Checking radiation exposure after Fukushima—but it’s hard to gauge how much is dangerous
Few have heard of the Swedish medical physicist Rolf Maximilian Sievert, but his name has been much in use following the Fukushima nuclear leak. To honour his work, the international unit of radiation exposure adopted in 1979 was named the sievert. It is also one of the most ad hoc scientific units ever devised, underlining the fact that gauging the dangers of radiation to the human body is an inexact science.
The sievert is a useful but piecemeal attempt to quantify radiation’s biological effects. There are three types of radiation, each with a different propensity to damage tissue by ionising its atoms (making them electrically charged). The dangers are themselves dependent on the type of tissue exposed. The sievert is an attempt to bundle all these factors into a single measure.
High doses of radiation are obviously hazardous, although the effect of a particular “dose equivalent,” measured in sieverts, depends on how fast it is incurred: a sudden large dose can be worse than the same amount accumulated gradually. There is no consensus on the danger of low doses. Humans evolved against a background of natural radiation; some think the body can cope with small exposure (we have enzymes to repair DNA damaged by ionisation) and that it might even be beneficial. Others say that even minimal increases are risky.
That’s the basic problem in assessing the dangers. Far away from Fukushima—in Tokyo, say, 220km to the southwest—leakage from the plant has only a marginal impact. The average person receives 2-3 millisieverts (mSv) of background radiation a year, and the US Nuclear Regulatory Commission recommends that exposure shouldn’t exceed this by more than 1 mSv. But a single CT scan (X-rays are ionising too) can take you over that limit, while the US maximum exposure level for radiation workers is 50 mSv annually. And even a 5 mSv dose can be fatal if received at once.
At least the numbers supply some context. The peak radiation dose measured inside the Fukushima plant on 15th March, 400 mSv per hour, is terrifying; and you’d do well to avoid 31 mSv, the average total dose of evacuees from Chernobyl in 1986. Perhaps most frustrating for hazard assessment is the variation in radiation levels around the Japanese plant. Within a 30km to 60km radius, these have been found to vary at least a hundredfold, from 0.0005 to 0.05 mSv per hour. Fluctuations are equally wild within the plant’s perimeter, partly depending on the spread of fires.
This makes it very hard to assess the exposure of radiation workers or the local population. And Chernobyl shows that time won’t necessarily tell. The disaster took its toll, particularly in the raised incidence of thyroid cancers in children. Confirmed deaths from direct exposure are around 60, but projections of further deaths vary wildly. While in 2005 the WHO estimated that 4,000 of the 600,000 exposed could eventually die of radiation-related illnesses, other sources put the eventual death toll at hundreds of thousands. Similar disputes dog efforts to determine the cancer risk among people living close to nuclear plants: the validity of a new study commissioned in the US is already being questioned by health physicists. At these low and variable exposure levels, it is extremely hard to link cause and effect.
Selling cells The recent recommendation by the European Court that innovations derived from human embryonic stem cells should not be patentable threatens to undermine all EU work in this field if it is upheld by the European Court of Justice. Without such protection on intellectual property, the research—now starting to prove its value—would largely become untenable. The ruling stems from a case brought by Greenpeace against a German scientist who had patented a technique for turning stem cells into nerve cells. Adjudication of the appeal fell to the conservative advocate-general Yves Bot, who decided that embryonic stem cells should be considered legally equivalent to embryos, a tendentious interpretation both ethically and scientifically.
A final decision may take six months, but in the meantime it casts a shadow over Europe’s leadership in this area, where there has been little of the controversy that has hampered research in the US.
Flaky science? On its final flight, launched on 16th May, Nasa’s Endeavour space shuttle carried a $2bn experiment to test a theory that many experts don’t believe. The Alpha Magnetic Spectrometer (AMS) will look for cosmic rays, free from the interfering effects of Earth’s atmosphere. Cosmic rays are one of the hottest areas in astrophysics: they are thought to originate in extremely energetic processes such as supernovae or immense black holes. But the mission’s spokesman, Nobel laureate Samuel Ting, thinks some may be created by annihilation of matter colliding with stars and galaxies made of antimatter. Few believe such regions exist.
There’s other good science for AMS to do, but it’s ironic that it has been sold as part of the goal to prove the scientific value of the International Space Station, where it will be housed. With the space shuttles retired, soon all of the ISS, good or bad, will be dependent on Russian rockets.
Few have heard of the Swedish medical physicist Rolf Maximilian Sievert, but his name has been much in use following the Fukushima nuclear leak. To honour his work, the international unit of radiation exposure adopted in 1979 was named the sievert. It is also one of the most ad hoc scientific units ever devised, underlining the fact that gauging the dangers of radiation to the human body is an inexact science.
The sievert is a useful but piecemeal attempt to quantify radiation’s biological effects. There are three types of radiation, each with a different propensity to damage tissue by ionising its atoms (making them electrically charged). The dangers are themselves dependent on the type of tissue exposed. The sievert is an attempt to bundle all these factors into a single measure.
High doses of radiation are obviously hazardous, although the effect of a particular “dose equivalent,” measured in sieverts, depends on how fast it is incurred: a sudden large dose can be worse than the same amount accumulated gradually. There is no consensus on the danger of low doses. Humans evolved against a background of natural radiation; some think the body can cope with small exposure (we have enzymes to repair DNA damaged by ionisation) and that it might even be beneficial. Others say that even minimal increases are risky.
That’s the basic problem in assessing the dangers. Far away from Fukushima—in Tokyo, say, 220km to the southwest—leakage from the plant has only a marginal impact. The average person receives 2-3 millisieverts (mSv) of background radiation a year, and the US Nuclear Regulatory Commission recommends that exposure shouldn’t exceed this by more than 1 mSv. But a single CT scan (X-rays are ionising too) can take you over that limit, while the US maximum exposure level for radiation workers is 50 mSv annually. And even a 5 mSv dose can be fatal if received at once.
At least the numbers supply some context. The peak radiation dose measured inside the Fukushima plant on 15th March, 400 mSv per hour, is terrifying; and you’d do well to avoid 31 mSv, the average total dose of evacuees from Chernobyl in 1986. Perhaps most frustrating for hazard assessment is the variation in radiation levels around the Japanese plant. Within a 30km to 60km radius, these have been found to vary at least a hundredfold, from 0.0005 to 0.05 mSv per hour. Fluctuations are equally wild within the plant’s perimeter, partly depending on the spread of fires.
This makes it very hard to assess the exposure of radiation workers or the local population. And Chernobyl shows that time won’t necessarily tell. The disaster took its toll, particularly in the raised incidence of thyroid cancers in children. Confirmed deaths from direct exposure are around 60, but projections of further deaths vary wildly. While in 2005 the WHO estimated that 4,000 of the 600,000 exposed could eventually die of radiation-related illnesses, other sources put the eventual death toll at hundreds of thousands. Similar disputes dog efforts to determine the cancer risk among people living close to nuclear plants: the validity of a new study commissioned in the US is already being questioned by health physicists. At these low and variable exposure levels, it is extremely hard to link cause and effect.
Selling cells The recent recommendation by the European Court that innovations derived from human embryonic stem cells should not be patentable threatens to undermine all EU work in this field if it is upheld by the European Court of Justice. Without such protection on intellectual property, the research—now starting to prove its value—would largely become untenable. The ruling stems from a case brought by Greenpeace against a German scientist who had patented a technique for turning stem cells into nerve cells. Adjudication of the appeal fell to the conservative advocate-general Yves Bot, who decided that embryonic stem cells should be considered legally equivalent to embryos, a tendentious interpretation both ethically and scientifically.
A final decision may take six months, but in the meantime it casts a shadow over Europe’s leadership in this area, where there has been little of the controversy that has hampered research in the US.
Flaky science? On its final flight, launched on 16th May, Nasa’s Endeavour space shuttle carried a $2bn experiment to test a theory that many experts don’t believe. The Alpha Magnetic Spectrometer (AMS) will look for cosmic rays, free from the interfering effects of Earth’s atmosphere. Cosmic rays are one of the hottest areas in astrophysics: they are thought to originate in extremely energetic processes such as supernovae or immense black holes. But the mission’s spokesman, Nobel laureate Samuel Ting, thinks some may be created by annihilation of matter colliding with stars and galaxies made of antimatter. Few believe such regions exist.
There’s other good science for AMS to do, but it’s ironic that it has been sold as part of the goal to prove the scientific value of the International Space Station, where it will be housed. With the space shuttles retired, soon all of the ISS, good or bad, will be dependent on Russian rockets.
