Health concerns have been rising in Japan after the government found unacceptable radiation levels in milk and vegetables from several regions and in drinking water in Tokyo. The radiation comes from the crippled Fukushima Daiichi nuclear plant. But how worrisome are the radiation levels? And when will the food and water supply be safe again?
According to reports  today, two tap water samples tested in Tokyo contained 190 and 210 becquerels/kilogram (Bq/kg) of iodine-131. That level is roughly twice the limit of 100 Bq/kg for infants that's considered safe by Japan's health officials.
In response, Japanese authorities advised parents not to give babies tap water or use it in formula. Children are of special concern because any ingested iodine-131 will be absorbed by their developing thyroid gland and can lead to thyroid cancer. (The safe level is three times higher for adults.)
Still, the risk to babies is low, says epidemiologist Richard Wakeford of the University of Manchester in the United Kingdom. According to his calculations, an infant drinking tap water for a year that contained twice the safe level for iodine-131 would receive a dose of about 0.8 millisieverts. By comparison, the dose from natural background sources is 2.5 millisieverts a year, he says.
Nuclear engineer Shih-Yew Chen of the U.S. Department of Energy's Argonne National Laboratory in Illinois says he's "not at all surprised" that iodine-131 is showing up in Tokyo's water because if the wind blows southward from the plant, it could carry radiation that far. Without knowing the source of Tokyo's drinking water, however, he says it's hard to say exactly how the iodine entered the water supply. But most likely it came from airborne dust or fell in rain or snow that fed lakes, streams, and reservoirs.
The good news is that iodine-131 has a half-life of only 8 days, so any radiation from the Fukushima plant will be gone from the water within a couple of months once the leaks are stopped.
Japan has also reportedly banned  sales  of raw milk and certain vegetables from Fukushima prefecture and some other prefectures after unsafe levels of iodine-131 and cesium-137 were detected in samples. (The United States has banned imports of dairy products and produce from several regions.) Milk likely became contaminated when cows fed in pastures dusted with fallout. Spinach, which had some of the highest detected radiation levels, has large leaves that collect more radioactive dust than nonleafy vegetables.
As with tap water, safety levels are set based on the ingestion of large amounts of the food over a long time. "Unless you consumed it continuously at a high level," the exposure level would be minimal, Chen says.
The threat from iodine-131 in food will also fade quickly once the releases stop. But cesium-137 is a different story. Once the cesium enters the soil, its half-life of 30 years becomes a "long-lasting problem for sure," Chen says, and it will show up in vegetables, meat, and milk.
How much of a problem depends on the soil type, says retired Colorado State University radioecologist F. Ward Whicker. Clay binds the metal and keeps plants from taking it up. But if the soil is sandy and low in clay, cesium "can be recycled from soil to plant to soil for a long, long time," Wicker says. Authorities dealt with the problem after the 1986 Chernobyl nuclear disaster by turning over a deep layer of soil to bury the radioactive dust.