One of the ways to understand the spectrum of electromagnetic energy that produces nonionizing radiation is to think of a rainbow. At the far right is ionizing radiation. As you begin to move from right to left, you pass through a spectrum that includes cosmic rays, gamma rays, X-rays and some ultraviolet light. All these forms of ionizing radiation are high energy. They earn their name because they are strong enough to knock small, negatively charged subatomic particles known as electrons from the orbit of atoms. When these electrons are knocked off, normal atoms become charged, which makes them able to disrupt other molecules.

Nonionizing radiation -- which begins at about the top of rainbow -- is unable to knock electrons from their tightly held orbits. The spectrum of nonionizing radiation starts at the edge of the ultraviolet band, continues through visible light and also includes infrared light and the microwaves used in radar and ovens. Then there are the ultra-high-frequency (UHF) and very-high-frequency (VHF) bands used for television. Next come the FM radio waves, followed by short wave, AM radio waves and extremely-low-frequency (ELF) signals used for submarine communication. Finally there are the alternating electric currents that bring electricity to homes and offices.

Except for infrared rays, all these forms of nonionizing radiation "readily penetrate the body," says Dr. Ruey Lin of the Maryland Department of Health and Mental Hygiene. However, how much of this radiation is absorbed "varies depending on frequency of waves and type of tissue. Systems of the body which are particularly vulnerable to electromagnetic waves include the central nervous system, the blood and immune system and the cardiovascular and endocrine systems."