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IONIZING RADIATION

Herman Cember Ph.D. PE CHP^†, and Thomas E. Johnson Ph.D. CHP

INTRODUCTION

      Ionizing radiation may be defined as electromagnetic (X‐rays and gamma rays) and particulate radiation (alpha and beta particles, electrons, neutrons, and protons) with sufficient energy to disrupt an atom or a molecule. This is done by knocking out an electron, thereby “ionizing” the atom or molecule.

      Knowledge of radiation and safety problems associated with its use goes back more than 100 years, with the discovery of X‐rays in 1895 by Wilhelm Roentgen. On the same day that Roentgen announced his discovery, Emile Grubbe, a physicist working in Chicago with an apparatus similar to Roentgen's, developed severe skin burns after handling an energized cathode ray tube similar to the one used by Roentgen. This was the first work‐related injury from radiation. In 1896, Antoine Henri Becquerel discovered radioactivity while working with an ore called pitchblende. Investigation revealed three different radiations that originated in the ore. Ernest Rutherford named the first two radiations alpha and beta rays. P.V. Villard later named a third type of radiation, the gamma ray (later investigations showed that X‐rays and gamma rays were the same type of radiation). Alpha and beta particles are still occasionally referred to as alpha rays and beta rays as a result of this discovery.

      The discovery of these radiations opened new fields of scientific investigation and uses for these radiations. In parallel with the studies of the physics and chemistry of radioactive elements and the associated radiation, researchers were also examining the biomedical effects. With this expanded use of radiation, there were further reports of harmful effects, such as skin burns and hair loss when X‐rays were used in medical diagnosis. In 1899, the first case of a cancer, a basal cell carcinoma on a woman's face, was cured by X‐rays. In 1906, two French physiologists, Bergonie and Tribondeau, published their classical paper on the relative radiosensitivity of different cells and tissues. They found that the less differentiated a cell was, and the more frequently it divided, the more radiosensitive it was. Although to date we have found nothing to contradict their observations, much is understood now about the molecular biology basis for their findings. Since the time of their publication, an enormous amount of information on the nature of the interaction of radiation with living tissue and on the dose–effect and dose–response relationships has been amassed. Sources of information, such as the experiences of the radium dial painters, early radiologists, and uranium miners, showed that occupational overexposures led to harmful effects. Other major sources of information include populations that had been medically exposed to diagnostic and therapeutic radiation, survivors of the atomic bombings in Japan, data from radiation accidents, and epidemiological studies of populations exposed to low‐level radiation from nuclear facilities and from natural background. This body of knowledge forms the scientific basis for the radiation safety standards currently in use. After radiation safety guidelines were developed, occupational exposures that were within the radiation safety guidelines did not lead to harmful biomedical effects.

2 SOURCES OF IONIZING RADIATION

One is exposed to radiation from both naturally occurring and anthropogenic sources. Natural background radiation includes cosmic radiation that originates in the sun and in the stars, and terrestrial radiation that is emitted from radioactive minerals on or within the earth. These naturally occurring background radiations must be accounted for when making radiation measurements and decisions, and in certain instances may pose a potential health threat.

      2.1 Natural Radiation

      Cosmic radiation, when it enters the earth's atmosphere, consists mainly (90%) of high kinetic energy protons, with the remaining 10% being alpha particles, neutrons, and electrons. The interaction of these very high‐energy particles with the atmosphere leads to the production of certain radioactive isotopes (notably tritium (³H) and radiocarbon (¹⁴C)), muons (extremely high‐energy, heavy electrons