Background radiation: natural and man-made

Abstract

A brief overview and comparison is given of dose rates arising fromnatural background radiation and the fallout from atmospheric testing ofnuclear weapons. Although there are considerable spatial variations inexposure to natural background radiation, it is useful to give estimates ofworldwide average overall exposures from the various components of thatbackground. Cosmic-ray secondaries of low linear energy transfer (LET),mainly muons and photons, deliver about 280 µSv a−1.Cosmic-ray neutrons deliver about another 100 µSv a−1.These low- and high-LET exposures are relatively uniform to thewhole body. The effective dose rate from cosmogenic radionuclides isdominated by the contribution of 12 µSv a−1 from14 C.This is due to relatively uniform irradiation of all organs and tissues from low-energyβparticles. Primordial radionuclides and their progeny (principally the 238 U and 232 Th series, and40 K)contribute about 480 µSv a−1of effective dose by external irradiation. This is relatively uniform photonirradiation of the whole body. Internally incorporated 40 Kcontributes a further 165 µSv a−1of effective dose in adults, mainly from β particles, but with asignificant γcomponent. Equivalent doses from 40 Kare somewhat higher in muscle than other soft tissues, but the distinction isless than a factor of three. Uranium and thorium series radionuclidesgive rise to an average effective dose rate of around 120 µSv a−1. Thisincludes a major αparticle component, and exposures of radiosensitive tissues in lung, liver, kidney and theskeleton are recognised as important contributors to effective dose. Overall, these varioussources give a worldwide average effective dose rate of about 1160 µSv a−1. Exposureto 222 Rn,220 Rnand their short-lived progeny has to be considered separately. This isvery variable both within and between countries. For 222 Rn andits progeny, a worldwide average effective dose rate is about 1105 µSv a−1. For220 Rnand its progeny, the corresponding value is 91 µSv a−1.In both cases, the effective dose is mainly due to α particleirradiation of the bronchial tissues of the lungs. Overall, the worldwide averageeffective dose rate from natural background is about 2400 µSv a−1 or2.4 mSv a−1.For comparison, worldwide average effective dose ratesfrom weapons fallout peaked at 113 µSv a−1(about 5% of natural background) in 1963 and have since fallen to about 5.5 µSv a−1(about 0.2% of natural background). These values perhaps serve to emphasisethat even gross insults to the natural environment from anthropogenic releases ofradioactive materials are likely to be of limited significance when set in thecontext of the ambient radioactive environment within which all organisms,including humans, have developed.