Japanese officials confirmed overnight that samples of Tokyo tap water showed elevated radiation levels more than twice the recommended limit for infants.
Concerns remain about contamination of food grown in the region around the Fukushima nuclear plant. Our colleagues at the UK Science Media Centre gathered the comments below on the latest situation from UK-based scientists. Feel free to use their comments in your articles.
To talk to a local expert on radiation and food and water contamination, contact the NZ SMC.
Regarding Tokyo tap water precautions:
Dr Mike Thorne, independent consultant in radiological and environmental science, said:
“If we take 500 Bq per litre of I-131 and assume that this concentration is maintained for about two weeks (it cannot be longer because of the 8 day radioactive half life of I-131) we can calculate the total intake. Total fluid intakes in adults (excluding milk) are around 1.65 litres per day (though typically only about 0.15 litres of this is tap water). However, if we take 1.65 litres on the assumption that they make all their drinks with tap water, we get an intake of 1.65*14*500 = 11550 Bq.
“The effective dose per unit intake of I-131 for adults is 2.2E-8 Sv/Bq. Therefore, the effective dose that could be received from 500 Bq per litre of I-131 in drinking water is 0.25 mSv. This is around one tenth of the natural annual dose received from background sources. [See Prof Wakeford’s comment below.] The dose factor for infants is larger at 1.8E-7 Sv per Bq, but their intake of drinking water would be rather less than for adults, so their dose would not be very much larger.
“However, the decrease in consumption rate does not fully compensate for the higher value of dose per unit intake, hence the need for stricter standards for infants. Overall the situation is rather different from the case of milk, where infants consume as much (or rather more) than adults.”
What are UK safety levels?
Prof Steve Jones, independent nuclear and environmental consultant, said:
“In terms of ‘safe’ levels of 131I in water, the EU Community Food Intervention Levels are 500 Bq kg-1 for dairy produce and liquid foods, 150 Bq l-1 for baby foods. The figure I have for 131I in Tokyo tap water is 210 Bq l-1, reported by Associated Press today. So I guess if the water was used to make up baby milk formula, the CFILs would say it shouldn’t be consumed.”
What might effects of the reported radiation levels be?
Prof Richard Wakeford, Dalton Nuclear Institute and Visiting Professor of Epidemiology, University of Manchester, said:
“No early effects, the consequent doses are way too low – the conservative assumption is that water contaminated at the level of the limit is consumed at a reference rate over a prolonged period (a year) and the consequent radiation dose is calculated accordingly – the resulting effective dose after one year drinking water at the Japanese limits is 0.4 mSv for an infant and 0.3 mSv for an adult. This compares with the annual effective dose limit (excluding medical and natural sources) of 1 mSv. The annual effective dose from natural background sources is about 2.5 mSv.”
Why different advice for children and adults?
Dr Jim Smith, Reader in Environmental Physics at the University of Portsmouth said:
“Children have different body sizes and water consumption rates than adults; also, they are more vulnerable to radiation risk because they are growing. Radiation tends to damage rapidly dividing cells more.”
Prof Richard Wakeford, Dalton Nuclear Institute and Visiting Professor of Epidemiology, University of Manchester, said:
“The primary objective is to limit the radiation dose to the thyroid gland of infants and young children, because it is well established that infants and young children are at the greatest risk from the accumulation of radioactive iodine in the thyroid. The contamination limits keep the resultant doses to tolerable levels.”
Is it safe to bathe or wash food in contaminated tap water?
Prof Richard Wakeford, Dalton Nuclear Institute and Visiting Professor of Epidemiology, University of Manchester, said:
“Yes, because only small amounts of water would be ingested from these practices; but the advice of local authorities should be followed. If infants or young children have already consumed some water this will not be a problem since the limits are based on prolonged consumption of contaminated water. The rise in radioactive iodine levels in water probably results from a change in wind direction (and possibly rain) and is likely to be a transient problem when the wind swings from Tokyo (which it is forecast to do).”
Dr Jim Smith, Reader in Environmental Physics at the University of Portsmouth said:
“Following the finding of up to 210 Bq/l of radioactive iodine in tap water in Tokyo, the recommendation that infants are not given tap water is a sensible precaution. But it should be emphasised that the limit is set at a low level to ensure that consumption at that level is safe over a fairly long period of time. This means that consumption of small amounts of tap water – a few litres, say – at twice the recommended limit would not present a significant health risk. I would expect that the recommendation not to drink tap water would also extend to women who are pregnant or breastfeeding.”
Further comment on food and water contamination:
Prof Steve Jones, independent nuclear and environmental consultant, said:
“I have seen one press report of much higher levels of radioactivity in green vegetables than those we have so far seen: 82,000 Bq/kg of 137Cs and 15,000 Bq/kg of 131I (in the vegetable ‘kukitachma’). Apart from being very much higher than those previously reported for spinach and leeks, the ratio of caesium to iodine is very much higher – this suggests that something has changed in the form of the release.
“Further, the IAEA monitoring update for yesterday reports some figures for ground deposition 28 to 50 km from the plant, at 200,000 to 900.000 Bq m-2. On a very rough calculation these suggest a release from the plant of 1015 to 1016 becquerels – 1,000 to 10,000 terabequerels. This would be rather larger than the release in the 1957 Windscale Pile Fire, and about 1% of the release from Chernobyl.”
Dr Jim Smith, Reader in Environmental Physics at the University of Portsmouth said:
“Whether the release was near the scale of Chernobyl is difficult to confirm without more information on measured radiation dose rates and radionuclide levels, particularly in the area of land and sea close to Fukushima. Information from the IAEA technical briefing (http://www.iaea.org/newscenter/news/tsunamiupdate01.html) suggests very significant contamination and external radiation dose rates in some areas outside the 20 km evacuated zone.
“In view of this, I would not be surprised to see further evacuations of some areas outside the 20 km zone around the plant. It is not really well known what the composition of different radionuclides is, so it is difficult to make any predictions as to how much of a health risk, and how long-lived the contamination is going to be. This should become clearer in the coming days as more monitoring information becomes available. In particular, information is needed on the level and composition of contamination in the vicinity of the plant. Such information, to my knowledge, has not so far been published.”