Flight-Time Equivalent Dose – A Concept to Contextualize Radiological Dose

People are exposed to low levels of radiation every day from many sources including natural radioactivity in soil, rocks, air, food and from cosmic rays. Medical procedures, flying in an airplane, elements within our bodies and even some objects around the house may additionally expose people to small amounts of radiation.The International System of Units (SI) uses the sievert (Sv) as the unit to express the health effect of low levels of ionizing radiation. The sievert, however, is not a commonly understood unit.The worldwide population-weighted average annual effective dose from natural sources of radiation is approximately 2.4 mSv per year [1]. At sea level, approximately 17% of the dose is due to cosmic radiation entering the earth’s atmosphere from space. From the earth’s surface, cosmic radiation will typically double with every increase of 1800 m in altitude. Thus, an individual flying in a commercial jet airplane at an altitude of 10 000 m will receive more radiological dose than an individual on the earth’s surface.By relating the dose received during a commercial flight at altitude to the time spent on the flight, radiological dose from a large variety of sources can be expressed in the more readily understood context of time. For example, an individual flying in an airplane at 10 000 m will receive an average radiological dose of approximately 0.004 mSv per hour, thus a dose of 0.004 mSv is approximately equivalent to a flight-time equivalent dose (FED) of 1 hour. Similarly, the annual worldwide average natural background radiation of 2.4 mSv can be expressed as approximately equivalent to an FED of 600 hours, and a 0.01 mSv dental x-ray as an FED of 2½ hours.The concept of FED provides an excellent benchmark to contextualize radiological dose, especially in communications with the general public. Using this concept, radiological dose can be expressed in readily understandable units of time spent in an airplane at altitude.