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; that is, mutations.

## Units of measurement

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).

An assortment of typical radiation doses (in mSv)
Used to destroy the bone marrow in preparation for a marrow transplant (given over several days) 10,000
Approximate lethal dose ("LD50") if no treatment and given to the entire body in a short period 4,500
Causes radiation sickness (when absorbed in a short period) >1,000
When delivered in a single dose, increases the risk of developing cancer by 1% 100
Increase in lifetime dose to most heavily exposed people living near Chernobyl 430
Annual dose (excluding natural background) permitted for U.S. radiation workers 50
Average annual dose (excluding natural background) for medical X-ray technicians 3.2
Maximum permissible annual dose (excluding natural background and medical exposure) to general public 1.7
Average annual dose of natural background radiation, worldwide 2.4
Natural background, Boston, MA, USA (per year)(excluding radon) 1.02
Natural background, Denver, CO, USA (per year)(excluding radon) 1.8
Additional annual dose if you live in a brick rather than a wood house 0.07
Annual dose in some houses in Ramsar, Iran >130
Average dose to person living within 10 miles of Three-Mile Island (TMI) caused by the accident of 28 March 1979 0.01
Most heavily exposed person (a fisherman) near TMI <1.0
Approximate dose received by a person spending 1 year at the fence surrounding a nuclear power station 0.001–0.006
Average dose to each person in the U. S. population from nuclear power plants (per year) 0.00002
Received by the brain during a set of dental x rays 0.005
Received by the colon during a barium enema 15
Received by the lungs during a chest x ray 0.01—0.15
Screening mammogram 0.5
Total dose received by the people living near the Fukushima Daiichi Nuclear Power Station in Japan during the first year after the reactors were damaged by a devastating tsunami. 12–25
Dose from a typical set of full-body computed tomography (CT) scans 20
Cardiac stress test using radioactive thallium 36
Typical dose received by the abdomen during a CT scan to diagnose appendicitis 10
Typical PET scan 14.0
Airline passenger crossing the U.S. 0.04
Flight crew flying regularly between New York and Tokyo (per year) 9
Hourly dose to skin holding piece of the original "Fiesta Ware" (a brand of pottery) 2–3
Annual dose to each person in the U. S. population from fallout (former weapons testing plus Chernobyl) 0.0006

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.)