The Fukushima plant was struck by a tsumani following a magnitude-9 earthquake on 11th March, 2011. A 14-meter tsunami wave neutralized the plant's emergency power supply, resulting in a meltdown in three of the facility's six reactors.
Doses of 10 millisieverts a year and 50mSv may have been received by residents of two nearby towns, namely Namie which lies within the 12-mile evacuation zone, and Litate, which is within 25 miles of the nuclear plant.
In an interview with Reuters, Gregory Hartl, a WHO spokesperson, said of the infants in Namie who received thyroid radiation doses estimated to be between 100 and 200mSv a year:
"That would be one area because of the estimated high dose that we would have to keep an eye on. Below 100mSv, the studies have not been conclusive."
Iodine-131Our bodies absorb iodine in food; it is preferentially concentrated in the thyroid gland - the gland needs iodine to function properly. When (radioactive) Iodine-131, also known as 131I is present at high levels in the environment due to radioactive fallout, humans can adsorb it through contaminated food - and it builds up in the thyroid.
As 131I decays, the thyroid gland can become damaged. Exposure to high levels of 131I raises the risk of later in life developing radiogenic thyroid cancer. There is also a higher risk of non-cancerous growths and thyroiditis.
The younger people are when exposed to 131I, the higher is their lifetime risk of thyroid cancer.
Three Fukushima Daiichi reactors overheated, causing meldowns and the release of radioactive material into the air. Reactor Unit 3 (right) and Unit 4 (left), on 16th March, 2011
None of the plant workers' deaths linked to radiationAnother (interim) report carried out by the United Nation's scientific committee says that there was no association between the deaths of six workers at the plant during the accident and radiation exposure. Even though a number of workers had had radiation exposure to their skin, no detectable health effects were present. The committee said that the six people's deaths were linked to either previous health problems or non-radiation accidents. One of the people who died had acute leukemia, while another fell off a ladder - in both cases, they are believed to have died from drowning when the tsunami hit the facility.
The scientists say that exposure to higher levels of radiation raises lifetime cancer risk 'slightly'. Acute radiation syndrome can occur at 1Sv (1000mSv).
Health authorities, as well as other public bodies in Japan say they can keep people's exposure levels below 20mSc annually. Their aim is to bring levels down to below 1mSv in nearby areas. A level of 2.4mScv per year is what natural background radiation worldwide typically exposes us to.
The preliminary report - entitled "Preliminary dose estimation from the nuclear accident after the 2011 Great East Japan Earthquake and Tsunami" - explains that other parts of Fukushima prefecture had exposure rates of between 1 and 10mSv, compared to 0.1-1 mSv in the rest of the country.
The report adds that radiation from the accident that reached other countries, such as South Korea, were at levels below 0.01mSv, less than half of what a patient receiving a chest X-ray is exposed to.
AWHO measured radiation levels in the water, soil, air and food supplies; it emphasized that after the accident "In these most affected locations, external exposure is the major contributor to the effective dose."
WHO added that every effort had been made by the scientists to make sure there was no underestimation of doses. However, the Agency added that their estimates are based on limited data.
WHO explained that the 1986 Chernobyl accident exposed people to much higher lifetime doses of radiation, compared to the Fukushima one.
The authors wrote:
"The experience of the Chernobyl accident was that about 30% of the lifetime dose was delivered during the first year and about 70% during the first 15 years.
On the basis of environmental activity concentration data, it can be expected that the fraction of the lifetime dose beyond the first year will be lower for the Fukushima Daiichi nuclear power plant accident than for the Chernobyl accident."
Written by Christian Nordqvist