Warning on Tokyo tap water

23 March 2011


UPDATE 1: 2.57pm GMT, Context from other Fukushima readings

UPDATE 2: 3.36pm GMT, New information on food restrictions
UPDATE 3: 5.11pm GMT, Context on radiation doses
UPDATE 4: 12.00pm GMT, 24 March, addition of link to story on lifting of advice


Parents in Tokyo have been recommended to avoid giving tap water to infants under one year of age, although no health effect would be expected. Restrictions on food have also been expanded. 


The Ministry of Health, Labour and Welfare reported radioactivity readings in tap water from various parts of Tokyo including 103, 137 and 174 bequerels per kilogram. One measurement for iodine-131 rated 210 becquerels per kilogram.


These remain below the Nuclear Safety Commission of Japan's indices for emergency situations, at present enforced as regulation in Japan. The body has set 300 becquerels per kilogram from iodine-131 or 200 becquerels per kilogram from caesium-137 as the current standards for drinking water that would not cause health effects if consumed for one year. However, there is a separate level of 100 becquerels per kilogram for iodine-131 in milk to be used in baby formula.


With readings falling among the standards, authorities have recommended that parents avoid giving tap water to babies. The ministry said that potential for health effects after using water that temporarily exceeds index values is very low and it is still safe to use if there is no alternative. The water can be used for handwashing, bathing and normal domestic uses.


An infant could receive a radiation dose of about 10 microsieverts from drinking one litre of the tap water, meaning one would have to drink a litre per day for a year to receive a dose of between 1 and 10 millisieverts. The dose for an adult would be about ten times lower. The infant does would be higher than the normal 2.4 millisieverts per year from background radiation, but meaningful comparisons cannot be made because the emergency situation is not expected to continue for a such a period. In addition, the levels of iodine are expected to decrease within a few weeks once releases to the atmosphere at Fukushima Daiichi have ceased. The doses are too low to present an immediate health concern and would be meaningful only for a large population for a period longer than a few weeks.


The health concern with iodine-131 is that the body draws it to the thyroid gland, which can result in the development of thyroid cancer. However, the isotope has a half-life of about eight days, meaning its risk reduces fairly quickly after a release. Caesium-137, meanwhile, has a half life of almost 31 years and so could potentially present a longer-term concern, although particulates of the isotope will be dispersed more quickly by rainfall and ground penetration.


Nearer the Fukushima Daiichi power plant, Public broadcaster NHK has reported higher levels, including 965 becquerels from iodine-131 per kilogram at Iitate. In Tamura, levels were 365 bequerels per kilogram from iodine-131 on 17 March but these fell to 161 becquerels per kilogram two days later.


One factor in the rise in detections of radioactivity in Tokyo may be recent rainfall, which would have brought airborne radionuclides to ground and washed them into reservoirs.


Food controls expand  


A range of vegetables are now controlled due to the possibility of elevated levels of iodine-131. "At the moment these vegetables are not harmful to people's health," said chief cabinet secretary Yukio Edano. However, the current situaiton could continue for a time, he said, and that was the reasoning behind a warning not to distribute or consume some goods from Fukushima prefecture.


Prime minister Naoto Kan has asked the governor of Fukushima prefecture to restrict distribution and consumption of any leafy vegetables (e.g. spinach, cabbage) or any flowerhead brassicas (e.g. broccoli, cauliflower) for the time being. In Ibaraki prefecture the governor has been asked to restrict distribution of milk and parsley.


Edano said that if someone were to eat the vegetables for ten days then they would be exposed to about half of one year's background radiation. This was no risk to future health, he said.


Researched and written 

by World Nuclear News