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About the Health Effects Fact Sheets

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The chemical fact sheets contained in this notebook provide a hazard summary for each of the hazardous air pollutants specified in the Clean Air Act Amendments of 1990. These brief summaries are intended to provide a quick reference for pertinent toxicity information and to indicate where more comprehensive and primary information can be found. This information is targeted to those concerned with toxic air pollutants at the State and local levels. The information in the fact sheets is summarized from U.S. Environmental Protection Agency (EPA) databases, and, where EPA data are lacking, information from other sources has been added for a more complete picture of possible toxicity concerns. The fact sheets should be used as one of many information sources for understanding a pollutant's human health hazard potential. In the case of listed compound categories, one or more representative compounds have been selected for the fact sheet summary.

The fact sheets do not represent EPA policy on health hazards and they should not be construed as constituting an EPA judgment about a hazard where a judgment does not already exist. In addition, they cannot be cited or quoted as a primary source of EPA health hazard information.

We have developed separately a glossary of health, exposure, and risk assessment terms and the definitions of acronyms used in the graphs that accompany the fact sheets.  Also available is a hazardous air pollutants' Fact Sheet chemical name and CAS number cross reference file, which contains an alphabetical listing of all the chemicals and their most common synonyms contained in this Notebook.

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General Information

This health effects notebook contains fact sheets on hazardous air pollutants. These chemicals include volatile organic chemicals, chemicals used as pesticides and herbicides, inorganic chemicals, and radionuclides. Many of these chemicals are used for a variety of purposes in the United States today. Other chemicals, although not in use today, were used extensively in the past and may still be found in the environment.

These fact sheets were developed based on available human and/or animal data. The human data consist primarily of epidemiological studies, which are studies that examine the incidence, distribution, and control of disease in the human population. The primary type of epidemiologic studies used in the development of these fact sheets were occupational studies, which examined the effects of a chemical on a group of workers over time. The animal data are from studies that experimentally exposed animals to chemicals to observe and measure toxicity and disease development. These studies include effects data related to acute (short-term) exposures, subchronic (medium-length) exposures, and chronic (long-term) exposures.

The primary sources of information used to develop the fact sheets are EPA's Integrated Risk Information System (IRIS),(, a database that summarizes available toxicity data and contains EPA's assessment of the data, and secondary sources, such as EPA's Health Assessment Documents, and the Agency for Toxic Substances and Disease Registry (ATSDR) Toxicological Profiles( In addition, databases such as the Hazardous Substances Data Bank (HSDB), which contains summaries of peer-reviewed literature, and the Registry of Toxic Effects of Chemical Substances (RTECS), which lists toxic effects of chemicals and is not peer reviewed, may be used.

Each bulleted piece of information on the fact sheet contains one or more reference numbers after it which correspond to the numbered references at the end of each fact sheet. Complete reference information is provided in the reference section of each fact sheet.

When available, numerical data are presented in the fact sheets. The numerical data presented are primarily those numbers that are found in the IRIS database. IRIS numbers are EPA- verified values; i.e, they have been subjected to extensive review by EPA scientists and a consensus was reached on their validity. These numbers include the Oral Reference Dose (RfD) and the Inhalation Reference Concentration (RfC) (see the Chronic Effects section) and the risk specific concentration (see the Cancer Risk section). They additionally include Minimal Risk Levels (MRLs) developed by the Agency for Toxic Substances and Disease Registry (ATSDR) and Reference Exposure Levels (RELs) developed by the California Environmental Protection Agency. These additional numbers have been developed in similar manner as the IRIS reference values and subjected to the similar degree of review and consensus, which is reflected in their prioritization by the EPA Office of Air Quality Planning and Standards.

The quality and decisiveness of the knowledge base underlying each of the agents portrayed in the fact sheets vary according to the abundance or paucity of high-quality health science data. The strength or confidence in particular hazard conclusions or inferences similarly varies according to the amount and quality of data and also according to the inherent methodologic generated uncertainties that go along with health hazard assessment. The overall effect of these circumstances is that each agent has its unique combination of uncertainty elements that are not explicitly listed in the fact sheets. A responsible user of these fact sheets must appreciate the implications of the underlying uncertainties, which take on more or less meaning depending on how the fact sheets are used.

The fact sheets focus on the health effects of each chemical in humans and laboratory animals (mammals). No information is presented on the ecological effects of the chemicals in nonmammalian species. 

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Fact Sheet Organization

The fact sheets are organized as follows:

Chemical Name and CAS Number

The chemical name and Chemical Abstract Service (CAS) registry number given on the fact sheet are the same as that used in the list of 189 hazardous air pollutants in the Clean Air Act Amendments of 1990. For listed compound categories, CAS numbers for one or more representative compounds have been selected.

Hazard Summary

The hazard summary is an overview of the key information provided in the Health Hazard Information section of the fact sheet. 


This section presents the major uses of the chemical in the United States today. In addition, past uses of the chemical may be summarized.

Sources and Potential Exposure

This section contains information on how the general public may be exposed to the chemical, e.g., through water, food, or air, as well as information on occupational exposure. In addition, this section tells which chemicals have been listed as pollutants of concern to EPA's Great Waters Program and the reason for the listing. The purpose of the Great Waters Program is to evaluate the atmospheric deposition of air pollutants to the Great Lakes, Lake Champlain, Chesapeake Bay, and coastal waters. All of the pollutants of concern possess certain common characteristics, including persistence in the environment and the potential for long-distance transport.

Assessing Personal Exposure

This section describes the medical tests available to determine whether a person has been exposed to the chemical.

Health Hazard Information

This section summarizes the key toxicology information on the chemical, emphasizing the inhalation (air) route of exposure. If available, oral and dermal (skin) exposure data are also presented.

Acute Effects

Information on effects reported from acute animal tests and/or acute human studies are presented in this section. Acute human studies may be controlled exposure studies or may consist of case reports from accidental poisonings or industrial accidents. All of these studies help to define the acute exposure levels at which effects are seen in humans.

Acute animal studies often consist of LD50 and LC50 tests, which present the median lethal dose (or concentration) to the animals. The results from these tests are divided into the toxicity categories as shown below.

Type of Dose/Concentration Extreme High Moderate Low
Oral LD50 <50 mg/kg >50 thru 500 mg/kg >500 thru 5,000 mg/kg >5,000 mg/kg
Dermal LD50 <200 mg/kg > 200 thru 2,000 mg/kg >2000 thru 20,000 mg/kg >20,000 mg/kg
Inhalation LC50 <200 mg/m3 > 200 thru 2,000 mg/m3 >2,000 thru 20,000 mg/m3 >20,000 mg/m3

Based on 40 CFR 156.62.

Chronic Effects

This section summarizes the major chronic noncarcinogenic effects seen from exposure to the chemical. Chronic animal studies usually range from 90 days' to 2 years' duration. Human studies investigating effects ranging from exposure of a few years to a lifetime are also included. In addition, subchronic studies may be included in this section. Subchronic studies are usually animal studies of several weeks' to 90 days' duration.

The Inhalation RfC (or MRL or REL) is presented in this section, with information on the critical effect and species upon which it is based. The RfC is an estimate (with uncertainty spanning perhaps an order of magnitude) of the daily exposure of a chemical to the human population by inhalation (including sensitive subpopulations) that is likely to be without deleterious effects during a lifetime of exposure. It is based on the available toxicological data, using uncertainty factors to account for data gaps such as experimental animal-to-human extrapolation and extrapolation to sensitive members of the population. The RfC is not a direct or absolute estimator of risk, but rather a reference point to gauge the potential effects. Exposures at or below the RfC are not likely to be associated with any adverse health effects. However, exceedance of the RfC does not imply that an adverse health effect would necessarily occur. As the amount and frequency of exposures exceeding the RfC increase, the probability that adverse effects may be observed in the human population also increases. If available, the oral RfD is also presented in this section. The RfD is the oral equivalent of the RfC.

EPA's confidence in the RfC and/or RfD is also presented in this section. In the past, EPA ranked each RfC and RfD as either low, medium, or high in three areas: (1) confidence in the study on which the RfC or RfD was based; (2) confidence in the database; (3) overall confidence in the RfC or RfD. As available, all three rankings are presented in this section.

Reproductive and Developmental Effects

This section presents the results of reproductive and developmental studies on the effects of the chemical in animals and humans. Examples of female reproductive effects include reduced fertility, a decrease in the survival of offspring, and alterations in the reproductive cycle. Male reproductive effects include a decrease in sperm count or an increase in abnormal sperm morphology. Developmental effects are adverse effects on the developing organism that result from exposure prior to conception (either parent), during prenatal development, or postnatally to the time of sexual maturation. Examples include altered growth, death of the developing organism, and malformations or birth defects. Reproductive and developmental effects may be observed after short-term or long-term exposure to the chemical because some effects can be attributed to one-time or short-term exposures during a critical biological cycle.

Cancer Risk

The results of available cancer studies in animals and/or humans are presented in this section. In addition, the EPA weight of evidence characterization of the evidence with regard to the chemical's potential to be carcinogenic to humans is included.

The cancer risk section also includes the inhalation unit risk for a chemical, which is the increased probability of a person developing cancer from breathing air containing a specified concentration of the chemical for a lifetime. The inhalation unit risk is derived using mathematical models that assume a nonthreshold approach; i.e., there is some risk of cancer occurring at any level of exposure. The methods used to derive these values result in an "upper bound" estimate; i.e., the true risk is unlikely to exceed this value and may be much lower. The lifetime risk-based concentration, an estimate of the chemical concentration for which a lifetime continuous exposure corresponds to a specified level of cancer risk, is also presented in this section. The risk-based concentrations corresponding to a one-in-a-million, one-in-a-hundred thousand, and one-in-ten thousand excess risk attributed to exposure to the chemical are presented. This means that EPA has estimated that, if an individual were to breathe air containing these concentrations of the chemical over his or her entire lifetime, that person would theoretically have no more than a one-in-a-million, one-in-a-hundred thousand, or one-in-ten thousand increased chance of developing cancer as a direct result of breathing air containing the chemical. The use of the risk-based concentration should include recognition of the weight-of-evidence. 

Physical Properties

This section provides the physical state (solid, liquid, or gas) of the chemical at ambient conditions, as well as solubility information and the odor threshold of the chemical (if available). The chemical formula, molecular weight, vapor pressure, and octanol/water partition coefficient are also provided in this section.

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Following each fact sheet is a graph that visually presents the available toxicity or health risk numbers, showing the concentration (on a logarithmic scale). The numbers labeled as "toxicity and health risk" on the graph are LC50 and LD50 numbers, the RfC (or MRL or REL), RfD, lowest-observed-adverse-effect level (LOAEL) and no-observed-adverse-effect level (NOAEL), which are the basis for the RfC, MRL, REL RfD; lifetime risk-based concentrations developed based on cancer risk estimates cited in EPA's IRIS (or California EPA). Relevant regulatory and advisory numbers are also included on the graph. Regulatory numbers consist of values that have been incorporated in government regulations, such as EPA and State air regulations or the Occupational Safety and Health Administration's (OSHA's) occupational standards. Advisory numbers are nonregulatory values that are provided by the Federal Government, State Government, or other groups as advice. Examples include EPA's Health Advisories and the American Conference of Governmental and Industrial Hygienist's (ACGIH's) threshold limit values (TLVs). The graphs present health data for inhalation exposure only. For those chemicals that lack inhalation data, a graph on health data from oral exposure has been included.

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Data Sources

The data sources relied on for each fact sheet are specified in the References section of each sheet.

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Updated July 2016