BOX 4.1
Units of Measurement

Specific Activity

The curie (Ci) is the traditional unit of measure of nuclear transformations (disintegrations) per second of unit mass. It is a concentration defined as the ratio of the amount of radioactivity divided by the mass or volume of radioactive substance. The International System unit for specific activity is the becquerel (Bq).

Absorbed Dose

The gray (Gy), formerly the rad, is the unit that describes the magnitude of absorbed radiation in terms of energy deposited on a tissue. However, the amount of energy deposited in tissue does not account for differences in the biological effects of different radiation types.

Dose Equivalent

The rem (Roentgen-equivalent-man) is the traditional unit of measure that incorporates the relative biological damage caused by different radiation types and deposition mechanisms. The International System unit for the biologically effective dose, dose equivalent, is the sievert, (Sv).


Specific Activity

Absorbed Dose

Biologically Effective Dose


curie (Ci)

becquerel (Bq)

gray (Gy)

rad (old standard unit)


sievert (Sv)


1 Bq = 1 transformation or disintegration per second = 2.7 × 10−11 Ci

1 Gy = 100 rad

1 mSv = 0.001 Sv

1 Sv = 100 rem


SOURCE: ATSDR, 1999a,b.

approximately 40 percent lower than that of naturally occurring uranium (25.4 mBq/μg) and considerably lower than that of enriched uranium (approximately 1,750 mBq/μg) (Harley et al., 1999). However as discussed above, the chemical properties of depleted uranium are the same as those of the enriched and natural forms.

The chemical and physical properties of depleted uranium are ideal for many military and commercial uses. It is 65 percent more dense than lead (with a density of 18.9 g/cm3), has a high melting point (2070°F, 1132°C), is highly pyrophoric (it ignites when it fragments), has a tensile strength comparable to most steels, and is chemically highly reactive (Kirk, 1981). The density of DU and its ability to self-sharpen are properties that attracted the attention of the Department of Defense (DoD) beginning in the late 1950s, as the military

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