Workers at the stricken Fukushima Daiichi nuclear power plant fighting to keep additional radioactive iodine, cesium, strontium and other harmful elements from being released into the environment are monitored daily for exposure to radiation. The same is true of the police and firefighters scouring the area within 10 kilometers of the plant for missing people.

The general population in northeastern Japan, however, has considerably less access to accurate, non-invasive radiation dose measuring equipment, a troubling situation made more so by Wednesday's announcement by Japan's science ministry that small amounts of cancer-causing radioactive strontium have been detected in soil and plants outside the 30-kilometer zone around the plant where the government has advised people to stay indoors.

One possible solution to quickly measure a population's exposure to radiation in the event of a nuclear disaster or some other large-scale leak of radioactive material—such as a so-called "dirty bomb" attack—would be to scan the body in places where that material is most readily absorbed. The human body takes up strontium, for example, as if it were calcium, which is why the radioactive form of the element can collect in teeth, nails and bones, causing serious health problems such as bone cancer.

Teeth and nails are good for measuring radiation because they pick up free radicals (atoms, or ions, with unpaired electrons) created by ionizing radiation and can retain them for long periods of time, says Harold Swartz, a Dartmouth Medical School professor of radiation oncology and director of the Dartmouth Biodosimetry Center for Medical Countermeasures against Radiation. Teeth, in particular, can hold onto radioactive materials for hundreds of thousands of years, which is why archaeologists often study them to ascertain the date of a fossil find.

Swartz and his colleagues are developing electron paramagnetic resonance (EPR) dosimetry that can measure electromagnetic signals, or wave forms, given off by teeth, fingernails and toenails of people exposed to radiation. "Once you are irradiated, you have a permanent record in your teeth in the form of free radicals," Swartz says. In small amounts "they don't do you any harm, but they are very specific and the only thing that can cause them at a certain magnitude is ionizing radiation."

A dosimeter that measures exposure to ionizing radiation via the teeth is the furthest along. It consists of a magnet, a source of (harmless) microwave radiation, and a device that senses the reflected microwaves. The person examined places his head between the poles of the magnet and rests his mouth on a bar. The tip of a resonator is placed against a tooth, which it scans about a dozen times over a period of three minutes. Free radicals in that tooth will absorb some portion of this microwave radiation. The dosimeter can determine the level of free radicals by comparing the amount of microwave radiation emitted with the amount that is reflected. The less reflected radiation, the more free radicals in the tooth.

There are currently five prototypes of Swartz's EPR dosimeters. Three of these are portable (with 27-kilogram magnets) whereas the other two are designed for clinics and use 680-kilogram magnets. One of these large dosimeters is in Japan now—having been delivered there two years ago to help the country's National Institute of Radiological Sciences with grant support from the government; it is being used to evaluate survivors of World War II's Hiroshima and Nagasaki atomic bomb blasts. Another of the prototypes can analyze nail clippings. The researchers are also working on a dosimeter that can examine fingertips and toes directly, without the need for clippings.

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