March 15th, 2011
02:29 PM ET

What you should know about radiation

Japanese workers are scrambling to control a nuclear plant after a fire that may have burned several fuel rods. Officials said that the readings at the Fukushima Daiichi nuclear plant Tuesday morning were not at levels to cause “harm to human health,” but anxiety about impending disaster persists. Here's more on radiation sickness and how to combat it.

Radiation levels at the plant Tuesday were between 100 and 400 millisieverts, Japan’s Chief Cabinet Secretary Yukio Edano said. To put that in perspective, in the United States, a person typically gets a radiation dose of 6.2 millisieverts per year.

That dose would quickly dissipate with distance from the plant, and radiation levels quickly fell back to levels where they would be no immediate public health threat, Edano said. Still, people as far as 30 kilometers (18.6 miles) were warned to stay inside.

At the higher end of that spectrum at the Japan plant, exposure to millisieverts for three hours would lead to radiation sickness, and eight hours would be fatal, said Ira Helfand of Physicians for Social Responsibility. But in general, in an emergency situation, keeping it below 500 millisieverts is pretty safe, said Nolan E. Hertel, nuclear engineering expert at Georgia Institute of Technology. And the further away you are from a radiation source, the lower exposure you will have.

Are you confused about what this all means? Here’s a bit of basic science to help explain the science and health effects of radiation.

What is radiation and why is it dangerous?

Atoms become radioactive when they are unstable. In a process called "decay," the atom can split into smaller pieces and release a lot of energy. The types of subatomic particles emitted during a radioactive decay include gamma rays, neutrons, electrons, and alpha particles that shoot through space. Some of these can travel through the human body, damaging cells and posing a hazard for humans.

Alpha particles are relatively heavy and, when emitted, cannot penetrate human skin or clothing, but are harmful if they get into the body otherwise. Beta radiation can cause skin injury and are also harmful to the body internally. Gamma and X-rays are high-energy invisible light that can damage tissue and are most hazardous to humans.

Some elements such as uranium, commonly used as fuel in nuclear power plants, are always radioactive; there is no nonradioactive form of them that naturally occurs in nature.

Radiation from the Japanese power plant is getting into the atmosphere because of the explosions that have happened there, and because it is necessary to vent some of it in order to prevent further damage.

How is radioactivity measured?

Radioactivity of a given sample of a substance is measured in terms of how many atoms are spontaneously disintegrating (“decaying”) every second, according to the U.S. Department of Energy. Specific instruments are designed to detect particular kinds of radiation that get released with each decay.

When we talk about people being exposed to radiation, we use units called rems or sieverts, which measure the biological effect for whole-body radiation. But pay attention to the time period per unit: The longer the exposure, the more dangerous it is.

Geiger counters detect radiation emitted from decaying atoms.

What are some common sources of radiation?

The average annual radiation exposure a person gets from natural sources is about 3.1 millisieverts; in the United States, a person typically gets a total of 6.2 millisieverts in a year because of medical diagnostic procedures and other man-made sources of radiation. The limit for occupational radiation exposure among workers who deal with radioactive material is 50 millisieverts in a year. Here's more on that from the FDA.

To give you a sense of what that means, a chest X-ray delivers a dose of about .02 millisieverts (what you'd get from the natural background radiation in 2.4 days), and a CT to the abdomen carries 8 millisieverts (what you'd get naturally in 2.7 years). But note that these procedures don't last very long; prolonged exposure would be more dangerous.

Although high doses of radiation can lead to cancer, radiation therapy targeted at tumors is used to treat cancer. That dose is about 10,000 times that typical exposure rate for the U.S., but it is applied to the cancerous tissue; the total-body exposure during radiation therapy is much less.

What is the danger level for humans and radioactivity?

Radiation does not affect everyone in the same way, so it’s impossible to say that a particular level is fatal in all cases. But, according to the NRC, scientists believe that half a population would die within 30 days after a full-body dose of 3,500 to 5,000 millisieverts from a few minutes to hours.

The more than 130 plant workers and firefighters who developed acute radiation sickness at Chernobyl received doses of 800 to 16,000 millisieverts.

Studies of populations exposed to high levels of radiation have shown increased risk of cancer. High-dose exposure, above 500 millisieverts, has been associated with leukemia, breast, bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma, and stomach cancers, according to the United States Nuclear Regulatory Commission.

But there is no hard evidence proving that low dose radiation, below 100 millisieverts per year, is likely to cause cancer. People who live in places that have higher levels of background radiation than normal, such as Denver, Colorado, with levels above 10 millisieverts per year, have not shown adverse biological effects, the NRC said. In general, the higher up you are in the atmosphere, the more radiation (rays from the sun and other cosmic sources) you're exposed to, which is why airline pilots and flight attendants also get more radiation exposure than the average person.

CNN's Madison Park and Sabriya Rice contributed to this report.

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