Human exposures from dioxin in soil—a meeting report

Abstract

A 1984 risk assessment identified 1 part per billion (ppb) of dioxin in soil as a “level of concern” at Times Beach, Mo. The authors of the assessment had to rely on many assumptions in their analysis, but since that time, a number of investigators have obtained data that bear directly on estimating exposures from substances in soil. Partly because of the assumptions and partly because of the site‐specific nature of their analysis, the authors of the Times Beach risk assessment cautioned against the adoption of 1 ppb as a delineator between acceptable and nonacceptable levels of contamination. Those cautions have been more frequently ignored than honored, and 7 ppb has become a de facto standard.

In November 1989, the Center for Risk Management at Resources for the Future hosted 50 experts at a workshop that heard and discussed published and new research about exposure estimates and measures. The 1984 assessment identified soil ingestion by toddlers as the single most important source of exposure to dioxin in soil; it assumed that toddlers ingested 10 g soil daily. Research discussed at the workshop shows that the average child ingests about 0.04 g soil daily, but that 1 of 320 studied children ingested 5 g. These findings leave open the risk management decision about whether acceptable exposure levels should be established to protect the average child or the extreme child. Furthermore, the absence of children from commercial and industrial sites led to suggestions that higher concentrations of dioxin are acceptable in soil at such sites. Some workshop participants objected to those suggestions because of the difficulty of assuring that such sites would not revert to residential use in the future. Whether deed restrictions are sufficient to prevent such reversions was seen as an important research topic. Workshop participants repeatedly discussed the importance of site‐specific data in estimating exposures: Measured half‐lives of dioxin in different soils range from 18 mo to greater than 100 yr; bioavailability from various soils differs by at least 20‐fold; and the amounts of soil ingested by grazing cattle can differ 20‐fold depending on conditions.

Workshop participants agreed upon some suggestions for research and generally favored the development of guidelines for exposure assessment that would allow consideration of site‐specific information. Furthermore, they generally agreed that no single concentration should be taken as a level of concern. Instead, levels should be established that consider the planned uses of the sites.