Physics » Medical Applications of Nuclear Physics » Biological Effects of Ionizing Radiation

# Summarizing Biological Effects of Ionizing Radiation

## Biological Effects of Ionizing Radiation Summary

• The biological effects of ionizing radiation are due to two effects it has on cells: interference with cell reproduction, and destruction of cell function.
• A radiation dose unit called the rad is defined in terms of the ionizing energy deposited per kilogram of tissue:

$$1 rad=\text{0.01 J/kg}.$$

• The SI unit for radiation dose is the gray (Gy), which is defined to be $$1 Gy = 1 J/kg = 100 rad.$$
• To account for the effect of the type of particle creating the ionization, we use the relative biological effectiveness (RBE) or quality factor (QF) given in this table and define a unit called the roentgen equivalent man (rem) as

$$\text{rem}=\text{rad}×\text{RBE}.$$

• Particles that have short ranges or create large ionization densities have RBEs greater than unity. The SI equivalent of the rem is the sievert (Sv), defined to be

$$\text{Sv}=\text{Gy}×\text{RBE}\text{and 1 Sv}=1\text{00 rem.}$$

• Whole-body, single-exposure doses of 0.1 Sv or less are low doses while those of 0.1 to 1 Sv are moderate, and those over 1 Sv are high doses. Some immediate radiation effects are given in this table. Effects due to low doses are not observed, but their risk is assumed to be directly proportional to those of high doses, an assumption known as the linear hypothesis. Long-term effects are cancer deaths at the rate of $$\text{10}/{\text{10}}^{6}\phantom{\rule{0.25em}{0ex}}\text{rem·y}$$and genetic defects at roughly one-third this rate. Background radiation doses and sources are given in this table. World-wide average radiation exposure from natural sources, including radon, is about 3 mSv, or 300 mrem. Radiation protection utilizes shielding, distance, and time to limit exposure.

## Glossary

### gray (Gy)

the SI unit for radiation dose which is defined to be $$1 Gy=1 J/kg=\text{100 rad}$$

### linear hypothesis

assumption that risk is directly proportional to risk from high doses

the ionizing energy deposited per kilogram of tissue

### sievert

the SI equivalent of the rem

### relative biological effectiveness (RBE)

a number that expresses the relative amount of damage that a fixed amount of ionizing radiation of a given type can inflict on biological tissues

### quality factor

same as relative biological effectiveness

### roentgen equivalent man (rem)

a dose unit more closely related to effects in biological tissue

### low dose

a dose less than 100 mSv (10 rem)

### moderate dose

a dose from 0.1 Sv to 1 Sv (10 to 100 rem)

### high dose

a dose greater than 1 Sv (100 rem)

### hormesis

a term used to describe generally favorable biological responses to low exposures of toxins or radiation

### shielding

a technique to limit radiation exposure

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