Bioavailability of soil-bound TCDD: oral bioavailability in the rat

The implications to the public health of trace amounts of 2,3,7,8-TCDD in the environment are under evaluation by regulatory agencies in the United States and Western Europe. One major consideration in such evaluations is the contribution to human exposure via ingestion of TCDD-contaminated soil. An 80% figure is under consideration by some regulators for estimated human exposure. A contractor for one agency has, in fact, used a value of 100% bioavailability for estimating human bioavailability. Several studies have investigated the oral bioavailability of TCDD from contaminated soil in animals. Most have reported estimates of 25-50%, although one has reported less than 0.5 and 85%, depending on the source of the contaminated soil. This paper reports an oral bioavailability of approximately 43% in the rat dosed with three environmentally contaminated soil samples from Times Beach, Missouri. This figure did not change significantly over a 500-fold dose range of 2 to 1450 ng TCDD/kg of body weight for soil contaminated with approximately 2, 30, or 600 ppb of TCDD. The relevance of animal oral bioavailability data for the human remains to be evaluated. However, since regulatory agencies use animal data for extrapolating to humans, the 43% or 25-50% figure would be more accurate than the 80 or 100% estimates.