Health concerns have increased in Japan after the government found unacceptable levels of radiation in milk and vegetables from several regions and in drinking water Tokyo. The radiation comes from the crippled Fukushima Daiichi nuclear plant. But how worrying are the levels of radiation? And when the food and water supply is safe again?
According to reports today, two tap water samples tested at Tokyo contained 10 and 210 becquerels / kilogram (Bq / kg) of iodine-131. This level is almost double the limit of 100 Bq / kg for infants that is considered safe by officials of Japan's health.
In response, the Japanese authorities advised parents not to give babies tap water or use it in the formula. Children are of particular concern because any iodine-131 will be absorbed by ingestion of development of the thyroid gland and can lead to thyroid cancer. (The security level is three times higher for adults.)
However, the risk to babies is small, said epidemiologist Richard Wakeford of the University of Manchester in the UK. According to his calculations, a child drinking water tap for a year that contained twice the level of security for iodine-131 would receive a dose of about 0.8 millisieverts. By comparison, the dose from natural background sources is 2.5 millisieverts per year, he said.
nuclear engineer Shih-Chen Yew US Department of Argonne National Laboratory Energy in Illinois says he is "not at all surprised" that iodine-131 is showing in water from Tokyo because if the wind blows to the south of the plant, it could carry radiation far. Without knowing the source of drinking water in Tokyo, however, he said it is difficult to say exactly how iodine entered the water supply. But most likely, it came from airborne dust or fell in the rain or snow fed lakes, rivers and reservoirs .
the good news is that iodine-131 has a half-life of only eight days, so that all radiation from the Fukushima plant will have disappeared from the water within a couple months after the leaks are stopped.
Japan has also banned the sale of raw milk and some vegetables from Fukushima Prefecture and other prefectures after dangerous levels of iodine-131 and cesium-137 were detected in the samples. (The United States banned imports of dairy products and produce from several regions.) Milk is likely became infected when fed cows in pastures sprinkled fallout. Spinach, which had some of the highest detected radiation levels, has large leaves that collect the most radioactive dust that nonleafy vegetables.
As with tap water, the security levels are defined according to ingesting large quantities of food over a long period. "Unless you have consumed continuously at a high level," the level of exposure would be minimal, said Chen.
The threat of iodine-131 in food will also quickly disappear once cessation of discharges. But cesium-137 is a different story. once the cesium enters the soil, its half-life of 30 years became a "long-term problem for sure," Chen said, and it will appear in vegetables, meat and milk.
How much of a problem depends on the soil type, the retirement said Colorado State University radioecologist F. Ward Whicker. Clay binds the metal and keeps the plants to take up. But if the soil is sandy and clay poor, cesium "can be recycled from the soil for planting in the ground for a long, long time," says Wicker. The authorities dealt with the problem after the Chernobyl nuclear disaster in 1986 turning on a deep layer of soil to bury the radioactive dust.
for our complete coverage of the crisis in Japan, see our Japan Earthquake page . to Science s answers to readers' questions about the crisis, see our Quake issues .
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