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.
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.
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.