The new DARI (Dose Annuelle due aux Radiations Internes) unit proposed by Georges Charpak and Richard Garwin (Physics Today, June 2002, page 24) has been defined elsewhere as a measure of the internal “irradiation experienced during a single year by an individual due to the radiation emitted by the radioactive materials present in the human body that have nothing to do with any line of work.” 1 Equal to precisely 0.2 millisieverts, the DARI takes into account, as the Physics Today article states, “the biological effects of different decay particles.” This exposure arises principally from the body’s natural levels of potassium-40 and carbon-14; both are sources of sparsely ionizing radiation. Because the sievert numerically equals the gray for such radiation, one DARI equals 0.2 milligray. The sievert is not a directly measurable quantity, because it relies on radiation- and tissue-weighting factors that have been set arbitrarily by committee consensus. 2 However, the gray, the unit of absorbed dose, is a ratio of measurable quantities—the joule and the kilogram.

I suggest that the DARI be related to the gray rather than the sievert. Moreover, I recommend that DARI be interpreted as a threshold of individual sparsely ionizing radiation exposure that would elicit any harmful effect in humans or in any of their tissues. Such a threshold value would be quite reasonable, considering our present knowledge that no harmful effects to humans occur at exposures less than 20 milligray. A threshold based on the DARI being equal to 0.2 milligray would thus contribute to the current discussion on developing a system of radiation protection that is based on risk to the individual. 4 The nonmeasurable sievert and the linear no-threshold hypothesis—impossible to test at exposures low enough to call into question the concept of dose as an amorphous average quantity 5 —are the real sources of confusion in the system used presently.

1.
G.
Charpak
,
R. L.
Garwin
,
Europhysics News
, Jan./Feb.
2002
, p.
14
.
2.
International Commission on Radiological Protection,
Annals of the ICRP
, ICRP Pub. No. 60,
Pergamon Press
,
New York
(
1991
).
3.
United Nations Scientific Committee on the Effects of Atomic Radiation,
UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes
,
United Nations
,
New York
(
2000
).
4.
R. H.
Clarke
,
J. Radiol. Prot.
19
,
107
(
1999
) .
5.
R.
Katz
,
F. A.
Cucinotta
, “Low Fluence,” paper presented at the World Space Congress 2002, Houston, Texas, 10–19 Oct. 2002 (abstract COSPAR02 A-170).