Estimation of variation among individuals of biological half-time of cadmium calculated from accumulation data.

For heavy metals, many experimental studies using animals have revealed short biological half-times (BHTs) to be 1 year or less by administration of heavy metals. Tsuchiya and Sugita, however, devised an original method and first reported the possibility of a long BHT for cadmium (Cd) calculated from Cd accumulation in postmortem human organs and tissues by age using a nonlinear regression analysis employing a differential equation. According to their reports, the estimated values of Cd BHT were 12.1-22.7 years in the kidney. The estimated Cd BHTs are point estimators. The point estimator of Cd BHT has been used indiscriminately to derive safety levels for Cd in the air of work environments or foodstuffs in the general population without taking into consideration individual variation of Cd BHT. In the present study, the estimated variation among individuals of Cd BHT ranges from a few years to at least 100 years in the kidney. It is concluded that the estimated average BHT according to mathematical calculation should not be used indiscriminately to derive the safety level of cadmium exposure.