Dioxin and dioxin-like compounds in soil, Part II: Technical support document for ATSDR interim policy guideline

Are Mexican agricultural farmlands PCDD/F soil reservoirs?

Our aim was to identify polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzo-p-furans (PCDFs) in agricultural farmland soils in the Northwest of Mexico. We obtained ≈50 g of soil in five Yaqui Valley (VY) agricultural fields in the north-western Mexican State of Sonora and in five Culiacán Valley (VC) agricultural fields in the north-western Mexican State of Sinaloa. Fields with minimal tillage, with ferti-irrigation, and those with intensive aerial and manual tillage were included. All soil samples were subjected to the chemical activated luciferase gene expression (CALUX(®)) test to determine PCDD/F. On average, samples contained 4.2 ± 1.2 PCDD/F ppt TEQ; VY soil samples contained 4.72 ± 1.23 PCDD/F ppt TEQ, while VC soil samples showed 3.6 ± 1.1 PCDD/F ppt TEQ (p = 0.47). On considering tillage-type, in agricultural fields catalogued as intensive tillage, PCDD/F concentrations were 4.40 ± 0.43 in agricultural fields catalogued as intensive tillage, while in farmlands of another tillage-type these concentrations were slightly higher (5.53 ± 0.8).

Identifying soil cleanup criteria for dioxins in urban residential soils: how have 20 years of research and risk assessment experience affected the analysis?

This article reviews the scientific evidence and methodologies that have been used to assess the risks posed by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and presents a probabilistic analysis for identifying virtually safe concentrations of TCDD toxicity equivalents (TEQ) in residential soils. Updated data distributions that consider state-of-the-science cancer and noncancer toxicity criteria, child soil ingestion and dermal uptake, bioavailability in soil, and residential exposure duration are incorporated. The probabilistic analysis shows that the most sensitive determinants of dose and risk are childhood soil ingestion, exposure duration, and the selected TCDD cancer potency factor. It also shows that the cancer risk at 1 per 100,000 predicted more conservative (lower) soil criteria values than did the noncancer hazard (e.g., developmental and reproductive effects). In this analysis, acceptable or tolerable soil dioxin concentrations (TCDD TEQ) ranged from 0.4 to 5.5 ppb at the 95th percentile for cancer potency factors from 9600 to 156,000 (mg/kg/d)(-1) with site-specific adjustments not included. Various possible soil guidelines based on cancer and noncancer risks are presented and discussed. In the main, the current toxicology, epidemiology, and exposure assessment data indicate that the historical 1 ppb TEQ soil guidance value remains a reasonable screening value for most residential sites. This analysis provides risk managers with a thorough and transparent methodology, as well as a comprehensive information base, for making informed decisions about selecting soil cleanup values for PCDD/Fs in urban residential settings.

Public health assessment of hexachlorobenzene

Recently, hexachlorobenzene (HCB) was proposed for inclusion in the system of toxicity equivalency factors (TEFs) currently used for dioxin-like compounds. In this paper, we explore the practical implications of the proposition to the Agency for Toxic Substances and Disease Registry (ATSDR) programs by comparing respective health guidance values for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and HCB (expressed as total toxicity equivalents [TEQs]), reviewing possible interactions between HCB and dioxin-like chemicals, and by providing information on actual co-existence of HCB and dioxin-like chemicals at hazardous waste sites. We found a good correlation between the TEF-adjusted oral exposure guidance values for HCB and guidance values for TCDD. The combination of HCB and other dioxin-like compounds was not found in soil, air, or water media at hazardous waste sites. Based on this fact, it is not necessary to include HCB in the total TEQ count at hazardous waste sites at this time.

Reducing uncertainty in the derivation and application of health guidance values in public health practice. Dioxin as a case study

We were requested by the U.S. Environmental Protection Agency (EPA) to clarify the relationships among the minimal risk level (MRL), action level, and environmental media evaluation guide (EMEG) for dioxin established by the Agency for Toxic Substances and Disease Registry (ATSDR). In response we developed a document entitled "Dioxin and Dioxin-Like Compounds in Soil, Part I: ATSDR Interim Policy Guideline"; and a supporting document entitled "Dioxin and Dioxin-Like Compounds in Soil, Part II: Technical Support Document". In these documents, we evaluated the key assumptions underlying the development and use of the ATSDR action level, MRL, and EMEG for dioxin. We described the chronology of events outlining these different health guidance values for dioxin and identified the areas of uncertainty surrounding these values. Four scientific assumptions were found to have had a great impact on this process; these were: (1) the specific uncertainty factors used, (2) the toxicity equivalent (TEQ) approach, (3) the fractional exposure from different pathways, and (4) the use of body burdens in the absence of exposure data. This information was subsequently used to develop a framework for reducing the uncertainties in public health risk assessment associated with exposure to other chemical contaminants in the environment. Within this framework are a number of future directions for reducing uncertainty, including physiologically based pharmacokinetic modeling (PBPK), benchmark dose modeling (BMD), functional toxicology, and the assessment of chemical mixture interactions.