Radiation's Effect Depends on Amount

Japanese authorities warned Wednesday morning that they believed that the population living in the area immediately surrounding the stricken nuclear reactors now faced a health risk from further increases in radiation leaks.
Authorities said that radiation levels there had surged to levels that will "clearly have impact on the human body."

On average, an American will receive an annual radiation dose of about 620 millirem, or 6,200 microsieverts, a dose that isn't deemed to be harmful, says the U.S. Nuclear Regulatory Commission.

Half of this radiation exposure comes from natural background sources, such as radioactive materials that naturally exist in rocks, soil and other sources, as well as cosmic rays.

The other half originates from human sources, mainly diagnostic medical procedures such as computer tomography scans, which emit roughly 1,500 microsieverts of radiation, or a full set of dental X-rays, about 400 microsieverts.

Television sets, smoke detectors and luminous watches can also contribute tiny amounts. Natural radioactivity even occurs in foods such as carrots and bananas, and in beer.

Radioactive materials are made of unstable atoms. Such atoms give off excess energy until they become stable; the emitted energy is known as radiation.

To put the reports coming out of Japan in perspective, the NRC limits occupational radiation exposure for adults working with radioactive material to 50,000 microsieverts a year.

The Federation of Electric Power Companies of Japan said that on Monday morning, a radiation level of 3,130 microsieverts per hour was recorded at the Fukushima Daiichi Nuclear Power Station, about six times the legal limit. Later in the morning, a radiation level of 326 micro sievert was recorded there.

Significant radiation exposure boosts cancer risk. Damage that occurs at the cellular or molecular level can disrupt the body's natural control processes and allow an uncontrolled growth of cells, or cancer.

Ionizing radiation can bring this about by breaking chemical bonds in atoms and molecules.

The NRC says there are no data to reliably estimate the occurrence of cancer following exposure to low doses and dose rates below about 100,000 microsieverts.

But high doses are dangerous. The U.S. Environmental Protection Agency says exposure to five to 10 rems of radiation will alter a person's blood chemistry, while 55 rems will also bring on nausea. and fatigue. (One rem is equal to 10,000 microsieverts.)

Vomiting and hair loss occur at 70 rem and 75 rem respectively, while exposure to 400 rem can mean possible death in two months. With even higher doses, the onset of death is quicker.