Three weeks after the earthquake and tsunami that crippled Japan's Fukushima Daiichi nuclear power plant workers have made some headway in cooling the facility's overheated fuel rods. But overall, the situation remains "very serious," according to the International Atomic Energy Agency (IAEA). Despite the ongoing work to stabilize the plant and fears that radioactive materials had contaminated tap water as far away as Tokyo, 240 kilometers to the south, most of the recommended restrictions on drinking water have been lifted. Still, the presence of radioactive iodine and cesium in the water supply—four villages in Fukushima Prefecture are still recommending that children less than one year old not be given tap water—raises public health questions moving forward.

Exposure to or ingestion of radioactive isotopes, or radionuclides, (species of the same element with differing numbers of neutrons and hence differing atomic weights) can lead to a number of health problems, including nausea and vomiting at low doses, and central nervous system damage and internal bleeding at high doses. Radioactive iodine, in particular, concentrates in the thyroid and can result in thyroid cancer, particularly in children.

The past has offered little guidance on the impact of nuclear power plant emergencies on drinking water. At the Chernobyl plant in Ukraine, which exploded in April 1986, the greatest impact observed was an increase in thyroid cancer cases, most of which came from drinking contaminated milk. Inhalation and water consumption were less important health issues at Chernobyl, Scott Davis, a professor of epidemiology at the University of Washington in Seattle, recently told CNN. Radioactive contamination of surface waters throughout much of Europe declined quickly following the Chernobyl incident through dilution, physical decay and absorption of radionuclides in sediment beds and catchment (water-catching) soils, according to a World Health Organization report issued in 2005. Elevated concentrations of radioactive cesium were found in fish from lakes as far away as Germany and Scandinavia, however.

Radioactivity reduced
The levels of radioactive iodine and cesium in northeastern Japan's drinking water were highest one week after the tsunami devastated Fukushima's reactor cooling systems. The University of Tokyo Hospital Radiation Oncology Team had reported finding 170 becquerels per kilogram of iodine 131 in Fukushima's drinking water on March 18, although the level has since dropped significantly. One becquerel represents the rate of radioactive decay—or radiation emitted by a substance—as one disintegration, or count, per second. The March 18 number is significant because the Japanese government mandates that infants less than one year old not be given water with a concentration of greater than 100 becquerels per kilogram. (A liter of water weighs one kilogram.) Adults should avoid any water with more than 300 becquerels per kilogram, according to the government's current standards.

Based on reports collected by the Ministry of Education, Culture, Sports, Science and Technology, there have not been significant levels of radioactive iodine or cesium in most of Japan's 47 prefectures studied since March 24, when 110 becquerels per kilogram of iodine 131 were found in Tochigi, a city about 100 kilometers north of Tokyo and about 160 kilometers from Fukushima. The Institute of Public Health had reported a level of 210 becquerels per kilogram in Tokyo's drinking water on March 23 but the Tokyo Metropolitan Government later stated that reading was incorrect and much higher than the actual level, which was 79 becquerels per kilogram the following day.

Despite reassurances that Japan's water is, for the most part, safe for drinking, there was a run on bottled water last week in areas of the capital, leading stores to restrict purchases to one bottle per customer. Tokyo's government last week promised to distribute 3.5-liter bottles of mineral water to 80,000 households in the city with infants, according to Japanese public broadcaster NHK.

Radioactive removal
Most radioactive radionuclides—including iodine 131 and cesium 134 and 137—can be removed from water. Others, such as tritium, a heavy form of hydrogen that is the most ubiquitous radioactive pollutant produced by nuclear power plants, cannot be filtered out of water. Whether and how an isotope could and would be removed depends on which it is. Unlike drinking water contaminated with microbial pathogens such as Escherichia coli, giardia or cryptosporidium, water containing radioactive material cannot be made potable by boiling, bleach or exposure to ultraviolet light. Instead, isotopes must be removed using activated charcoal filtration, reverse osmosis or water softening, to name a few methods. Radioactive material may also fall out of a water supply by settling to the bottom of a reservoir or via adsorption (adhesion) onto the surface of soil particles in a reservoir. Another option with some radioactive contaminants is to wait until they radioactively decay to safe levels—a period known as half-life, which greatly varies with the particular isotope, from seconds to tens of thousands of years.

There is no single approach to removing radioactive iodine, cesium or other radionuclides from water. "You need to know what's in the water because that will help tell you whether you can remove it or not, and what kind of water treatment process you need to perform," says David Ozonoff, an environmental health professor and chair emeritus of the Boston University School of Public Health's Department of Environmental Health .

One difficulty with filtering out radioactive isotopes is that the filters and membranes would become radioactive waste that must then be disposed of carefully, says Rob Renner, executive director of the Water Research Foundation. If the waste had a relatively short half-life of less than 30 days—iodine 131's half-life is eight days, for example—the filters or charcoal could be stored for a few months to ensure the material is safe to handle. "If elements had a higher level of activity and had a longer half-life, it could be a problem," Ozonoff says.

Given the relatively short half-life of iodine 131, officials are more likely to wait out the decay process. "Probably [iodine 131] turnover in reservoirs is slow enough that much of the activity deposited decays before the water gets to the consumers," says Keith Baverstock, an environmental scientist at the University of Eastern Finland who has studied Chernobyl extensively. "It is only necessary to protect children (adult thyroids are quite insensitive to radiation), and the better solution is to arrange for bottled water for them."

As for isotopes like cesium 137, however, its decay half-life is around 30 years.