Infinitesimal radioactive isotopes can be carried along on the breeze, landing unseen on the ground, clothes and skin. These tiny products of nuclear reactions are capable of causing large-scale bodily damage if they make it inside through inhalation, ingestion or even a cut. And many fear that such isotopes spewed from the damaged Fukushima Daiichi nuclear plant are traveling inter-continentally—and in higher quantities than Japanese officials are reporting.

This invisible threat has spurred people as far from Japan as the U.S. and Europe to snatch up and swallow potassium iodine pills in hopes of staving off any accumulation of radioactive iodine 131 particles in their thyroid glands (which can lead to cancer).

Although the most pressing immediate health concern is the powerful direct gamma radiation that threatens workers at the plant, "we need also to focus on the radioactive isotopes that are being dispersed at some distance from the plant," Ira Helfand, a former president of Physicians for Social Responsibility, said at a Wednesday news conference organized by that group, which is opposed to nuclear power.

The Japanese government has maintained that residents of Tokyo, some 220 kilometers south of the compromised Fukushima Daiichi nuclear plant, are not facing dangerous levels of radiation. Winds are expected to shift slightly more southerly at the end of the weekend, which could move more radioactive isotopes toward the capital city, but a rainstorm predicted for Sunday could help knock many of these harmful particles out of the air.

Some nuclear experts are concerned that "even if the total radiation dose is not real high downwind from a plant, the concentration of these isotopes can pose a very serious health problem," Helfand said. 

Elemental issues
The correlations among total body radiation exposure, ingested or inhaled isotopes and cancer are convoluted at best. The major studies in this field have used event-based samples, such as survivors of the atomic bombs dropped on Japan by the U.S. in World War II, to measure and map cancer rates years out.

The clean "linear relationship between your dose of total body radiation and the effect on your health is really lost when you're talking about low-dose radiation at some distance from the source," Helfand said. "You can have a very small total body radiation dose and end up getting thyroid cancer, or ingest some radioactive strontium and end up getting leukemia."

But the likelihood of this event causing large-scale cancer increases in the Tokyo population is slim, says Jeffrey Clanton, director of radiopharmacy services at Vanderbilt University Medical Center.

Iodine 131 is relatively short-lived (with a half-life of about eight days, compared with strontium 90, which decays to half its potency in 29 years). So the spent fuel stores would likely have already lost the majority of their iodine 131 before last Friday's earthquake and tsunami damaged the plant, Clanton explains. And any that is released "will most likely fall out of the atmosphere fairly quickly."

Strontium 90 isotopes, however, are a concern because the body treats them like calcium, integrating them into the bone, which can cause leukemia. But, Clanton says, they are not likely to travel more than a few kilometers—"five miles, probably, max."

Plutonium is of graver concern because of its exceptionally long half-life (about 24,000 years) and its propensity to cause lung cancer if inhaled. (There is plutonium in the fuel rods used at reactor No. 3 at the Fukushima complex.)

Clanton, however, maintains that a person would have to inhale or ingest "a reasonably large amount" of one or more radioactive isotopes to see negative effects. "It would take more than I've seen published from the area."

People in the U.S. purchasing prophylactic potassium iodine pills (which saturate the thyroid with regular iodine in hopes of preventing the uptake of radioactive iodine 131) "are wasting their money," Clanton says.

Winds of concern
No matter how much radioactive material is released, the worrisome isotopes from the Fukushima Daiichi facility are not particularly likely to follow a Chernobyl-scale distribution, Clanton notes. Even if the fuel temperatures fail to drop significantly, it is unlikely to produce the type of catastrophic explosion that launched so much radioactive material into the atmosphere over Chernobyl 25 years ago. The material at Fukushima also lacks the graphite-tipped control rods that were elemental in dispersing the isotopes from the Chernobyl event.

The distribution patterns depend on both the winds and the release intensities from the reactors. And although the current evacuation zones are concentric circles around the plant, it is more likely that dispersed radioactive material will fall in narrow bands, David Richardson, an epidemiologist at the University of North Carolina at Chapel Hill's Gillings School of Global Public Health, said during the Wednesday briefing. He added, "It's going to be quite a while until we have anything more than a crude understanding of the magnitude and the distribution."

Gathering enough data to create sturdy predictions, however, has so far proved impossible. "The infrastructure is so bad over there, I can't really tell what's going on," Clanton says. "It's very frustrating in terms of determining what is actually happening."

Nevertheless, the U.S. Nuclear Regulatory Commission (NRC) and President Obama have reassured Americans that there will not be harmful levels of radiation reaching the U.S. "Whether it's the west coast, Hawaii, Alaska or U.S. territories in the Pacific, we do not expect harmful levels of radiation," Obama said Thursday.

Even as higher-than-normal levels of background radiation are detected in the U.S., it is unlikely to pose a health risk. Simply living at high altitudes or frequent air travel exposes people to extra radiation (via the sun's rays)—commercial flight crews, for example, can in a year accumulate more than three times normal annual U.S. background radiation found at sea level. And safety standards for nuclear industry employees allow for more than six times average U.S. background radiation, which is still far below levels considered carcinogenic.

Fallout fish?
In Japan, winds have been primarily blowing out to sea since the March 11 earthquake and tsunami, which is good news for the millions of people who live in and around Tokyo. This does mean, however, that most of the radioactive material would be falling out of the air over the ocean, not far from Japan's east coast. "And that means that marine life will pick up this material—and if people eat fish, they will in turn intake this material," Marvin Resnikoff, a consultant at Radioactive Waste Management Associates, said during the press conference.

But Japan's fisheries are unlikely to become another Chernobyl milk incident (in which local cows grazed on contaminated grass produced isotope-tainted milk that has been implicated in many of the 1,800 cases of thyroid cancer documented in children from the affected area).

The Pacific Ocean is no small-town Ukrainian meadow. "The solution to pollution is dilution," Clanton says, quoting the adage and acknowledging the risk of sounding flippant. "The ocean's a big place." Even under the improbable scenario of a large fire in reactor No. 4, in which "it's likely there would be at least a momentary release of radioactive material into the atmosphere," he says, combined with strong winds that carried that material into the ocean, "there's a slight potential that you could detect it in fish later—but even that would be small."

Any ichthyic-bound isotopes would be so few that "they would be very hard to detect," Clanton says, comparing the amounts with the naturally occurring isotope potassium 40 found in bananas.

Dangers of displacement

The NRC's recommendation earlier this week that U.S. residents in Japan relocate to at least 50 miles from the Fukushima facility has caused confusion, given its relatively extreme distance compared with Japanese recommendations—and even local U.S. nuclear disaster contingency plans.

Clanton shares the sentiment of many radiation experts that given the comparatively few number of U.S. residents in the area, "why not be conservative?" For the Japanese government to move the evacuation boundary the additional 30 kilometers would mean dislocating a lot more people. Some 80,000 people within the 20-kilometer boundary have already been evacuated, and many more beyond that zone had left for fear of radiation contamination, The New York Times reported.

And on a population-wide scale, "there's probably much more [health] risk [in displacement] than of the reactor," Clanton says. "Just trying to house and feed them in an environment that's near freezing every night—that's got to be an untenable issue."