In recent years there has been a growing interest in natural radiation and the environment from the perspectives of metrology, dosimetry and health effects. According to UNSCEAR (2000), if radiotherapy and radiation accidents are excluded, annual effective doses arising from natural radiation in the environment at about 2.4 mSv year~(-1) contribute on average about 85% of the dose received by the general population worldwide. Of the natural doses, the largest and most variable component is due to residential indoor radon exposure, which has been the subject in recent years of extensive epidemiolog-ical investigations. The significance of the health impact of radon worldwide is currently highlighted by the International Radon Project of the World Health Organisation. Papers on radon in this special issue are therefore timely and appropriate.
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