Having described the units in which radiation is measured, and discussed how radiation relates to biological damage even in the most severe cases, it is necessary to put this information into perspective with the background radiation that is continuously received from the natural environment and then to relate to the allowable radiation doses set by regulatory bodies.

5.1.3.1 Background Radiation

The natural environment gives every individual a dose of approximately 100-600 mrem per year due to cosmic rays and due to radioactive materials in the Earth’s crust. Table 5.3 demonstrates the origin of this natural background.

The natural cosmic ray-induced radiation varies with height above sea level, such that a resident in Denver, Colorado could better than halve his

exposure to cosmic radiation by moving to a new home at sea level. Areas at high altitude such as the Himalayan areas of Tibet are particularly high in this form of background radiation.

Terrestrial radiation also varies with location, the granite areas of France being ten times as high in radioactive background as that of the continental USA, while Kerala, India is another factor 5 higher still.

Added to this natural background are man-made radioactive sources. Table 5.4 shows the origin and levels of some man-made sources. The high rate from medical x rays is being now reduced by the education of operators, more use of shielding, and the use of discriminating x-ray beams. A great deal still remains to be done to limit the indiscriminate use of medical and dental x rays.

In the past, highly radioactive sources were presented to the public out of an ignorance of the consequences: diagnostic x-ray machines for shoe fitting, the use of uranium compounds for coloring yellow bathtubs, further use of uranium compounds in jewelry and watch dials. Many of these sources had very high radioactive levels: the yellow pigment colored bath­tubs gave an average clean individual exposure of some 100 mrem/yr. Many of these practices have now been discontinued with the imposition of modern licensing radiation standards.

Nevertheless, it is apparent that even today man may apparently be content to double his background radiation levels by his own acceptance of man-made radioactivity from such things as x rays and TV sets.

5.1.3.2 The International Commission on Radiological Protection (ICRP)

The ICRP has recommended limits to be placed on the dose that an indi­vidual may incur in the course of his work. These limits, published in 1967, are listed in Table 5.5 (7).