Its short wavelength permits an x-ray to penetrate the body and produce on image on film. Its high energy causes electrons to be dislodged from atoms and molecules when they are struck by x-rays. This results in ion formation and causes tissue damage and, potentially, cancer. Therefore x-rays are known as ionizing radiation. When x-rays pass through the body, they are absorbed by tissues to different degrees.
Bone absorbs the most and the air in the lungs and the intestines absorbs the least.
When a bone is in the path of the beam, few x-rays strike the film so it looks white. Conversely, many x-rays strike the film when air is in the path of the beam so that it looks black. Half of the dose of ionizing radiation received by the U.S. population is due to medical diagnostic or therapeutic x-rays. The remainder is due to background radiation from cosmic rays and naturally occurring radioactive substances. Other than for pregnant women, the risk of exposure to appropriately performed diagnostic x-rays is exceedingly small and is far outweighed by the benefit of accurate diagnosis.