We all experience radiation exposure. Much of our exposure is from natural sources such as radon, cosmic sources in space, and the soil around us. The rest is from manmade sources like medical procedures, nuclear medicine, and a variety of consumer products such as smoke detectors. This chart from the EPA shows the various sources and percentages.
The reports from Japan and the Fukushima Daiichi nuclear power plant in the weeks that followed the March 2011 earthquake and nuclear disaster thrust many technical terms into the news. A term that is used very frequently is the sievert. What is it? Another link here describes the sievert.
The sievert is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of millionths of a sievert, or micro-sievert.
A unit used in defining the sievert is the Joule/kg. The kilogram (kg) is a quantity of mass of material. It is nearly equal to the mass of 2.25 pounds of ground beef. The Joule (J) is a quantity of energy. It is nearly equal to the energy needed to lift an apple from the floor back up to the kitchen counter against the force of gravity. What the sievert unit does is describe how much energy is deposited in a specific quantity of human flesh by exposure to radiation. The location and nearness of the radiation dosage to vital tissue such as reproductive or intestinal lining are factors that can determine how dangerous the dosage might be.
The deposition of 1 Joule of energy doesn’t sound like much. But, when concentrated on relatively small volumes of tissue, the odds increase for damage to cell structures or DNA genetic material. We all are exposed to radiation doses continuously. These exposures are unavoidable and are quite low in the millionths or thousandths of sieverts (micro-sieverts and milli-sieverts) ranges. Average exposure of U.S. residents from natural and man-made radiation sources is 6.2 milli-sieverts per year.
This video from the University of Nottingham presents the technical definition in understandable terms with some demonstrations of radiation measurement.