# 10.3: Radiation and its effect on DNA

For biologists, the most significant forms of radiation are light, heat, and ionizing radiation. Ionizing radiation can penetrate cells and create ions in the cell contents. These, in turn, can cause permanent alterations in DNA (i.e., mutations). Ionizing radiation includes X rays, gamma rays, neutrons, electrons ("beta" particles), and alpha particles (helium nuclei).

### Units of measurement

• rad: The rad represents a certain dose of energy absorbed by 1 gram of tissue. It is a unit of concentration. So if we could uniformly expose the entire body to radiation, the number of rads received would be the same whether we were speaking of a single cell, an organ (e.g., an ovary) or the entire body (just as the concentration of salt in sea water is the same whether we consider a cupful or an entire ocean).
• rem: Some forms of radiation are more efficient than others transferring their energy to the cell. To have a level playing field, it is convenient to multiply the dose in rads by a quality factor (Q) for each type of radiation. The resulting unit is the rem ("roentgen-equivalent man"). Thus, rem = rad x Q. X rays and gamma rays have a Q about 1, so the absorbed dose in rads is the same number in rems. Neutrons have a Q of about 5 and alpha particles have a Q of about 20. An absorbed dose of, say, 1 rad of these is equivalent to 5 rem and 20 rem respectively.
• The sievert (Sv) and gray (Gy): Despite the years of high-quality research reported in rems and millirems (mrem, 10-3 rem), the International Commission on Radiation Units and Measurements wants us to give up the rad in favor of the gray (Gy), a unit 100 times larger. Similarly, the rem is to be replaced by the sievert (Sv), again so that 100 rem = 1 Sv. So I will try to express all radiation doses in a single unit, the millisievert (mSv).

Estimated average annual radiation exposure from various sources (in millisieverts) of an inhabitant of the United States (total = 5.86 mSv). Individual exposures, especially to radon and medical sources, vary widely from these average values. The use of medical imaging in the United States (some 67 million CT scans were performed here in 2006) has increased greatly in recent years. As for radon, only the lungs are exposed as the alpha particles emitted by radon cannot penetrate other tissues. (Data from the National Council on Radiation Protection and Measurements, Bethesda, MD.)