RADIOACTIVITY

The radiation dose absorbed by a person is measured in units called millirems; the US citizen gets an average dose of 248 mrems per year; 130 from nature, and another 118 from man-made sources.

The natural background radiation varies with location and altitude; it amounts to 52 mrems/year in Aiken, S.C., and to 157 mrems/year in Denver, Colo. A typical total of 110 mrems from natural sources might be broken down to 35 from cosmic rays (this component roughly doubles for every mile of altitude), 35 from building materials, 25 from food, 11 from the ground, and 5 from the air. The food intake results in a normal level of potassium 40 in human blood which gives its owner an annual dose of 20 mrem.

Of the 118 mrems/year from man-made sources, the lion's share (103 rems) is due to X-ray diagnostics, and most of the remainder also goes for medical equipment (therapeutic 6, radiopharmaceutical 2); 4 mrems are due to global fallout, 1 comes from color television.

These figures are based on data published by the EPA, the US AEC, and the International AEC, which are in turn based on decades of intensive research by prestigious scientific institutions.

Since radioactivity arises by the decay of matter, a given quantity of atoms will eventually stop being radioactive when all their nuclei have been broken down. The halflife of a certain isotope is the time during which half of the original amount has decayed; another halflife will leave half of the previous half, or a quarter; a third halflife will leave 1/8, and so on. The halflives of different nuclei vary from fractions of a second to millions of years.

Funnily enough, the longer the halflife of some radioactive isotope, the more nervous and shrill the environmentalist lamentations, though for a given amount of atoms, a longer halflife must obviously result in a lower intensity of radiation. By the environmentalist logic, it is better to have a 20gallon barrel of water dropped on one's head than to take a 10 minute shower.

Excessive exposure to radioactivity can have two health effects, radiation sickness and cancer. A third effect, genetic mutations in offspring, has been observed in animals, but not in humans (not even in Hiroshima and Nagasaki, in spite of very thorough investigations). Cases of radiation sickness, which can kill victims within weeks (but leaves no symptoms if they survive), could occur only in a major nuclear disaster, in which event they would be few compared to the far larger threat of cancer - a subject to which we will return in our next issue. Until then, we will just note that fossil-burning plants are already causing cancers, and on a far larger scale than nuclear power is ever likely to do.