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Glossary of Key Terms

1 in a million cancer risk:
A risk level of 1 in a million implies a likelihood that up to one person, out of one million equally exposed people would contract cancer if exposed continuously (24 hours per day) to the specific concentration over 70 years (an assumed lifetime). This risk would be an excess cancer risk that is in addition to any cancer risk borne by a person not exposed to these air toxics. Note that this assessment looks at lifetime cancer risks, which should not be confused with or compared to annual cancer risk estimates. If you would like to compare an annual cancer risk estimate with the results in this assessment, you would need to multiply that annual estimate by a factor of 70 or alternatively divide the lifetime risk by a factor of 70.


"N" in a million cancer risk:
A risk level of "N" in a million implies a likelihood that up to "N" people, out of one million equally exposed people would contract cancer if exposed continuously (24 hours per day) to the specific concentration over 70 years (an assumed lifetime). This risk would be an excess cancer risk that is in addition to any cancer risk borne by a person not exposed to these air toxics. Note that this assessment looks at lifetime cancer risks, which should not be confused with or compared to annual cancer risk estimates. If you would like to compare an annual cancer risk estimate with the results in this assessment, you would need to multiply that annual estimate by a factor of 70 or alternatively divide the lifetime risk by a factor of 70.


Activity pattern data:
In an inhalation exposure assessment, activity pattern data depict both the actual physical activity (including an associated inhalation exertion level) ; the physical location; and the time of day the activity takes place (e.g., at midnight while sleeping at home , jogging in the park at 8 a.m., or driving in a car at 6 p.m). The Hazardous Air Pollution Exposure Model (HAPEM) extracts activity pattern data from EPA's Comprehensive Human Activity Database (CHAD).


Air toxics:
Also known as toxic air pollutants or hazardous air pollutants are those pollutants known to or suspected of causing cancer or other serious health problems. Health concerns could be associated with both short and long term exposures to these pollutants. Many are known to have respiratory, neurological, immune or reproductive effects, particularly for more susceptible or sensitive populations such as children. Five important air pollutants are not included in the list of air toxics because the Clean Air Act addresses them separately as "criteria pollutants”. These are particulate matter (PM), nitrogen oxides (NOx), sulfur oxides (SOx), ozone, and carbon monoxide. Lead is both a criteria pollutant and an air toxic. These pollutants with the exception of lead are not addressed in NATA.


Ambient:
Surrounding, as in the surrounding environment. In NATA assessments, ambient air refers to the outdoor air surrounding a person through which pollutants can be carried. Therefore, the ambient concentrations estimated by NATA are those concentrations estimated in the outdoor environment. NATA also estimates exposure concentrations that result from an individual's movement through various microenvironments, including the indoor environment.


Area and other sources:
Include sources that generally have lower emissions on an individual basis than "major sources" and are often too small or ubiquitous to be inventoried as individual sources. "Area sources" include facilities that have air toxics emissions below the major source threshold as defined in the air toxics sections of the Clean Air Act and thus emit less than 10 tons of a single toxic air pollutant or less than 25 tons of multiple toxic air pollutants in any one year. Area sources include smaller facilities, such as dry cleaners.


ASPEN:
A computer simulation model used to estimate toxic air pollutant concentrations. The ASPEN model takes into account important determinants of pollutant concentrations, such as: rate of release, location of release, the height from which the pollutants are released, wind speeds and directions from the meteorological stations nearest to the release, breakdown of the pollutants in the atmosphere after being released (i.e., reactive decay), settling of pollutants out of the atmosphere (i.e., deposition), and transformation of one pollutant into another (i.e., secondary formation or decay). The model estimates toxic air pollutant concentrations for every census tract in the United States, Puerto Rico, and the Virgin Islands. For more detailed information, see ASPEN Model.


Atmospheric transformation (Secondary Formation):
The process by which hazardous air pollutants (HAP) are transformed in the air into other chemicals. When a HAP is transformed, the original HAP no longer exists, but it is replaced by one or more chemicals. Compared to the original HAP, the newer reaction products may have more, less, or the same toxicity. Transformations and removal processes affect both the fate of the HAP and its atmospheric persistence. Persistence is important because human exposure to HAPS is influenced by the length of time the HAP remains in the atmosphere. Note that in NATA, the terms atmospheric transformation and secondary formation are used interchangeably.


Background:
For NATA, the contributions to outdoor air toxics concentrations resulting from natural sources, persistence in the environment of past years' emissions, and long-range transport from distant sources. Background concentrations could be levels of pollutants that would be found in a particular year, such as 1996 or 1999, even if there had been no recent manmade emissions. Background concentrations must be added to the modeled concentrations. See more information on the background concentrations in Section 2 of the Technical Support Document.


Cancer Risk
The probability of contracting cancer over the course of a lifetime (assumed to be 70 years for the purposes of NATA risk characterization).


Carcinogen:
A chemical or physical agent capable of causing cancer.


Chemical Abstracts Service (CAS) Number:
A unique number assigned to a chemical by the Chemical Abstracts Service, a service of the American Chemical Society that indexes and compiles abstracts of worldwide chemical literature called "Chemical Abstracts." The purpose is to make database searches more convenient, as chemicals often have many names.


Census tracts:
Land areas defined by the U.S. Census Bureau. Tracts can vary in size but each typically contains about 4,000 residents. Census tracts are usually smaller than 2 square miles in cities, but are much larger in rural areas.


Chromium:
Chromium sources of emissions include the combustion of coal and oil, electroplating, vehicles, iron and steel plants, and metal smelters. The emissions reflected in NATA assessments are based on state and local agency reporting of chromium as "chromium and compounds," individual chromium compounds, and chromium ions. In the 1996 NATA assessment, because of inconsistent reporting, all chromium emissions reported were lumped together for dispersion modeling as "Chromium VI" using the assumption that 34 percent of the reported chromium is hexavalent chromium (which is the most toxic form) based on information from past inventorying efforts. For 1999, a more refined approach was used to estimate emissions of hexavalent chromium that did not utilize an across-the-board assumption about the percentage of chromium that was hexavalent. Individual compounds of chromium reported in the inventory were identified as either hexavalent or trivalent based on their chemical formulas. Any compounds reported as either "chromium" or "chromium and compounds" were then speciated using source category specific speciation data. The particular speciation data used are documented in Appendix C of the Emissions Modeling System for Hazardous Air Pollutant (EMS-HAP) User’s Guide. For source categories where speciation data were not available, the U.S. Environmental Protection Agency (EPA) continued to assume that 34 percent of the chromium is hexavalent.


Cohort:
Generally defined as a group of people within a population who are assumed to have identical exposures during a specified exposure period. The use of cohorts is a necessary simplifying assumption for modeling exposures of a large population. For the exposure assessment, the population is divided into a set of cohorts such that (1) each person is assigned to one and only one cohort, and (2) all the cohorts combined encompass the entire population.


Critical effect:
The first adverse effect, or its known precursor, that occurs to the most sensitive species as the dose rate of an agent increases.


Diesel particulate matter:
Diesel particulate matter (diesel PM) is a mixture of particles that is a component of diesel exhaust (DE). EPA lists DE as a mobile source air toxic due to the cancer and noncancer health effects associated with exposure to whole DE. Diesel PM (expressed as grams diesel PM/m3) has historically been used as a surrogate measure of exposure for whole DE. Although uncertainty exists as to whether diesel PM is the most appropriate parameter to correlate with human health effects, it is considered a reasonable choice until more definitive information about the mechanisms of toxicity or mode(s) of action of DE becomes available. Note that in the risk results presented, diesel PM only presents noncancer results. The non-diesel PM conponent (i.e., the gaseous component with air toxics such as benzene), does provide cancer and noncancer results. For these results, see the Onroad and Nonroad Mobile Cancer Risk files.


Dispersion model:
A computerized set of mathematical equations that uses emissions and meteorological information to simulate the behavior and movement of air pollutants in the atmosphere. The results of a dispersion model are estimated outdoor concentrations of individual air pollutants at specified locations.


Emission density:
Represents tons of emitted air toxics per year within a given area on a per-square-mile basis. In NATA, total county emissions are divided by the area (in total square miles) of the county. Emission density is often used to show emissions information graphically because it provides a more consistent basis for comparison than emissions totals alone.


Emissions Modeling System For Hazardous Air Pollutants (EMS-HAP):
A modeling system that processes the National Emissions Inventory to provide model-ready emissions for input into dispersion models. These inputs consist of tract-level emissions and point source emissions for each toxic air pollutant, temporalized into eight 3-hour time blocks for an annually-averaged year. For purposes of NATA, the EMS-HAP temporalized emission outputs are summed into annual emissions.


Exposure assessment:
Identifying the ways in which chemicals might reach individuals (e.g., by breathing); estimating how much of a chemical an individual is likely to be exposed to; and estimating the number of individuals likely to be exposed.


Hazard index (HI):
The sum of hazard quotients (HQs) for substances that affect the same target organ or organ system. Because different pollutants can cause similar adverse health effects, it is often appropriate to combine HQs associated with different substances. EPA has drafted revisions to the national guidelines on mixtures that support combining the effects of different substances in specific and limited ways. Ideally, HQs should be combined for pollutants that cause adverse effects by the same toxic mechanism. However, because detailed information on toxic mechanisms was not available for most of the substances in this assessment, EPA aggregates the effects when they affect the same target organ regardless of the mechanism. The hazard index (HI) is only an approximation of the aggregate effect on the target organ, (i.e., lungs) because some of the substances might cause irritation by different, (i.e., non-additive,) mechanisms. As with the HQ, aggregate exposures equal to or below an HI of 1.0 derived using target organ specific hazard quotients likely will not result in adverse noncancer health effects over a lifetime of exposure and would ordinarily be considered acceptable. However, an HI greater than 1.0 does not necessarily suggest a likelihood of adverse effects. Because of the inherent conservatism of the reference concentration (RfC) methodology, the acceptability of exceedances must be evaluated on a case-by-case basis, considering such factors as the confidence level of the assessment, the uncertainties, the slope of the dose-response curve (if known), the magnitude of the exceedance, and the numbers or types of people exposed at various levels above the RfC. Furthermore, the HI cannot be translated to a probability that adverse effects will occur and is not likely to be proportional to risk.


Hazard quotient (HQ):
The ratio of the potential exposure to the substance and the level at which no adverse effects are expected. If the HQ is calculated to be equal to or less than 1, then no adverse health effects are expected as a result of exposure. If the HQ is greater than 1, then adverse health effects are possible. The HQ cannot be translated to a probability that adverse health effects will occur and it is unlikely to be proportional to risk. It is especially important to note that an HQ exceeding 1 does not necessarily mean that adverse effects will occur.


Hazardous Air Pollutant Exposure Model (HAPEM):
A computer model that has been designed to estimate inhalation exposure for specified population groups and air toxics. Through a series of calculation routines, the model makes use of census data, human activity patterns, ambient air quality levels, climate data, and indoor/outdoor concentration relationships to estimate an expected range of inhalation exposure concentrations for groups of individuals. For more detailed information, see HAPEM Model.


High end:
Describing a person living at the centroid of a census tract defined as a reference point that is usually but not always located at the geographic center of a census tract) and engaging in a range of activities (indoors and outdoors) that tend to produce higher exposures and risks than are typical. These activities were chosen to represent the 90th percentile of individuals, meaning 90 percent of individuals are expected to engage in activities that put them at lower risk. Important to bear in mind, however, is that the full variation in exposures among individuals is not reflected in the current assessment, because all individuals are placed at the centroid of a census tract.


Inhalation:
Breathing. Once inhaled, contaminants can be deposited in the lungs, taken into the blood, or both.


Lifetime cancer risk
The probability of contracting cancer over the course of a lifetime (assumed to be 70 years for the purposes of NATA risk characterization).


Major sources:
Defined by the Clean Air Act as those stationary facilities that emit or have the potential to emit 10 tons of any one toxic air pollutant or 25 tons of more than one toxic air pollutant per year.


Maximum likelihood estimate (MLE):
The most accurate maximum likelihood estimate is, by definition, the mode of a data set (i.e., the most frequent observation). When data are too limited to identify a clear mode, the average or the median of the data is usually substituted. For some air toxics for which adequate human data exist, EPA has based the unit risk estimate on the MLE for response data or for fitted curves.


Median:
The middle value of a set of ordered values (i.e., half the numbers are less than or equal to the median value). A median is the 50th percentile of the data.


Microenvironment:
A small space in which human contact with a pollutant takes place. A microenvironment can be treated as a well-characterized, relatively homogenous location with respect to pollutant concentrations for a specified time period. For NATA, the Hazardous Air Pollutant Exposure Model (HAPEM) considers cohort activities in 37 ME locations that include: (1) indoor locations (e.g., residence, office, store, school, restaurant, church, manufacturing facility, auditorium, health care facility, service station, other public building, garage); (2) outdoor locations (e.g., parking lot/garage, near road, motorcycle, service station, construction site, residential grounds, school, sports arena, park/golf course); and (3) in-vehicle locations (e.g., car, bus, truck, other, train/subway, airplane).


Microgram:
One-millionth of a gram. One gram is about one twenty-eighth of an ounce.


National-Scale Air Toxics Assessments (NATA)
EPA's ongoing comprehensive evaluation of air toxics in the United States. These activities include the expansion of air toxics monitoring, the improvement and periodic updating of emission inventories, the improvement of national- and local-scale modeling, the continued research on health effects and exposures to both ambient and indoor air, and the improvement of assessment tools.


National Emissions Inventory (NEI):
EPA prepares a national database of air emissions information with input from numerous state and local air agencies, from tribes, and from industry. This database contains information on stationary and mobile sources that emit criteria air pollutants and their precursors, as well as hazardous air pollutants (HAPs). The database includes estimates of annual emissions, by source, of air pollutants in each area of the country, on an annual basis. The NEI includes emission estimates for all 50 states, the District of Columbia, Puerto Rico, and the Virgin Islands. Emission estimates for individual point or major sources (facilities), as well as county level estimates for area, mobile and other sources, are available for years 1985 through 1999 for criteria pollutants, and for all years beginning in 1996 for HAPs.


National Toxics Inventory (NTI):
EPA's compilation of quantitative information concerning the mass of air toxics emitted into the atmosphere (through smokestacks, tailpipes, vents). Starting in 1996, EPA included the National Toxics Inventory in the National Emissions Inventory which also includes information on criteria air pollutants and their precursors.


Noncancer risk:
The risk associated with effects other than cancer, based on the reference concentration (RFC) which is an estimate, with uncertainty spanning perhaps an order of magnitude, of an inhalation exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risks of deleterious effects during a lifetime.


Nonroad mobile sources:
Mobile sources not found on roads and highways (e.g., airplanes, trains, lawn mowers, construction vehicles, farm machinery).


Onroad mobile sources:
Vehicles found on roads and highways (e.g., cars, trucks, buses).


Overall confidence:
EPA has assigned an overall confidence level for each pollutant based on consideration of the combined uncertainties from emissions estimation, ambient concentration modeling, and exposure modeling. These judgments refer to the relative confidence between two air toxics compounds. A judgment of "higher" means the confidence is higher for this compound than for compounds assigned a "medium" or "lower confidence." The confidence also depends on the geographic scale considered. As larger geographic areas are considered, and the exposure is averaged over census tracts in that region, the confidence in estimates of these averages generally will increase. The confidence ratings apply to the nationwide estimates and not to smaller scales (e.g., state or county-level).


Oral exposure:
Eating food and drinking water (and pollutants), and their entry into the digestive tract.


Percentile:
Any one of the points dividing a distribution of values into parts that each contains 1/100 of the values. For example, the 75th percentile is a value such that 75 percent of the values are less than or equal to it. In this assessment, the distribution of values represented (national, state or county percentiles) depends on the presentation format of the results (map, bar chart, or data table).


Polycyclic organic matter (POM)
Defines a broad class of compounds that includes the polycyclic aromatic hydrocarbons (PAHs). POM compounds are formed primarily from combustion and are present in the atmosphere in particulate form. Sources of air emissions are diverse and include vehicle exhausts, forest fires and wildfires, asphalt roads, coal, coal tar, coke ovens, agricultural burning, residential wood burning, and hazardous waste sites. Not all POM reported to EPA's National Emission Inventory (NEI) is speciated. As a result, EPA applied some simplifying assumptions to model and assess the risk from the individual pollutants that comprise POM. See Approach for Modeling POM (PDF 6 pp., 541 KB) for more information.


7-PAH (Polycyclic Aromatic Hydrocarbons):
The 7-PAH group includes 7 chemical species: benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, chrysene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene. The 7-PAH are a subset of 16-PAH (16-PAH is referred to as polycyclic organic matter or "POM" in the presentation of results for the assessment). The 7 species that make up 7-PAH are in the cancer weight-of-evidence group “suggestive evidence of carcinogenic potential”.


Reference Concentration (RfC)
An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious noncancer health effects during a lifetime. The inhalation reference concentration is for continuous inhalation exposures and is appropriately expressed in units of mg/m3. It can be derived from various types of human or animal data, with uncertainty factors generally applied to reflect limitations of the data used.


Risk:
The probability that damage to life, health, or the environment will occur as a result of a given hazard (such as exposure to a toxic chemical). Some risks can be measured or estimated in numerical terms (e.g., one chance in a hundred).


Rural:
Consistent with the definition EPA used in the analyses to support the Integrated Urban Air Toxics Strategy, a county is considered "rural" if it does not contain a metropolitan statistical area with a population greater than 250,000, and the U.S. Census Bureau does not designate more than 50 percent of the population as "urban." The 1999, 2002 and 2005 NATAs used the 2000 Census data. The 1996 NATA relied on the 1990 Census data to make this determination. Note that this definition does not necessarily apply for any regulatory or implementation purpose.


Science Advisory Board (SAB)
A panel of scientists, engineers, and economists who provide EPA with independent scientific and technical advice.


Secondary Formation:
See "Atmospheric transformation (Secondary Formation)"


Secondary Sources:
See "Atmospheric transformation (Secondary Formation)"


Stationary sources:
The NEI inventory typically identifies emissions as being emitted from "major" sources or "area" sources based on the 10 ton or 25 ton emission definitions contained in the Clean Air Act (CAA) (see descriptions above). For presentation purposes, the NATA results are identified as "point" and "nonpoint" sources rather than "major" and "area" sources. The point and nonpoint designations reflect the way each source of emissions were modeled. Some smaller sources that are area sources in the inventory (based on the amount of their emissions), were modeled as point sources because the location of their emissions was identified with latitude and longitude coordinates.


Susceptibility:
An increased likelihood of an adverse effect, often discussed in terms of relationship to a factor (e.g., lifestage, demographic feature, or genetic characteristic, that can be used to describe a human subpopulation).


Toxicity-weighting:
A relative risk evaluation tool that normalizes the emissions rates of each HAP to a hypothetical substance with an inhalation unit risk value of 1 per µg/m3 (for carcinogenic effects) or a reference concentration (RfC) of 1 mg/m3 (for noncancer effects). It is entirely emissions- and toxicity-based, and does not consider dispersion, fate, receptor locations, and other exposure parameters. It may be calculated based on the emissions data for all HAPs released from a facility or source being assessed. It is particularly useful if there are a large number of HAPs and there is a desire to focus the risk analysis on a smaller subset of HAPs that contribute the most to risk.


Typical:
Describes a hypothetical person living at the census tract centroid (defined as a reference point that is usually but not always located at the geographic center of a census tract) and engaging in a range of activities (indoors and outdoors) that are representative of those in which individuals residing in that tract might engage. To characterize the risk that this person might experience, NATA divides the population as a whole into cohorts (groups who are assumed to have identical exposures during a specified exposure period) based on where they live, how old they are, whether they are male or female, and what their daily activity patterns might be. For each combination of residential census tract, age, and gender, various age- and gender-appropriate daily activity patterns are selected to represent the range of exposure conditions for residents of the tract. A population-weighted typical exposure estimate is calculated for each cohort, and this value is used to estimate representative risks for a “typical” individual residing in that tract.


Upper-bound:
A plausible upper limit to the true value of a quantity, usually not a true statistical confidence limit.


Upper-bound lifetime cancer risk:
A plausible upper limit to the true probability that an individual will contract cancer over a 70 year lifetime as a result of a given hazard (such as exposure to a toxic chemical). This risk can be measured or estimated in numerical terms (e.g., one chance in a hundred).


Unit risk estimate (URE):
The upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent at a concentration of 1 microgram per cubic meter (µg/m3) in air. The interpretation of the URE would be as follows: if the URE = 1.5 x 10-6 per µg/m3, 1.5 excess tumors are expected to develop per 1,000,000 people if they were exposed daily for a lifetime to 1 microgram of the chemical in 1 cubic meter of air. Unit risk estimates are considered upper bound estimates, meaning they represent a plausible upper limit to the true value. (Note that this is usually not a true statistical confidence limit.) The true risk is likely to be less, but could be greater.


Upper confidence limit (UCL):
The upper bound of a confidence interval around any calculated statistic, typically an average. For example, the 95 percent confidence interval for an average is the range of values that will contain the true average (i.e., the average of the full statistical population of all possible data), 95 percent of the time. In other words, one could say with 95 percent certainty that the "true" average will exceed the UCL only 2.5 percent of the time. EPA has based most UREs on the UCL of response data or of fitted curves, to avoid underestimating the true URE in light of the uncertainty.


Urban:
Consistent with the definition EPA used in the analyses to support the Integrated Urban Air Toxics Strategy, a county is considered "urban" if it either includes a metropolitan statistical area with a population greater than 250,000 or the U.S. Census Bureau designates more than 50 percent of the population as "urban". The 1999, 2002, and 2005 national-scale assessments use the 2000 census data, and the 1996 national-scale assessment relied on 1990 census data to make this determination. Note that this definition does not necessarily apply for any regulatory or implementation purpose.


Weight-of-evidence for carcinogenicity:
The weight of evidence (WOE) narrative for carcinogenicity is a short summary that explains what is known about an agent's human carcinogenic potential and the conditions that characterize its expression. It should be sufficiently complete to stand alone, highlighting the key issues and decisions that were the basis for the evaluation of the agent's potential hazard. The WOE characterizes the extent to which the available data support the hypothesis that an agent causes cancer in humans. Under EPA's 1986 risk assessment guidelines, the WOE is described by categories "A through E", Group A for known human carcinogens through Group E for agents with evidence of noncarcinogenicity. The approach outlined in EPA's guidelines for carcinogen risk assessment (2005) considers all scientific information in determining whether and under what conditions an agent can cause cancer in humans, and provides a narrative approach to characterize carcinogenicity rather than categories. To provide some measure of clarity and consistency in an otherwise free-form, narrative characterization, standard descriptors are used as part of the hazard narrative to express the conclusion regarding the weight of evidence for carcinogenic hazard potential. There are five recommended standard hazard descriptors.

Carcinogenic to Humans: This descriptor indicates strong evidence of human carcinogenicity. It covers different combinations of evidence. This descriptor is appropriate when there is convincing epidemiologic evidence of a causal association between human exposure and cancer. Exceptionally, this descriptor may be equally appropriate with a lesser weight of epidemiologic evidence that is strengthened by other lines of evidence. It can be used when all of the following conditions are met: (a) there is strong evidence of an association between human exposure and either cancer or the key precursor events of the agent's mode of action but not enough for a causal association, and (b) there is extensive evidence of carcinogenicity in animals, and (c) the mode(s) of carcinogenic action and associated key precursor events have been identified in animals, and (d) there is strong evidence that the key precursor events that precede the cancer response in animals are anticipated to occur in humans and progress to tumors, based on available biological information.

Likely to Be Carcinogenic to Humans: This descriptor is appropriate when the weight of the evidence is adequate to demonstrate carcinogenic potential to humans but does not reach the weight of evidence for the descriptor "Carcinogenic to Humans." Adequate evidence consistent with this descriptor covers a broad spectrum. At one end is evidence for an association between human exposure to the agent and cancer and strong experimental evidence of carcinogenicity in animals; at the other, with no human data, the weight of experimental evidence shows animal carcinogenicity by a mode or modes of action that are relevant or assumed to be relevant to humans. The use of the term "likely" as a weight of evidence descriptor does not correspond to a quantifiable probability. Moreover, additional information (e.g., on mode of action, might change the choice of descriptor for the illustrated examples).

Suggestive Evidence of Carcinogenic Potential: This descriptor of the database is appropriate when the weight of evidence is suggestive of carcinogenicity; a concern for potential carcinogenic effects in humans is raised, but the data are judged not sufficient for a stronger conclusion. This descriptor covers a spectrum of evidence associated with varying levels of concern for carcinogenicity, ranging from a positive cancer result in the only study on an agent to a single positive cancer result in an extensive database that includes negative studies in other species. Depending on the extent of the database, additional studies may or may not provide further insights.

Inadequate Information to Assess Carcinogenic Potential: This descriptor of the database is appropriate when available data are judged inadequate for applying one of the other descriptors. Additional studies generally would be expected to provide further insights.

Not Likely to Be Carcinogenic to Humans: This descriptor is appropriate when the available data are considered robust for deciding that there is no basis for human hazard concern. In some instances, there can be positive results in experimental animals when there is strong, consistent evidence that each mode of action in experimental animals does not operate in humans. In other cases, there can be convincing evidence in both humans and animals that the agent is not carcinogenic. A descriptor of "not likely" applies only to the circumstances supported by the data. For example, an agent may be "Not Likely to Be Carcinogenic" by one route but not necessarily by another. In those cases that have positive animal experiment(s) but the results are judged to be not relevant to humans, the narrative discusses why the results are not relevant.

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