor building) of Fukushima Daiichi Nuclear Power Station was 15.5 μSv/hour at 9:00PM on May 25. In order to accumulate a 100-mSv dose, a person would have to stay at that gate for 268 days. Note that the measurement is in micro (μ) sieverts and the dose is in milli (m) sieverts. A reading of 350-400 μSv/hour south of the main building (TEPCO) would require a little more than a day to accumulate a 100-mSv dose.
Recently, the Japanese government decided to raise the permissible radiation exposure to school children from 1 mSv per year to 20 mSv per year. Parents protested, and the government rescinded the change. The Japanese decision to raise the limit was probably based on the recommendations issued by the International Commission on Radiation Protection, which state
When the radiation source is under control contaminated areas may remain. Authorities will often implement all necessary protective measures to allow people to continue to live there rather than abandoning these areas. In this case the Commission continues to recommend choosing reference levels in the band of 1 to 20 mSv per year, with the long-term goal of reducing reference levels to 1 mSv per year (ICRP 2009b, paragraphs 48-50).Children are more sensitive to radiation damage than adults, although the increase in sensitivity is not well known. For an adult, an increase in exposure to 20 mSv results in a fraction of a percent increased cancer risk. If we assume a tenfold increase in children’s sensitivity, which is probably high, then the children’s cancer risk increases by a few percent.
Measurements of environmental radioactivity are in becquerels per area or volume. Becquerels are indicative of the amount of a radioactive substance that is present and not simply translatable into dose received. The dose received also depends on the amount of time a person spends in a radiation zone. This is what makes evaluating the contamination in terms of cancer risk even more difficult.
It’s important to recognize that today’s measurements in Japan will decrease over time if there are no further significant releases from the reactors. The three primary isotopes accounting for the contamination are iodine-131, cesium-134, and cesium-137. Iodine-131’s half life is about eight days, so it should be approaching nondetectability in another month or so. Cesium-134’s half-life is about 2 years and cesium-137’s half-life is about 30 years, so they will http://www.blogger.com/img/blank.gifbe around longer, but their activity will decrease over time.
Setting radiation standards is an exercise of judgement. What percentage increase in cancers is acceptable? When the question is isolated like that, most people would say none. But most jobs have some hazards associated with them, even the hazard of inactivity in sitting at a desk in a clean office. The hazards of residual environmental radioactivity have to be balanced with removing people from their homes, which presents its own set of health hazards.
These are difficult decisions, made more difficult by the widespread lack of understanding of exactly what the radiation hazards are.
Cross-posted at the BMJ Blog.