The emission and propagation of energy through space or a medium in the form of electromagnetic waves (gamma or x-rays) or particles (alpha and beta). Electromagnetic radiation may be ionizing or non-ionizing; all particulate radiation is ionizing. Certain human body parts are more specifically affected by exposure to different types of radiation sources. Several factors are involved in determining the potential health effects of exposure to radiation. These include the size of the dose (amount of energy deposited in the body), the ability of the radiation to harm human tissue, and which organs are affected (Table R.1). The most important exposure factor is the amount of the dose the amount of energy actually deposited in your body. The more energy absorbed by cells, the greater the biological damage. Health physicists refer to the amount of energy absorbed by the body as the radiation dose. The absorbed dose, the amount of energy absorbed per gram of body tissue, is usually measured in units called rads. Another unit of radiation is the rem, or roentgen equivalent in man. To convert rads to rems, the number of rads is multiplied by a number that reflects the potential for damage caused by a type of radiation. For beta, gamma, and x-ray radiation, this number is generally one. For some neutrons, protons, or alpha particles, the number is 20.