The scientific basis of the radiation hazard.

Radiation hazards have been known for more than 60 years. Only a few months after Rontgen’s discovery in I 895, enthusiasts were burning themselves and their patients by liberal applications of the new X-rays. Soon the dangers of X-rays and radium became all too obvious, or so it seems to us. At the time, few people were much alarmed and no effective measures of control were organized until about 1920. More recently, some new radiation hazards have been identified and our assessment of the old ones has become keener. At the same time the subject has been confused by outbreaks of superstition, prejudice, ignorance and emotion, all serious obstacles to orderly argument and rational judgement. Against this highly coloured background it is possible to pick out a few features relevant to the subject of this Symposium. T o begin, it should be made clear that radiation is everywhere. We all receive a measurable dose from cosmic rays and from radioactive materials present in the earth’s crust, in building materials and even in drinking water. A further contribution comes from radioactive isotopes of potassium and carbon in our bodies. (See Table I). Next we should decide what is meant by the term radiation. For the present purpose, the word covers several different kinds of energy. Some are in the form of electromagnetic waves and some are kinetic energy (that is, energy of movement) of subatomic particles. The waves are either X-rays or y-rays. These two kinds are identical in properties, but different in origin. X-rays are made in an electrical-discharge tube and y-rays come from radioactive materials. Radioactivity is a spontaneous breaking up of the atomic nucleus, with the emission of electrons (almost invariably) and y-rays (very commonly). The IOO or so chemical elements have among them about 1500 isotopes of which some 1200 are radioactive. The reason why radioactive materials are not commoner and more important is that most of them do not last very long. The breaking up of the nucleus usually transforms the radioactive atom into a stable one with no further emanations. The decay process goes according to an exponential law, quickly at first and then more slowly. The rate of decay can best be denoted by the half-life, i.e. the time needed for half of the atoms