Outside the evacuation zone: Health effects of low dose radiation

Media coverage and commentary on the crisis in Japan’s Fukeshima Daiichi nuclear plant has focused intensely on possible acute effects from radiation exposure.

In response, a local expert has contacted the Science Media Centre with the following comments on a topic he thinks is more relevant for people outside the ailing nuclear plant’s evacuation perimeter, including Kiwis who are wrestling with whether or not to leave Japan.

What are the potential long term risks from exposure to low doses of radiation?

Dr Peter Roberts is a former senior scientist and manager with the NZ Institute for Geological and Nuclear Studies (1974-2001), now a consultant on ionising radiation for the International Atomic Energy Agency (IAEA), government and the private sector. He comments:

“The effects of high radiation doses over a short period are quite clear. But what of much lower doses over a longer period, the type of risk that the general public outside the immediate vicinity of the Fukushima nuclear plant may be exposed to?

“The unit of dose that measures the effect of radiation on the human body is the Sievert (Sv), but for low doses it is easier to think in terms of milliSievert, one-thousandth of a Sv or mSv.

“4000 mSv is a high dose that if given in a short time will kill about 50% of those receiving it in 30 days. Their bone marrow fails and, with it, the supply of blood cells so that massive infection occurs. Higher doses kill more quickly with other symptoms.

“At 1000 mSv, people become sick, but will recover. These high doses are in the range that the workers within the Chernobyl plant received and which some of the Fukishima workers fighting the reactor problems may be at risk of receiving.

“However, outside the plant boundaries such doses will not be received by members of the public. Doses will be far lower as they will not be exposed to the direct radiation from exposed fuel rods. They will be exposed to a rapidly dispersing radioactive plume. At doses well below 1000 mSv received over a period of time, people will not get sick, but their chance of contracting cancer over the next 5 to 40 years will be increased.

“To get a handle on these risks it is useful to think of the background radiation that everyone is exposed to from cosmic rays, and to natural radioactive materials in the soil, air and our food. This dose is approximately 2 mSv per year. However, it can range from 1-6 mSv per year in different locations of the world. Various medical diagnostic tests (simple x-rays to CT scans) can each be a fraction of annual background or several times greater.

“Radiation workers under routine conditions are allowed 20 mSv per year averaged over 5 years, with no more than 50 mSv in any single year. These limits are based on estimates of the risk of contracting cancer if the worker is at the limit for a working life of, say, 40 years. In this case the usual risk of cancer in one’s lifetime, say 20%, is raised a few percent. In practice workers are always well below their limits in routine situations, they are monitored and they receive some benefit from the work they do.

“The same estimates of risk can be used to estimate the risk of cancer from background radiation received over the first 50 years of life. It is in the region of 1% on top of risk from other causes. This is an estimate; the risk could be a bit higher or lower. But it is worth knowing that the no increase in cancer rates have been detected in areas of the world with high background levels; in fact in at least one high background area the risk is lower than ‘normal’.

“The risk estimates come from data from several sources such as Atomic-bomb survivors, uranium miners and other groups exposed to moderate doses. The data are good, but the extrapolation back to much lower doses is subject to argument.

“What does this mean in summary? No one should be exposed to any unnecessary dose of radiation, and a precautionary approach near the Fukushima plant is essential. However, if general radiation levels a little above background are received for a short period, then the increased risk is within the natural variation in cancer risk. At doses approaching those where people are acutely sick but survive, the future cancer risk is a few percent.

“One further note of caution concerns radioactive isotopes within the plume that target specific organs. These can concentrate the dose in these organs and must be a major concern of monitoring and remedial activities. The classic example is iodine-131 which locates to the thyroid and caused a few thousand thyroid cancer cases subsequently after Chernobyl. Iodine-131 decays away within a few weeks. It can be blocked by iodine-containing tablets.”