Eric J. Hall - 2002 Jul: An explanation of the types of radiation, the units used for measuring radiation, and the affect that various amounts of radiation have on people. Radiation is energy that travels through space. It can travel as a wave which has a frequency that determines penetration characteristics, or as a particle which has mass, size, and velocity that determine penetration ability. Life has evolved in a bath of radioactivity from the beginning, and today we add to this exposure with medical practices that have significant benefits. Radiation levels are measured in Becquerels (Bq) which consist of one atomic decay per second. There are several kinds of radiation: alpha, beta, gamma, and neutron. A radiation dose delivers energy into a human body. This is measured in grays, one joule of energy per kilogram of body. However, the effect of this energy transfer differs for each type of radiation. If this is taken into account, a radiation dose of one type can be compared with that of another type. This way of measuring radiation dose uses sieverts as the unit of measure. One sievert is the maximum dose that you can receive without being killed. Below 50 millisieverts (mSv) no effects of radiation have been measured. The general background level is around 3 mSv. Some places have backgrounds as high as 260 mSv without any evidence of health problems. For public exposure, 1 mSv per year averaged over five years is the typical regulated limit, over and above background levels and medical exposure.