The developing crisis at the Fukushima Daiichi nuclear power plant in the wake of the March 11 earthquake and tsunami has raised concerns over the health effects of radiation exposure: What is a "dangerous" level of radiation? How does radiation damage health? What are the consequences of acute and long-term low-dose radiation?

Though radioactive steam has been released to reduce pressure within the wrecked complex's reactors and there has been additional radiation leakage from the three explosions there, the resulting spikes in radiation levels have not been sustained. The highest radiation level reported thus far was a pulse of 400 millisieverts per hour at reactor No. 3, measured at 10:22 A.M. local time March 15. (A sievert is a unit of ionizing radiation equal to 100 rems; a rem is a dosage unit of x-ray and gamma-ray radiation exposure.) The level of radiation decreases dramatically as distance from the site increases. Radiation levels in Tokyo, about 220 kilometers to the southwest, have been reported to be only slightly above normal.

"We are nowhere near levels where people should be worried," says Susan M. Langhorst, a health physicist and the radiation safety officer at Washington University in Saint Louis.

According to Abel Gonzalez, vice chairman of the International Commission on Radiological Protection who studied the 1986 Chernobyl disaster, current information coming from Japan about levels of radiation leakage are incomplete at best and speculations about "worst-case scenarios" are as of yet irrelevant.

The health effects caused by radiation exposure depend on its level, type and duration.

Radiation level:
The average person is exposed to 2 to 3 millisieverts of background radiation per year from a combination of cosmic radiation and emissions from building materials and natural radioactive substances in the environment.

The U.S. Nuclear Regulatory Commission recommends that beyond this background level, the public limit their exposure to less than an additional one millisievert per year. The U.S. limit for radiation workers is 50 millisieverts annually, although few workers are exposed to anything approaching that amount. For patients undergoing medical radiation there is no strict exposure limit—it is the responsibility of medical professionals to weigh the risks and benefits of radiation used in diagnostics and treatment, according to Langhorst. A single CT scan, for example, can expose a patient to more than one millisievert.

Radiation sickness (or acute radiation syndrome) usually sets in after a whole-body dose of three sieverts—3,000 times the recommended public dose limit per year, Langhorst says. The first symptoms of radiation sickness—nausea, vomiting, and diarrhea— can appear within minutes or in days, according to the U.S. Centers for Disease Control and Prevention. A period of serious illness, including appetite loss, fatigue, fever, gastrointestinal problems, and possible seizures or coma, may follow and last from hours to months.

Radiation type:
Of concern in the current situation is ionizing radiation, which is produced by spontaneously decaying heavy isotopes, such as iodine 131 and cesium 137. (Isotopes are species of the same element, albeit with different numbers of neutrons and hence different atomic masses.) This type of radiation has sufficient energy to ionize atoms (usually creating a positive charge by knocking out electrons), thereby giving them the chemical potential to react deleteriously with the atoms and molecules of living tissues.

Ionizing radiation takes different forms: In gamma and x-ray radiation atoms release energetic light particles that are powerful enough to penetrate the body. Alpha and beta particle radiation is lower energy and can often be blocked by just a sheet of paper. If radioactive material is ingested or inhaled into the body, however, it is actually the lower energy alpha and beta radiation that becomes the more dangerous. That's because a large portion of gamma and x-ray radiation will pass directly through the body without interacting with the tissue (considering that at the atomic level, the body is mostly empty space), whereas alpha and beta radiation, unable to penetrate tissue, will expend all their energy by colliding with the atoms in the body and likely cause more damage.

In the Fukushima situation, the radioactive isotopes detected, iodine 131 and cesium 137, emit both gamma and beta radiation. These radioactive elements are by-products of the fission reaction that generates power in the nuclear plants.

The Japanese government has evacuated 180,000 people from within a 20-kilometer radius of the Fukushima Daiichi complex. They are urging people within 30 kilometers of the plant to remain indoors, close all windows, and to change clothes and wash exposed skin after going outside. These measures are mainly aimed at reducing the potential for inhaling or ingesting beta-emitting radioactive material.

Exposure time:
A very high single dose of radiation (acquired within minutes can be more harmful than the same dosage accumulated over time. According to the World Nuclear Association, a single one-sievert dose is likely to cause temporary radiation sickness and lower white blood cell count, but is not fatal. One five-sievert dose would likely kill half of those exposed within a month. At 10 sieverts, death occurs within a few weeks.

The effects of long-term, low-dose radiation are much more difficult to gauge. DNA damage from ionizing radiation can cause mutations that lead to cancer, especially in tissues with high rates of cell division, such as the gastrointestinal tract, reproductive cells and bone marrow. But the increase in cancer risk is so small as to be difficult to determine without studying a very large exposed population of people. As an example, according to Langhorst, 10,000 people exposed to a 0.01-sievert whole-body dose of radiation would potentially increase the total number of cancers in that population by eight. The normal prevalence of cancer, however, would predict 2,000 to 3,300 cancer cases in a population of 10,000, so "how do you see eight excess cancers?" Langhorst asks.

Chernobyl's lessons:
According to Gonzalez, some of the emergency workers at Chernobyl received several sieverts of radiation, and many were working "basically naked" due to the heat, allowing contaminated powder to be absorbed through their skin. In comparison, the Japanese workers are most likely very well-equipped and protected at least from direct skin doses.

The Tokyo Electric Power Co. (TEPCO), the plant's owners, has evacuated most of its workers, but 50 remain at the site to pump cooling seawater into the reactors and prevent more explosions. These workers are likely exposing themselves to high levels of radiation and braving significant health risks. "As a matter of precaution, I would limit the workers' exposure to 0.1 sievert and I would rotate them," Gonzalez says. The workers should be wearing personal detectors that calculate both the rate and total dose of radiation and that set off alarms when maximum doses are reached. "If the dose of the workers start to approach one sievert then the situation is serious," he says.

The thousands of children who became sick in the aftermath of the Chernobyl disaster were not harmed from direct radiation or even from inhalation of radioactive particles, but from drinking milk contaminated with iodine 131. The isotope, released by the Chernobyl explosion, had contaminated the grass on which cows fed, and the radioactive substance accumulated in cows' milk. Parents, unaware of the danger, served contaminated milk to their children. "Certainly this will not happen in Japan," Gonzalez says.

When it comes to radiation exposure, professionals who frequently work with radioactive materials, whether in a hospital or a nuclear power plant, abide by the ALARA principle: "as low as reasonably achievable". Radiation exposure limits are conservatively set well below the levels known to induce radiation sickness or suspected of causing long-term health effects. Temporary exposure to dosages many times these limits, however, is not necessarily dangerous.

News of the U.S. Navy repositioning its warships upwind of the reactor site, the distribution of potassium iodide pills by the Japanese government, and images of officials in hazmat suits using Geiger counters to measure radiation levels among babies may stoke the public's fears—but, for now, these measures are ALARA in action, or "good extra precautions," Gonzalez says. The idea here is to always err on the side of caution.