Reassuringly low doses of radiation (Image: Tayama Tatsuyuki/Gamma-Rapho via Getty Images)

Most children and workers exposed to radioactive fallout from the Fukushima Dai-Ichi nuclear disaster on 11 March last year received reassuringly low doses of radiation, but some highly-exposed workers will need close monitoring for health problems in the future.

That’s the main conclusion of an interim report on the health impacts of fallout from Fukushima issued yesterday in Vienna by the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).

“For the time being, we don’t see anything of major concern,” says Wolfgang Weiss, chairman of the committee of 70 scientists. “The levels of exposure are much lower than what we saw at Chernobyl.”


The committee evaluated data on how much radiation was released from the stricken plant, how much has been absorbed by plant workers, and how much reached the general public – especially children. They also looked at contamination levels among wildlife on land and in the sea.

The report reveals that children in villages with the heaviest fallout received relatively modest doses, although most exceeded the 20 millisieverts considered an acceptably safe annual cumulative dose for workers in the radiation industry.

Thyroid risk

The biggest risk faced by the children in closest proximity to the plant was accumulation of radioactive iodine in their thyroid glands, which can lead to cancer. Among those living near the site of the Chernobyl nuclear disaster there have been 6000 cases of thyroid cancer since 1986, although most have been successfully treated.

In the hotspots around Fukushima, thyroid radioactivity monitoring of 1080 children under 15 revealed that none received doses higher than 100 millisieverts, an amount 10 times smaller than the typical exposure levels of children around Chernobyl. Measurements outside the “hotspot” zones in Fukushima prefecture suggested that the maximum thyroid dose for children was around 35 millisieverts.

The committee has demanded more information and data on how the doses were assessed, and UNSCEAR should have detailed ultrasound thyroid scans on 300,000 children from the hotspots around Fukushima within the coming weeks. But Weiss says, “The data we’ve seen for the children is reassuring.”

For some of the 20,000 workers who’ve been labouring to bring the four stricken reactors at Fukushima under control, the outlook is less certain.

Reassuringly, two-thirds of the workers received total radiation doses lower than 10 millisieverts. But 167 workers received doses exceeding 100 millisieverts. Of these, six received doses exceeding 250 millisieverts. “That’s equivalent to 12 years of receiving a full dose that’s considered acceptable within the industry,” says Weiss.

Most in danger

Two workers received exceptional doses of around 680 millisieverts. “That gets towards the range where you see acute effects that can kill you,” says Weiss. “At Chernobyl, 28 workers more or less died on the spot, or within a couple of days, from acute radiation syndrome.”

“The levels we saw in those two workers are substantial, and we need to keep a very close watch on them,” says Weiss.

The committee has also demanded more information on how worker doses and exposures have been established, and what assumptions were made in calculating them. For instance, Weiss says that in the early days of the disaster, some workers may have been exposed to high-energy neutrons as fuel melted and went critical, but there’s no exposure data because their dosimeters can’t detect neutrons.

For the population as a whole, Weiss says the risks from radioactive iodine have long passed, as most of it decayed away within a month of the disaster. But the potential risks from two radioactive caesium isotopes remain. Half of any amount of caesium-134 decays away within two years, for caesium-137 it is 30 years.

The committee is due to deliver its final report to the UN next May, after incorporating additional data from the Japanese authorities, US military and navy vessels exposed to fallout from Fukushima, and local surveys of sediments, fish and other marine life. Also, to assess the risks from food, the committee will analyse 130,000 samples of produce from around Fukushima gathered since the disaster by the UN Food and Agriculture Organization.