Radiation sickness can be deadly

Q. I've been reading about "dirty bombs" that spread radiation. They can kill by giving you radiation sickness. What exactly is this?

Radiation sickness is an illness caused by too much exposure to radiation. How sick you get depends on the dose and the rate of exposure. Exposure to low-dose radiation, such as x-ray examinations, does not cause radiation sickness.

There are two basic radiation forms: nonionizing and ionizing. Nonionizing radiation usually does not cause tissue damage and comes in the form of light, radio waves, microwaves and radar.

Ionizing radiation is radiation that produces immediate chemical effects on human tissue. X-rays, gamma rays, and particle bombardment give off ionizing radiation.

Radiation exposure can be acute from a single large exposure, or chronic from a series of small exposures. Radiation sickness is usually linked to acute exposure. Chronic exposure is usually associated with delayed medical problems such as cancer.

Radiation sickness can be lethal, but it isn't common. Many suffered from it in the atomic bombings in Japan during World War II. There was a lot of radiation sickness caused by the 1986 nuclear reactor accident in Chernobyl, Ukraine.

There are natural sources of radiation that are around us all the time. Radon, a gas that emanates from the ground, produces two-thirds of this exposure. No adverse health effects have been discerned from doses arising from these levels of natural radiation exposure.

Radiation also comes from outer space and from within our own bodies. It is in air, food and water. Certain foods such as bananas and brazil nuts naturally contain higher levels of radiation than other foods. Brick and stone homes have higher natural radiation levels than homes made of other building materials such as wood.

Levels of natural radiation vary by place. For example, people in Colorado are exposed to more natural radiation than residents of the East Coast or West Coast because Colorado has more cosmic radiation at a higher altitude and more terrestrial radiation from soils enriched in naturally occurring uranium.

There are man-made sources of radiation from medical, commercial, and industrial activities. Medical x-rays are among the largest of these sources of exposure. In addition, tobacco, fertilizer, welding rods, gas mantles, luminous watch dials, and smoke detectors contribute radiation.

Natural radiation contributes about 82 percent of the annual dose to the population, while medical procedures contribute most of the remaining 18 percent.

The absorbed dose of radiation is measured in a unit called a gray (Gy). A safe radiation exposure of about 0.0025 Gy comes from a conventional dental X-ray. A typical whole-body computerized tomography (CT) scan produces about 0.012 Gy.

Signs and symptoms of radiation sickness usually appear with absorbed doses to the whole body of at least 1 Gy. Doses greater than 8 Gy are usually lethal.