home edit2 whole TSD November 2002 PDF format - OEHHA

PREFACE
The Technical Support Document for Describing Available Cancer Potency Factors (TSD) contains
cancer unit risks and potency factors for 121 of the 201 carcinogenic substances or groups of
substances for which emissions must be quantified in the Air Toxics Hot Spots program. The purpose
of this document is to provide a summary of the data supporting the carcinogenic potential of the
substance or group of substances and to provide the calculation procedure used to derive the estimated
unit risk and cancer potency factors.
The Risk Assessment Advisory Committee (RAAC) report made several recommendations regarding
harmonization within Cal/EPA and with US EPA. The TSD has made an effort to implement the
RAAC recommendations regarding harmonization both in spirit and in substance. For example, the
cancer unit risk and potency factor values developed by OEHHA that are contained in the TSD were
reviewed by a Cal/EPA working group to ensure agency-wide consistency and harmonization.
Currently, OEHHA uses a toxic equivalency factor procedure for polychlorinated dibenzo-p-dioxins
and dibenzofurans that differs slightly from the procedures used by US EPA. This document proposes
adoption of US EPA’s International Toxicity Equivalency Factors (ITEFs) for determining cancer unit
risk and potency values for these chemicals; this is discussed in Appendix A. The document proposes
to adopt 12 US EPA Integrated Risk Information System (IRIS) cancer unit risk values, in lieu of
developing or using other Cal/EPA values. This is summarized in the Unit Risk and Cancer Potency
Table on pages 2-6. In addition, the TSD has identified and evaluated 29 other chemicals for which
Cal/EPA cancer unit risk values differ from corresponding US EPA IRIS values; this is discussed on
page 18 and in Appendices E and F. Fifteen of these 29 chemicals were developed under the Toxic
Air Contaminant (TAC) program; the other 14 were developed under the Proposition 65 program. All
29 of the Cal/EPA values received external peer review and public comments prior to adoption by the
program of origin. Revising these numbers may require the original program to reconsider the value in
an open public process. For example, the Air Resources Board and Scientific Review Panel has
established a specific petition process to reconsider values adopted under the TAC program. The
procedures for requesting reevaluation of cancer potency values by the programs which originated those
values are listed in Appendix G. It should be noted that no new cancer unit risks or potency factors
were developed for this document; all such values contained here were previously developed in
documents by Cal/EPA or US EPA.
As indicated above, the document proposes for adoption 12 US EPA IRIS cancer unit risk values.
Currently, US EPA is considering revisions to its cancer risk assessment guidelines. Some of the
proposed changes may modify the cancer unit risk values only slightly. For example, we anticipate that
some changes in potency slopes and unit risk values will result from changing the body weight scaling
factor from 2/3 to 3/4 power. Additionally, US EPA has stated that cancer potency values listed in
IRIS will not be revisited solely for the purpose of incorporating changes in cancer potency value
calculation methods contained in the draft revised cancer risk assessment guidelines.
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