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 would be in addition to those cancer cases that would normally occur in an unexposed population of one million people. 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. A 1 in million lifetime risk to the public in 1996 was 250 cancer cases over a 70 year period.


"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 would be in addition to those cancer cases that would normally occur in an unexposed population of one million people. 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 the day the activity takes place (e.g., sleeping at home at midnight, jogging in the park at 8 a.m., or driving in a car at 6 p.m.). The HAPEM4 model extracts activity pattern data from the 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 may 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 sensitive populations such as children. Five important air pollutants are not included in the list of air toxics (http://www.epa.gov/ttn/atw/187polls.html) because the Clean Air Act addresses them separately as "criteria pollutants:" (http://epa.gov/air/criteria.html) particulate matter (PM), nitrogen oxides (NOx), sulfur oxides (SOx), ozone, and carbon monoxide. Lead is listed as a criteria pollutant whereas lead compounds are listed and regulated as an air toxic. These pollutants are not addressed in NATA.


Ambient:
Surrounding, as in the surrounding environment. In this assessment, ambient air refers to the air surrounding a person through which pollutants can be carried.


Area and other sources:
Include sources that generally have smaller emissions on an individual basis than "major sources" and are often too small or ubiquitous in nature 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. "Other sources" include sources such as wildfires and prescribed burnings that may be more appropriately addressed by other programs rather than through regulations developed under certain air toxics provisions (section 112 or 129) in the Clean Air Act. For example, wildfires and prescribed burning are being addressed through the burning policy agreed to by the Interim Federal Wildland Policy.


ASPEN(Assessment System for Population Exposure Nationwide) model:
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). The model estimates toxic air pollutant concentrations for every census tract in the continental United States, Puerto Rico and the Virgin Islands. However, the output for the model is presented at the county level. For more detailed information, see ASPEN Model.


Background concentrations:
In this context, EPA uses background concentrations to mean 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 1996 even if there had been no recent manmade emissions. To accurately estimate outdoor concentrations, it is necessary to account for the background concentrations by adding them to the modeled concentrations. In this assessment, except for diesel PM, background concentrations are based on values identified in the Cumulative Exposure Project (study which estimated 1990 ambient concentrations of air toxics). From that study, EPA used background concentration values reported in the technical literature for 13 of the air toxic pollutants, and for the rest, EPA assumed a value of zero. For diesel PM, instead of using monitored air quality data to estimate background concentrations, a modeling-based approach was used. For more detailed information, see Background Concentrations.


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


Census tracts:
They are land areas defined by the U.S. Bureau of the Census that vary in size but typically contain about 4,000 residents each. Census tracts are usually smaller than 2 square miles in size in cities, but 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 this assessment are based on state and local agency reporting of chromium as "chromium and compounds," individual chromium compounds and chromium ions. Because of the inconsistent reporting, all of the chromium was lumped together for dispersion modeling as "Chromium VI." EPA then based quantitative risk estimates on an assumption that 34 percent of the chromium is hexavalent chromium (which is the most toxic form) based on information from past inventorying efforts.


Cohort:
A cohort is 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 was 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.


Consolidated Human Activity Database (CHAD):
Based on actual daily diary summaries for more than 22,000 people nationwide which are coded by age, gender, and race.


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 (PM) is a mixture of particles that is a component of diesel exhaust. EPA lists diesel exhaust as a mobile source air toxic due to the cancer and noncancer health effects associated with exposure to whole diesel exhaust. EPA believes that exposure to whole diesel exhaust is best described, as many researchers have done over the years, by diesel particulate concentrations.


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 per year within a given area on a per square mile basis. In this assessment, total county emissions are divided by the total square mileage 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.


Exposure assessment:
Identifying the ways in which chemicals may 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 for substances that affect the same target organ or organ system. Because different pollutants may cause similar adverse health effects, it is often appropriate to combine hazard quotients 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, hazard quotients 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 aggregated only the effects of different respiratory irritants. The HI for respiratory irritation is only an approximation of the aggregate effect on the respiratory system (i.e., lungs and air passages) because it is possible that some of the substances cause irritation by different (i.e., non-additive) mechanisms. As with the hazard quotient, aggregate exposures below a HI of 1.0 will likely not result in adverse noncancer health effects over a lifetime of exposure. However, an HI greater than 1.0 does not necessarily suggest a likelihood of adverse effects. Furthermore, the HI cannot be translated to a probability that adverse effects will occur, and is not likely to be proportional to risk. A respiratory HI greater than 1.0 can be best described as indicating that a potential may exist for adverse irritation to the respiratory system.


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


Hazardous Air Pollutant Exposure Model, Version 4 (HAPEM4):
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.


High end:
Means a person living at the centroid (center of the population mass) of a census tract and engaging in a range of activities (indoors and outdoors) that tend to produce higher exposures and risks than is typical. These activities were chosen to represent the 90th percentile of individuals, meaning 90% of individuals are expected to engage in activities that put them at lower risk. It is important to bear in mind, however, that the full variation in exposures between individuals is not reflected in the current assessment, since 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.


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 are usually substituted. For some air toxics for which adequate human data exist, EPA has based the Unit Risk Estimate on the Maximum Likelihood Estimate for response data or for fitted curves.


Median:
The middle value of a set of 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 (ME):
A small space in which human contact with a pollutant takes place which can be treated as a well-characterized, relatively homogenous location with respect to pollutant concentrations for a specified time period. In this national-scale assessment, HAPEM4 considers cohort activities in thirty-seven 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 Air Toxics Assessments (NATA)
EPA's ongoing comprehensive evaluation of air toxics in the U.S. These activities include expansion of air toxics monitoring, improving and periodically updating emission inventories, improving national- and local-scale modeling, continued research on health effects and exposures to both ambient and indoor air, and improvement of assessment tools.


National Toxics Inventory
EPA's compilation of quantitative information concerning the mass of air toxics emitted into the atmosphere (through smokestacks, tailpipes, vents, etc.)


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. More information on how EPA assigned this cumulative confidence level is available elsewhere in this Web site.


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


Percentile:
Any one of the points dividing a distribution of values into parts each of which contain 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 hydrocarbon compounds (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 and wildfires, asphalt roads, coal, coal tar, coke ovens, agricultural burning, residential wood burning, and hazardous waste sites. Because of limited emissions data, for this assessment Polycyclic Organic Matter (POM) data have been limited to either the group of 7 or group of 16 individual PAH species referred to as 7-PAH and 16-PAH, respectively. In this assessment POM refers to 16-PAH. The 16-PAH group includes the 7-PAH group.


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 probable human carcinogens.


Reference Concentration (RfC)
The RfC is an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups which include children, asthmatics and the elderly) that is likely to be without an appreciable risk of deleterious effects during a lifetime. 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, and/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 was considered "rural" if, based on 1990 census data, it does not contain a metropolitan statistical area with a population greater than 250,000, and the U.S. Census Bureau did not designate more than 50 percent of the population as "urban." 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.


Typical:
Does not refer to a specific individual or even the average over a group of individuals. It means a person living at the centroid (center of population mass) of a census tract and engaging in a range of activities (indoors and outdoors) that are representative of those in which individuals might engage.


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 Unit Risk Estimate is the upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent at a concentration of 1 µg/m3 in air. The interpretation of the Unit Risk Estimate would be as follows: if the Unit Risk Estimate = 1.5 x 10-6 per µg/m3, 1.5 excess tumors are expected to develop per 1,000,000 people if exposed daily for a lifetime to 1 µg 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 Confidence Limit is the upper bound of a confidence interval around any calculated statistic, most 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, we can say with 95 percent certainty that the "true" average will exceed the UCL only 2.5 percent of the time. EPA has based most Unit Risk Estimates on the Upper Confidence Limit of response data or of fitted curves, to avoid underestimating the true Unit Risk Estimate in the face of uncertainty.


Urban:
Consistent with the definition EPA used in the analyses to support the Integrated Urban Air Toxics Strategy, a county was considered "urban" if, based on 1990 census data, 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."" This definition does not necessarily apply for any regulatory or implementation purpose.


Weight-of-Evidence for Carcinogenicity:
A system used by EPA for characterizing 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 weight-of-evidence is described by categories "A through E," Group A for known human carcinogens through Group E for agents with evidence of noncarcinogenicity.

Each pollutant may be placed into one of the following five categories:

Group A (human carcinogen): Compounds for which human data are sufficient to demonstrate a cause and effect relationship between exposure and cancer incidence (rate of occurrence) in humans. In the national-scale assessment, the 7 air toxics classified as human carcinogens are: arsenic compounds, benzene, 1,3-butadiene, chromium compounds, coke oven emissions, nickel compounds, and vinyl chloride.

Group B (probable human carcinogen):

  • Group B1: Compounds for which limited human data suggest a cause and effect relationship between exposure and cancer incidence (rate of occurrence) in humans. In the national-scale assessment, the 5 air toxics classified as probable (B1) human carcinogens are: acrylonitrile, beryllium compounds, cadmium compounds, ethylene oxide, and formaldehyde.

  • Group B2: Compounds for which animal data are sufficient to demonstrate a cause-and-effect relationship between exposure and cancer incidence (rate of occurrence) in animals, and human data are inadequate or absent. In the national-scale assessment, the 15 air toxics classified as probable (B2) human carcinogens are: acetaldehyde, carbon tetrachloride, chloroform, 1,3-dichloropropene, ethylene dibromide, ethylene dichloride, hexachlorobenzene (HCB), hydrazine, lead compounds, methylene chloride, PCBs, polycyclic organic matter (POM), perchloroethylene, propylene dichloride, trichloroethylene.

Group C (possible human carcinogen): Compounds for which animal data are suggestive to demonstrate a cause-and-effect relationship between exposure and cancer incidence (rate of occurrence) in animals.

Group D (not classifiable as to human carcinogenicity): Compounds for which human and animal data are inadequate to either suggest or refute a cause-and-effect relationship for human carcinogenicity. In the national-scale assessment, only manganese compounds were considered to be not classifiable as to human carcinogenicity.

Group E (evidence of noncarcinogenicity): Compounds for which animal data are sufficient to demonstrate the absence of a cause-and-effect relationship between exposure and cancer incidence (rate of occurrence) in animals. In the national-scale assessment, no air toxics were classified as having evidence of noncarcinogenicity.