An Introduction to EPA’s Integrated Risk Information System and Risk Assessments

EPA’s Integrated Risk Information System, or IRIS, is heavily relied upon by regulatory decision-makers and trial attorneys facing contaminant-specific issues.  Through IRIS, EPA evaluates scientific studies and data on human health effects that may result from exposure to substances from releases to air, water, and land.  The IRIS assessment process is supported by a database of 540 chemical substances containing information on human health effects that may result from exposure by various pathways.[1]  IRIS, which is maintained by EPA’s National Center for Environmental Assessment (NCEA) within the Office of Research and Development (ORD), produces assessments of chemical substances that are used as the first two steps in the four-step risk assessment process.  Risk assessments form the scientific basis for an array of environmental regulatory actions, including setting effluent limitations and cleanup standards at hazardous waste sites.  Risk assessments can also be used in contaminant exposure litigation in a myriad of ways, by attorneys and expert witnesses alike.

This article outlines the basics of the IRIS process, the IRIS assessment, and discusses how IRIS assessments are utilized in risk assessments to inform regulatory decision-makers.  Most of this information is available in electronic format on 

The IRIS Process

The IRIS process can be broken into three general stages: (1) choosing the substance; (2) assessing the substance; and (3) reporting the results.

            Choosing Substance for IRIS assessment

EPA develops an annual list of substances for IRIS assessment development using input from EPA Program Offices, EPA Regions, and the public.  Substances are selected based on one or more of the following factors: potential public health impact; EPA statutory, regulatory, or program-specific implementation needs; availability of new scientific information or methodology that might significantly change the current IRIS information; interest to other governmental agencies or the public; and availability of other scientific assessment documents that could serve as a basis for an IRIS assessment.  Proposed assessments and status updates for in-process assessments are published by EPA in the bi-annual Regulatory Agenda.[1]  

            Assessing the Substance

The IRIS assessment process consists of: (1) a Federal Register announcement of EPA's IRIS agenda and call for scientific information from the public on the selected substances (also known as the “Scientific Literature Search and Data Call-In”); (2) a search and compilation of the current scientific literature followed by a Federal Register announcement that the literature search is available on the IRIS internet site, and a call to submit additional scientific information on the substance; (3) development of a draft Toxicological Review or other assessment document; (4) internal peer consultation; (5) internal agency review; (6) scientific consultation with other Federal agencies and White House offices; (7) external peer review, public comment and a listening session(s); (8) final internal agency review, interagency science discussion and ORD management approval; and (9) posting on the IRIS database.[2]

Public comment on the draft IRIS assessment is available at step seven.  The public has the opportunity to comment through written submissions to the Federal Register via, or orally during the public listening sessions.  Agency comment is available during interagency review (step six) and interagency science discussion (step eight).  

            Reporting the Results

The IRIS process results in a final report for each chemical that contains descriptive and quantitative information on cancer and non-cancer effects.  This information can be used, among other things, to support the first two steps (hazard identification and dose-response evaluation) of the risk assessment process.[3]  By December 2007, most of the ongoing assessments had been in progress for over five years.[4]  In an effort to streamline the process, EPA revamped IRIS in 2009 to make the entire process, from step one through step nine, take about two years to complete.[5]  The revisions appear to be working; EPA completed 16 assessments between 2009-2011, more than the total number of assessments completed between 2005-2009.[6]  Final assessments, like drafts, are published to the IRIS database and are accessible to the public.[7] 

IRIS Assessments

A final IRIS assessment typically consists of: (1) the chronic health hazard assessments for noncarcinogenic effects; (2) the carcinogenicity assessment for lifetime exposure; and (3) the bibliography of all studies considered in the assessment.

The assessment for chronic noncarcinogenic effects generally consists of the reference dose for chronic oral exposure (RfD) and the reference dose for chronic inhalation exposure (RfC).  The RfD and RfC are estimates of threshold levels below which there is no “appreciable risk of deleterious effects during a lifetime.”[8] 

The carcinogenicity assessment provides descriptive information on the likelihood that the substance is a human carcinogen, and quantitative estimates of risk from oral and inhalation exposure.  Carcinogenicity is described via one of five categories: (1) not likely to be carcinogenic to humans; (2) inadequate information to assess carcinogenic potential; (3) suggestive evidence of carcinogenic potential; (4) likely to be carcinogenic to humans; and (5) carcinogenic to humans.[9] 

The quantitative estimates of risk are presented as slope factors, unit risk and population risk.  Slope factors are extrapolations of risk across a dose-response curve presented as risk per exposure per day.  Unit risk is a quantitative risk estimate in terms of risk per unit of drinking water or unit of air breathed.  Population risk is the presentation of cancer risk per three population units (1 in 10,000, 1 in 100,000, and 1 in 1,000,000). 

How IRIS Assessments Are Used

Risk assessment is an attempt to quantify the potential adverse effects to humans of environmental exposures.[10]  Estimates of environmental exposure are combined with the known adverse effects of exposure to determine an overall estimate of the potential public health risk.[11]  Risk assessments are the foundation of many regulatory actions (e.g., Maximum Contaminant Levels in water), and are often used to great effect in litigation involving human exposure to chemical substances (e.g., by experts opining on the risk of exposure of a contaminant to a given population).                    

A complete risk assessment consists of four steps:

1.      Hazard identification;

2.      Dose-response assessment;

3.      Exposure assessment; and

4.      Risk characterization.

The IRIS assessment consists of the hazard identification and dose-response assessment steps.  Risk assessors combine the first two steps with site- and problem-specific assessments to determine overall risk.[12] 

Hazard identification involves gauging whether exposure to a substance can cause an increased incidence of an adverse health effect.[13]  Dose-response assessment is the characterization of the relationship between exposure (i.e., dose) and the incidence and severity of the adverse health effect.[14]  Exposure assessment is an individualized determination of the intensity, frequency, and duration of exposure.[15]  Risk characterization integrates the three prior steps and synthesizes an overall conclusion about risk for decision makers.[16]

Risk characterization is used to inform many state and federal agency decisions, like: how much of a substance a company may discharge into a river; which substances may be stored at a hazardous waste disposal facility; to what extent a hazardous waste site must be cleaned up; permit levels for discharge, storage, or transport; and levels for air emissions.


EPA IRIS Portal:

IRISTrack, online tracking tool for IRIS assessments:

Health & Environmental Research Online (HERO) database:

EPA Risk Assessment Portal:

[1] Twice a year, federal agencies publish a comprehensive report describing regulations currently under development or recently completed.  These reports are bundled together and published as the Unified Agenda. Each agency's contribution is called a Semiannual Regulatory Agenda.  The EPA Regulatory Plan and Semiannual Regulatory Agenda is available to purchase in hard copy at, and in electronic format at 

[2] An EPA flowchart graphically representing the process is available at (last accessed October 29, 2011). 

[3] According to EPA, a human health risk assessment “is the process to estimate the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future.”  Environmental Protection Agency, Human Health Risk Assessment, (last visited October 29, 2011).  The assessment includes four basic steps: (1) hazard identification, (2) dose-response assessment, (3) exposure assessment, and (4) risk characterization, which are discussed in more depth infra.

[4] IRIS Progress Report, National Center for Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency (August 2011), available at: (last accessed November 4, 2011).

[5] Id.

[6] Id.

[7] Assessments are available on the searchable IRIS database, at (last visited November 4, 2011).

[8] See, e.g., Environmental Protection Agency, IRIS Assessment of Formaldehyde (2011), available at: (last visited October 29, 2011).

[9] For a description of each category, visit the EPA IRIS Guidance page: (last visited October 29, 2011). 

[10] For the purposes of this 101 Practice Series article, only human health risk assessments are addressed.  Ecological risk assessments do not typically rely upon IRIS assessments, and as such, fall outside of the scope of this article.

[11] See generally, Environmental Protection Agency, Risk Characterization Handbook (December 2000), available at: (last accessed October 29, 2011).

[12] See Environmental Protection Agency, Human Health Risk Assessment, (last visited November 4, 2011).

[13] See Environmental Protection Agency, Human Health Risk Assessment Step 1 – Hazard Identification, (last visited November 4, 2011).

[14] See Environmental Protection Agency, Human Health Risk Assessment Step 2 – Dose-Response Assessment, (last visited November 4, 2011).

[15] See Environmental Protection Agency, Human Health Risk Assessment Step 3 – Exposure Assessment, (last visited November 4, 2011).

[16] See Environmental Protection Agency, Human Health Risk Assessment Step 4 – Risk Characterization, (last visited November 4, 2011).

[17] See Environmental Protection Agency, Human Health Risk Assessment Step 4 – Risk Characterization, (last visited November 4, 2011).