There are various ways of expressing levels of ionizing radiation.

## Activity

Radioactivity, in a narrow technical sense, is the decay rate of radioactive material with no regard to its effects. The SI unit for radioactivity is the becquerel (Bq): 1 Bq means one decay per second. This unit is also known as dps (disintegrations per second). It corresponds roughly to the number of impulses per second that are detected by a Geiger counter. [?]

Closely related units are dpm (disintegrations per minute) and the curie (Ci). 1 Bq = 1 dps = 60 dpm. 1 Ci = 3.7 × 1010 Bq = 37,000,000,000 Bq.

## Absorbed dose

Not all ionizing radiation is caused by radioactivity, a notable source for human exposure to radiation being medical x-rays. Moreover, different processes of radioactive decay may differ vastly in the energy of their emitted radiation. Therefore the becquerel (multiplied by the time of exposure) is not a suitable unit for measuring biological effects.

The SI unit for absorbed radiation doses is the gray (Gy), defined as one Joule of energy absorbed per kilogramme of matter. Since the absorption depends on the type of matter as well as the type and amount of radiation, absorbed radiation can only be measured with respect to a given material, which is usually but not always taken to be human tissue.

A closely related, mostly obsolete, unit is the rad. 1 Gy = 100 rad. Since the gray is a large unit, actual measurements are typically given in milligray or microgray.

## Equivalent dose

The biological effects of alpha-radiation are different from those of x-rays (gamma-radiation) and beta-radiation. To measure the potential hazards of exposure to radiation the absorbed radiation is therefore multiplied by a weighting factor that ranges from 1 (x-rays) to 25 (alpha-radiation). The resulting equivalent dose is measured by the SI unit sievert (Sv). 1 Gy of gamma-radiation contributes 1 Sv to the overall equivalent dose, but 1 Gy of alpha-radiation contributes 25 Sv. Acute exposure to an equivalent dose of 1 sievert causes organic damage and acute symptoms, so it is a large unit and measurements are typically given in millisievert or microsievert.

A closely related older unit is the roentgen equivalent man (rem). 1 Sv = 100 rem.