Friday, January 27, 2017

The Genetic Effects of Radiation

My local public library had their quarterly used book sale last weekend, and as usual I went to search for Isaac Asimov books. I collect Asimov's books in part to pay homage to the huge influence he had on me as a teenager. He was the main reason I became a scientist. No luck this time; I came back from the sale empty handed. It’s difficult for me to find Asimov books that I don’t have, because I have so many (most bought second-hand for a pittance). Nevertheless, he wrote over 500 books, and I own far fewer than that, so I always have a chance.

You would think that I would at least know about all his books, even if I don’t own a copy. Yet somehow, I was unaware of (or had forgotten about) his book The Genetic Effects of Radiation, although it is a topic closely related to Intermediate Physics for Medicine and Biology. If interested, you can download a pdf of the book free (and I think legally) here. Below I present an excerpt about natural background radiation.
Background Radiation

Ionizing radiation in low intensities is part of our natural environment. Such natural radiation is referred to as background radiation. Part of it arises from certain constituents of the soil. Atoms of the heavy metals, uranium and thorium, are constantly, though very slowly, breaking down and in the process giving off alpha rays, beta rays, and gamma rays. These elements, while not among the most common, are very widely spread; minerals containing small quantities of uranium and thorium are to be found nearly everywhere.

In addition, all the earth is bombarded with cosmic rays from outer space and with streams of high-energy particles from the sun.

Various units can be used to measure the intensity of this background radiation. The roentgen, abbreviated r, and named in honor of the discoverer of X rays, Wilhelm Roentgen, is a unit based on the number of ions produced by radiation. Rather more convenient is another unit that has come more recently into prominence. This is the rad (an abbreviation for "radiation absorbed dose") that is a measure of the amount of energy delivered to the body upon the absorption of a particular dose of ionizing radiation. One rad is very nearly equal to one roentgen.

Since background radiation is undoubtedly one of the factors in producing spontaneous mutations, it is of interest to try to determine how much radiation a man or woman will have absorbed from the time he is first conceived to the time he conceives his own children. The average length of time between generations is taken to be about 30 years, so we can best express absorption of background radiation in units of rads per 30 years.

The intensity of background radiation varies from place to place on the earth for several reasons. Cosmic rays are deflected somewhat toward the magnetic poles by the earth's magnetic field. They are also absorbed by the atmosphere to some extent. For this reason, people living in equatorial regions are less exposed to cosmic rays than those in polar regions; and those in the plains, with a greater thickness of atmosphere above them, are less exposed than those on high plateaus.

Then, too, radioactive minerals may be spread widely, but they are not spread evenly. Where they are concentrated to a greater extent than usual, background radiation is abnormally high.

Thus, an inhabitant of Harrisburg, Pennsylvania, may absorb 2.64 rads per 30 years, while one of Denver, Colorado, a mile high at the foot of the Rockies, may absorb 5.04 rads per 30 years. Greater extremes are encountered at such places as Kerala, India, where nearby soil, rich in thorium minerals, so increases the intensity of background radiation that as much as 84 rads may be absorbed in 30 years.

In addition to high-energy radiation from the outside, there are sources within the body itself. Some of the potassium and carbon atoms of our body are inevitably radioactive. As much as 0.5 rad per 30 years arises from this source.

Rads and roentgens are not completely satisfactory units in estimating the biological effects of radiation. Some types of radiation—those made up of comparatively large particles, for instance — are more effective in producing ions and bring about molecular changes with greater ease than do electromagnetic radiations delivering equal energy to the body. Thus if 1 rad of alpha particles is absorbed by the body, 10 to 20 times as much biological effect is produced as there would be in the absorption of 1 rad of X rays, gamma rays, or beta particles.

Sometimes, then, one speaks of the relative biological effectiveness (RBE) of radiation, or the roentgen equivalent, man (rem). A rad of X rays, gamma rays, or beta particles has a rem of 1, while a rad of alpha particles has a rem of 10 to 20.

If we allow for the effect of the larger particles (which are not very common under ordinary conditions) we can estimate that the gonads of the average human being receive a total dose of natural radiation of about 3 rems per 30 years. This is just about an irreducible minimum.
This is typical Asimov. Let me add a few observations.
  1. Asimov rarely wrote specifically about medical physics, although he wrote much about related topics. I think The Genetic Effects of Radiation is closer to IPMB than his other books.
  2. The Genetic Effects is over 50 years old; it is out of date. For example, it uses the archaic units of rad and rem instead of gray and sievert (100 rem = 1 Sv). Moreover, radon gas is now known to make the largest contribution to background radiation, but the word “radon” never appeared in The Genetic Effects. Yet, I was surprised how much has not changed. I think a reader of IPMB would still find much useful information in Asimov's book.
  3. The text is aimed at a general audience, rather than an expert. The book is not a replacement for, say, Radiobiology for the Radiologist, or even IPMB. Yet, for a 16-year old kid (as I was when devouring Asimov's books about science), the level is just right.
  4. The discussion is not mathematical. At times Asimov writes about mathematical results, but he rarely presents equations. Certainly Asimov’s writing has vastly fewer equations than IPMB.
  5. Asimov doesn't use a lot of figures. The Genetic Effects contains more pictures than most of his books.
  6. This excerpt illustrates the "clear, cool voice of Asimov." I admire the clarity of his writing. No one explains things better.
And now I have to return to my search for more Asimov books. I am particularly a fan of his science essay collections originally published in Fantasy and Science Fiction. I have most of them, but somehow missed the very first: Fact and Fancy (1962). Garage sale season will be here in a few months. Hope springs eternal.

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