Department of Toxic Substances Control Department of Toxic Substances Control

Human Health Risk (HERO)



Toxicity Criteria Selection for Risk Assessments, Screening Levels, and Remediation Goals.     

DTSC has released the draft regulations for selecting toxicity criteria for Risk Assessments, Screening Levels, and Remediation Goals. On August 28, 2017 in Sacramento, California, and on August 31, 2017 in Cypress, California, DTSC will explain the proposed regulations, and hold informal discussions to solicit input from the public on the proposed regulations.


Quarterly information from HERO with important recent updates on human health risk assessment.


Preliminary Endangerment Assessment Guidance Manual  (PEA Guidance Manual) (January 1994.  Revised October 2015).

This manual provides guidance on the basic information needed to determine if a release of hazardous substances to the environment presents a risk to human health or the environment.  It explains how a PEA ties into the cleanup process and provides technical recommendations for conducting a PEA site investigation and human health and ecological screening evaluations.  The manual also makes recommendations on how a PEA report should be organized and what information should be included in the report.   

Use of the Northern and Southern California Polynuclear Aromatic Hydrocarbon (PAH) Studies in the Manufactured Gas Plant Site Cleanup Process  (July 1, 2009) The purpose of this advisory is to describe how the ambient conditions for carcinogenic polynuclear aromatic hydrocarbons identified by the Northern or Southern California PAH Study (i.e., the ambient data sets) might be used as a pragmatic tool in various stages of the soil cleanup process at manufactured gas plant (MGP) sites.  Additional files available for download include the Northern and Southern California PAH studies and their corresponding ambient PAH datasets.

Arsenic Strategies: Determination of Arsenic Remediation - Development of Arsenic Cleanup Goals for Proposed and Existing School Sites 

During the site investigation, arsenic may be identified as a chemical of concern based on comparisons to naturally occurring background concentrations.   Once arsenic has been identified as a chemical of concern, a standard approach is needed to determine if remedial action is warranted and, if so, how to develop appropriate cleanup goals.

Ambient Metal Concentrations

This guidance document presents several useful principles for defining the local ambient data set, including pooling all data from all impacted sites and locating ambient conditions in the presence of possible contamination.

Background Metals at Los Angeles Unified School Sites - Arsenic

This guidance is intended to supplement the DTSC PEA Guidance Manual (DTSC1994), and provide a uniform and streamlined approach for evaluating background arsenic at LAUSD school sites.

Human Health Risk Assessment Note 1 - Default Human Health Exposure Factors
This note summarizes exposure factors which may be used as default values in human health risk assessments for California hazardous waste sites and permitted facilities.

Human Health Risk Assessment Note 2 - Dioxin,  April, 2017
Soil Remedial Goals for Dioxins and Dioxin-like Compounds for consideration at California Hazardous Waste Sites.  This note presents a suite of suggested Dioxin-TEQ soil remediation goals that have been developed for consideration at mitigation sites in California for the protection of human health. These goals may be revised in the future, as new scientific information becomes available.

Human Health Risk Assessment Note 3 – DTSC-Modified Screening Levels (DTSC-SLs), August 2017 Update.

HHRA Note Number 3 presents recommended screening levels (derived using DTSC-modified exposure and toxicity factors) for chemical constituents in soil, tap water, and ambient air. This August 2017 update incorporates the U.S. Environmental Protection Agency’s June 2017 release of their regional screening levels and other changes, and supersedes the June 2017 Interim Update to HHRA Note Number 3.

Links are provided for download of the HHRA Note 3 narrative and screening-level document, as well as links to the supporting Appendices that provide computational details. An additional link is provided to spreadsheet-based versions of the screening-level tables for users’ convenience. 


Human Health Risk Assessment Note 4 - Screening Level Human Health Risk Assessments

In a memorandum dated October 28, 1994, the Office of Human and Ecological Risk Assessment recommended guidelines for use of the U.S. EPA Region 9 Preliminary Remediation Goals (PRGs) at military sites. Subsequently, the U.S. EPA released Regional Screening Levels (RSLs) to replace the PRGs formerly available from several U.S. EPA Regional offices. In HHRA Note 3, HERO addressed the recommended methodology for use of U.S. EPA RSLs in the human health risk assessment process at DTSC sites. HHRA Note 4 outlines the current recommended methodology for conducting screening level human health risk assessments, and is an update which replaces our 1994 memorandum and earlier versions of Note 4. This revision incorporates clarification for sites with elevated chemical concentrations known to exist at depths greater than 10 feet below ground surface and typographic corrections.

Human Health Risk Assessment Note 5 – Indoor Air Action Levels for Trichloroethylene (TCE),  August 23, 2014

The U.S. EPA Region 9 released trichloroethylene (TCE) guidance on December 3, 2013 for expanded sample collection in the investigation of the Vapor Intrusion (VI) exposure pathway at specific National Priority List (NPL) sites in the San Francisco, CA South Bay.  Accelerated Response Action Levels and Urgent Response Level Action Levels for indoor air concentrations of TCE under residential, commercial/industrial (8-hour workday), and commercial/industrial (10-hour workday) exposure scenarios were presented in this document. Use of these Region 9 Interim Action Levels to sites beyond the NPL South Bay sites in San Francisco, California was provided in the June 30, 2014 U.S. EPA Region 9 Regional Toxicologist's memorandum, released under a July 9, 2014 transmittal memorandum from Enrique Manzanilla, Director of the Superfund Division, U.S. EPA Region 9.        

Human Health Risk Assessment Note 5 describes how HERO recommends implementation of the TCE Action Levels contained in this EPA Region 9 guidance, specifically on the issues of: 1) applicability to all sites where VI is being evaluated;  2) interim measures; and, 3) response actions. 

Human Health Risk Assessment Note 6 - Recommended Methodology for Evaluating Site-Specific Arsenic Bioavailability in California Soils

This note introduces the California Arsenic Bioaccessibility (CAB) method.  CAB is the recommended in vitro method for site-specific evaluation of arsenic bioavailability where arsenic levels in soil are 1500 mg/kg or less. It can accurately predict in vivo relative bioavailability (RBA) and can be used in place of expensive and time-consuming animal studies.  The use of site-specific RBA reduces the uncertainty of the risk assessment thereby improving remedial decision making.  Using the CAB method, where appropriate, often leads to a more effective use of the resources available for remediation without compromising the level of health protectiveness.   The CAB method is the outcome of a Brownfields Training, Research and Technical Assistance Grant from the US EPA (Brownfields Research Cooperative Agreement TR - 83415101) and was developed in collaboration with Nicholas Basta at The Ohio State University.  For more detailed information on how the method was developed and additional work completed under this grant please refer to our Study webpage at  



OEHHA "Schoolscreen" Spreadsheet

HERO no longer recommends the use of the OEHHA "Schoolscreen" spreadsheet (last updated January 2010) for evaluation of school sites, as the spreadsheet model is outdated and no longer supported by the OEHHA. Please consult with a HERO toxicologist for evaluating health risks in such cases.


LeadSpread is a tool for evaluating exposure and the potential for adverse health effects resulting from exposure to lead in the environment.  An updated version of LeadSpread has been developed (LeadSpread 8; 2011) to incorporate the updated CalEPA incremental lead toxicity criterion of 1ug/dL (OEHHA, 2007), as well as ensure that the model is adequately protective of women of child-bearing age.  The link to LeadSpread 7 is maintained for sites outside California being evaluated based on the 10 ug/dL total blood lead criterion.

Cancer Potency Factors and Reference Doses
Cancer Potency Factors and Reference Doses are available from the California Office of Environmental Health Hazard Assessment.



Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air - Final (October 2011; also known as the Vapor Intrusion Guidance)
DTSC's Vapor Intrusion Guidance provides a process for the investigation of subsurface vapor intrusion into indoor air and describes procedures for screening and site-specific evaluation of potential risks associated with this exposure pathway.  Indoor air concentrations estimated from soil gas or groundwater concentrations by default vapor attenuation factors, vapor intrusion modeling, and/or measured indoor air concentrations are used in the assessment.  Models for estimating indoor air concentrations include the DTSC modification of the U.S. EPA version of the Johnson and Ettinger vapor intrusion model with default and site-specific inputs.


Advisory - Active Soil Gas Investigations (July 2015). This Cal EPA Advisory was jointly revised and updated July 2015 by DTSC, Los Angeles Regional Water Quality Control Board (LARWQCB), and San Francisco Regional Water Quality Control Board. This updated Advisory provides technically consistent approaches and best practices for collecting and analyzing soil gas samples. Data obtained from soil gas investigations can be used to identify the spatial distribution of volatile contamination at a site and assist in the evaluation of vapor intrusion.

The Johnson and Ettinger (J&E) model (1991) predicts indoor air concentrations resulting from subsurface vapor migration into indoor air.  The model produces an attenuation factor "alpha" that represents the ratio of the indoor air concentration to the subsurface concentration.  In 1998, the United States Environmental Protection Agency (USEPA) programmed the J&E model into Microsoft EXCEL® and added a health risk component that calculates human health risks and/or hazards associated with inhalation of a specific contaminant at the estimated indoor air concentration.  The USEPA periodically revised the model to incorporate different assumptions about soil parameters and chemical properties, including new human health toxicity criteria developed by USEPA.  The USEPA model (last revised in 2004), a fact sheet, and User's Guide are available at:

The DTSC Human and Ecological Risk Office has modified the USEPA screening models for estimating indoor air concentrations from subsurface soil gas or groundwater data and incorporated human health toxicity criteria developed by the Cal/EPA Office of Environmental Health Hazard Assessment (OEHHA).  The toxicity criteria will be periodically updated as new California and USEPA criteria values are released: however it is the responsibility of the user to verify that all criteria in the models are current.  The models were revised in March 2014 to reflect recommendations in the Final DTSC Vapor Intrusion Guidance (2011), provide additional receptor scenarios and incorporate exposure time as a receptor exposure parameter, update toxicity criteria, and add features for user convenience.  The models were updated in December 2014 to incorporate USEPA and DTSC revisions in residential receptor exposure duration and noncancer averaging time.

The user can download these EXCEL® files and use the default input for screening and/or insert site-specific data to calculate indoor air concentrations and risk estimates for a specific site.  Microsoft EXCEL® or the Microsoft EXCEL® Viewer is required to view these files.  If the user does not have Microsoft EXCEL®, the EXCEL® Viewer is available at:

Johnson, P. and Ettinger, R., 1991.  Heuristic Model for Predicting the Intrusion Rate of Contaminant Vapors into Buildings.  Environmental Science and Technology, 25:1445-1452.

Page last updated September 29, 2017.