The media has reported that drinking water supplies in Tokyo following the nuclear accident at the Fukushima Daiichi plant have contained up to 210 becquerels of radioactive iodine (I-131) per liter. This exceeded the limit of 100 becquerels/L set by the Japanese government for drinking water for infants’ consumption, although it is still within the Japanese upper limit of 300 becquerels/L for adults. Drinking water I-131 levels in all affected areas of Japan have generally been decreasing over the past week.
Average intakes of drinking water by infants fed powder-based formulas are about 0.8 liters daily. Assuming a one-time contamination of drinking water at the I-131 levels detected in Tokyo, an infant’s thyroid gland could be exposed to approximately 9 mGy radioactive iodine. Isolated short-term exposure to the I-131 levels found in the Tokyo drinking water is extremely unlikely to cause a detectable increase in thyroid cancer risk for any age group.
Because iodine is concentrated in breast milk, and because breastfeeding women drink more water daily than other adults, women who are lactating are best advised to limit ingestion of water contaminated with I-131.
However, if exposure to this level of I-131 in drinking water cannot be avoided, it is still reasonable to continue breastfeeding.
Washing dishes or clothing in water with these I-131 levels does not pose a health risk. Similarly, it is safe to bathe in this water.
Boiling drinking water contaminated with I-131 is not advised. While up to about one third of the water’s radioactive iodine would be expected to be lost to evaporation, this would simply transfer the I-131 into the atmosphere, where it could be inhaled.
The public in Japan is advised to continue to follow public health advisories from the Japanese government. Because the situation around the Japanese reactors remains fluid, the American Thyroid Association will continue to monitor potential risks to health from this accident and will issue amended advisories as warranted.
Importantly, although trace amounts of I-131 have been detected in rainfall over the U.S., the nuclear accident in Japan is not expected to pose any risk to the U.S. water supply.