Worldwide Residential Soil Guidance Values for Total Dioxins and Dioxin-Like Compounds

Dioxins are well-known soil contaminants. Regulatory jurisdictions worldwide use regulatory guidance values (RGV) to help manage dioxin health risks. In the US, seven federal agencies, 41 states, and seven autonomous Native American jurisdictions have established values as have jurisdictions in at least 37 other nations. Dioxin soil guidance values are highly variable. The values for 2,3,7,8-tetrachlorbenzene-p-dioxin (or total dioxin expressed as TCDD) are well-dispersed over nearly 12 orders of magnitude from $4.91\times {10}^{-9}$ to $2.5\times {10}^3\mathrm{mg}/\mathrm{kg}$. The value spans for other dioxins and dioxin-like dibenzofurans are similar. The degree to which soil dioxin guidance value ranges can be attributed to uncertainty in direct contact cancer and noncancer risk models was analyzed. Based on bounded set uncertainty analysis, approximately 61% of the 304 TCDD RGVs identified fall within US Environmental Protection Agency risk model uncertainty bounds. Approximately 84% fall within noncancer risk model uncertainty bounds and, because the bounds overlap, approximately 60% could have been generated by either model. However, these bounds are conservative because they are based on improbable combinations of model coefficients. When Monte Carlo simulation was used to generate 95% confidence intervals, only 39% of dioxin RGVs fell below the upper bound at a target risk of $1\times {10}^{-6}$.