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Comparison values represent media-specific contaminant concentrations that are used to select contaminants for further evaluation to determine the possibility of adverse public health effects. The conclusion that a contaminant exceeds the comparison value does not mean that it will cause adverse health effects.

Cancer Risk Evaluation Guides (CREGs)
CREGS are estimated contaminant concentrations that would be expected to cause no more than one excess cancer in a million (10-6) persons exposed over their lifetime. ATSDR's CREGs are calculated from EPA's cancer potency factors (CPFs).

Maximum Contaminant Level (MCL)
The MCL is the drinking water standard established by EPA. It is the maximum permissible level of a contaminant in water that is delivered to the free-flowing outlet. MCLs are considered protective of public health over a lifetime (70 years) for individuals consuming 2 liters of water per day.

Environmental Media Evaluation Guides (EMEGs)
EMEGs are based on ATSDR minimal risk levels (MRLs) that consider body weight and ingestion rates. An EMEG is an estimate of daily human exposure to a chemical (in mg/kg/day) that is likely to be without noncarcinogenic health effects over a specified duration of exposure to include acute, intermediate, and chronic exposures.

Reference Media Evaluation Guides (RMEGs)
ATSDR derives RMEGs from EPA's oral reference doses. The RMEG represents the concentration in water or soil at which daily human exposure is unlikely to result in adverse noncarcinogenic effects.


The Agency for Toxic Substances and Disease Registry (ATSDR) is a federal public healthagency with headquarters in Atlanta, Georgia, and 10 regional offices in the United States.ATSDR's mission is to serve the public by using the best science, taking responsive public healthactions, and providing trusted health information to prevent harmful exposures and diseasesrelated to toxic substances. ATSDR is not a regulatory agency, unlike the U.S. EnvironmentalProtection Agency (EPA), which is the federal agency that develops and enforces environmentallaws to protect the environment and human health.

This glossary defines words used by ATSDR in communications with the public. It is not acomplete dictionary of environmental health terms. If you have questions or comments, callATSDR's toll-free telephone number, 1-888-42-ATSDR (1-888-422-8737).

The process of taking in. For a person or animal, absorption is the process of a substance getting into the body through the eyes, skin, stomach, intestines, or lungs.

Occurring over a short time [compare with chronic].

Acute exposure:
Contactwith a substance that occurs once or for only a short time (up to 14 days) [compare with intermediate duration exposure and chronic exposure].

Additive effect:
A biologic response to exposure to multiple substances that equals the sum of responses of all the individual substances added together [compare with antagonistic effect and synergistic effect].

Adverse health effect:
A change in body function or cell structure that might lead to disease or health problems.

Requiring oxygen [compare with anaerobic].

Surrounding (for example, ambient air).

Requiring the absence of oxygen [compare with aerobic].

A substance measured in the laboratory. A chemical for which a sample (such as water, air, or blood) is tested in a laboratory. For example, if the analyte is mercury, the laboratory test will determine the amount of mercury in the sample.

Analytic epidemiologic study:
A study that evaluates the association between exposure to hazardous substances and disease by testing scientific hypotheses.

Antagonistic effect:
A biologic response to exposure to multiple substances that is less than would be expected if the known effects of the individual substances were added together [compare with additive effect and synergistic effect].

Background level:
An average or expected amount of a substance or radioactive material in a specific environment, or typical amounts of substances that occur naturally in an environment.

Decomposition or breakdown of a substance through the action of microorganisms (such as bacteria or fungi) or other natural physical processes (such as sunlight).

Biologic indicators of exposure study:
A study that uses (a) biomedical testing or (b) the measurement of a substance [an analyte], its metabolite, or another marker of exposure in human body fluids or tissues to confirm human exposure to a hazardous substance [also see exposure investigation].

Biologic monitoring :
Measuring hazardous substances in biologic materials (such as blood, hair, urine, or breath) to determine whether exposure has occurred. A blood test for lead is an example of biologic monitoring.

Biologic uptake:
The transfer of substances from the environment to plants, animals, and humans.

Biomedical testing:
Testing of persons to find out whether a change in a body function might have occurred because of exposure to a hazardous substance.

Plants and animals in an environment. Some of these plants and animals might be sources of food, clothing, or medicines for people.

Body burden:
The total amount of a substance in the body. Some substances build up in the body because they are stored in fat or bone or because they leave the body very slowly.

See Community Assistance Panel.

Any one of a group of diseases that occurs when cells in the body become abnormal and grow or multiply out of control.

Cancer risk:
A theoretical risk of for getting cancer if exposed to a substance every day for 70 years (a lifetime exposure). The true risk might be lower.

A substance that causes cancer.

Case study:
A medical or epidemiologic evaluation of one person or a small group of people to gather information about specific health conditions and past exposures.

Case-control study:
A study that compares exposures of people who have a disease or condition (cases) with people who do not have the disease or condition (controls). Exposures that are more common among the cases may be considered as possible risk factors for the disease.

CAS registry number:
A unique number assigned to a substance or mixture by the American Chemical Society Abstracts Service.

Central nervous system:
The part of the nervous system that consists of the brain and the spinal cord.

[see Comprehensive Environmental Response, Compensation, and Liability Actof 1980]

Occurring over a long time (more than 1 year) [compare with acute].

Chronic exposure:
Contact with a substance that occurs over a long time (more than 1 year) [compare with acute exposure and intermediate duration exposure].

Cluster investigation:
A review of an unusual number, real or perceived, of health events (for example, reports of cancer) grouped together in time and location. Cluster investigations are designed to confirm case reports; determine whether they represent an unusual disease occurrence; and, if possible, explore possible causes and contributing environmental factors.

Community Assistance Panel (CAP):
A group of people, from a community and from health and environmental agencies, who work with ATSDR to resolve issues and problems related to hazardous substances in the community. CAP members work with ATSDR to gather and review community health concerns, provide information on how people might have been or might now be exposed to hazardous substances, and inform ATSDR on ways to involve the community in its activities.

Comparison value (CV):
Calculated concentration of a substance in air, water, food, or soil that is unlikely to cause harmful (adverse) health effects in exposed people. The CV is used as a screening level during the public health assessment process. Substances found in amounts greater than their CVs might be selected for further evaluation in the public health assessment process.

Completed exposure pathway:
[see exposure pathway].

Comprehensive Environmental Response, Compensation, and Liability Act of 1980(CERCLA):
CERCLA, also known as Superfund, is the federal law that concerns the removal or cleanup of hazardous substances in the environment and at hazardous waste sites. ATSDR, which was created by CERCLA, is responsible for assessing health issues and supporting public health activities related to hazardous waste sites or other environmental releases of hazardous substances.

The amount of a substance present in a certain amount of soil, water, air, food, blood, hair, urine, breath, or any other media.

A substance that is either present in an environment where it does not belong or is present at levels that might cause harmful (adverse) health effects.

Delayed health effect:
A disease or injury that happens as a result of exposures that might have occurred in the past.

Referring to the skin. For example, dermal absorption means passing through the skin.

Dermal contact:
Contact with (touching) the skin [see route of exposure].

Descriptive epidemiology:
The study of the amount and distribution of a disease in a specified population by person, place, and time.

Detection limit:
The lowest concentration of a chemical that can reliably be distinguished from a zeroconcentration.

Disease prevention:
Measures used to prevent a disease or reduce its severity.

Disease registry:
A system of ongoing registration of all cases of a particular disease or health condition in a defined population.

United States Department of Defense.

United States Department of Energy.

Dose (for chemicals that are not radioactive):
The amount of a substance to which a person is exposed over some time period. Dose is a measurement of exposure. Dose is often expressed as milligram (amount) per kilogram (ameasure of body weight) per day (a measure of time) when people eat or drink contaminatedwater, food, or soil. In general, the greater the dose, the greater the likelihood of an effect. An "exposure dose" is how much of a substance is encountered in the environment. An "absorbed dose" is the amount of a substance that actually got into the body through the eyes, skin, stomach, intestines, or lungs.

Dose (for radioactive chemicals):
The radiation dose is the amount of energy from radiation that is actually absorbed by the body. This is not the same as measurements of the amount of radiation in the environment.

Dose-response relationship:
The relationship between the amount of exposure [dose] to a substance and the resulting changes in body function or health (response).

Environmental media:
Soil, water, air, biota (plants and animals), or any other parts of the environment that can contain contaminants.

Environmental media and transport mechanism:
Environmental media include water, air, soil, and biota (plants and animals). Transport mechanisms move contaminants from the source to points where human exposure can occur. The environmental media and transport mechanism is the second part of an exposure pathway.

United States Environmental Protection Agency.

Epidemiologic surveillance:
The ongoing, systematic collection, analysis, and interpretation of health data. This activity also involves timely dissemination of the data and use for public health programs.

The study of the distribution and determinants of disease or health status in a population; the study of the occurrence and causes of health effects in humans.

Contact with a substance by swallowing, breathing, or touching the skin or eyes. Exposure may be short-term [acute exposure], of intermediate duration, or long-term [chronic exposure].

Exposure assessment :
The process of finding out how people come into contact with a hazardous substance, how often and for how long they are in contact with the substance, and how much of the substance they are in contact with.

Exposure-dose reconstruction:
A method of estimating the amount of people's past exposure to hazardous substances. Computer and approximation methods are used when past information is limited, not available, or missing.

Exposure investigation:
The collection and analysis of site-specific information and biologic tests (when appropriate) to determine whether people have been exposed to hazardous substances.

Exposure pathway:
The route a substance takes from its source (where it began) to its end point (where it ends), and how people can come into contact with (or get exposed to) it. An exposure pathway has five parts: a source of contamination (such as an abandoned business);an environmental media and transport mechanism (such as movement through groundwater); a point of exposure (suchas a private well); a route of exposure (eating, drinking, breathing, or touching), and a receptor population (people potentially or actually exposed). When all five parts are present, the exposure pathway is termed a completed exposure pathway.

Exposure registry:
A system of ongoing followup of people who have had documented environmental exposures.

Feasibility study:
A study by EPA to determine the best way to clean up environmental contamination. A number of factors are considered, including health risk, costs, and what methods will work well.

Geographic information system (GIS) :
A mapping system that uses computers to collect, store, manipulate, analyze, and display data. For example, GIS can show the concentration of a contaminant within a community in relation to points of reference such as streets and homes.

Grand rounds:
Training sessions for physicians and other health care providers about health topics.

Water beneath the earth's surface in the spaces between soil particles and between rock surfaces [compare with surface water].

Half-life (t):
The time it takes for half the original amount of a substance to disappear. In the environment, the half-life is the time it takes for half the original amount of a substance to disappear when it is changed to another chemical by bacteria, fungi, sunlight, or other chemical processes. In the human body, the half-life is the time it takes for half the original amount of the substance to disappear, either by being changed to another substance or by leaving the body. In the case of radioactive material, the half life is the amount of time necessary for one half the initial number of radioactive atoms to change or transform into another atom (that is normally not radioactive). After two half lives, 25% of the original number of radioactive atoms remain.

A source of potential harm from past, current, or future exposures.

Hazardous Substance Release and Health Effects Database (HazDat):
The scientific and administrative database system developed by ATSDR to manage datacollection, retrieval, and analysis of site-specific information on hazardous substances,community health concerns, and public health activities.

Hazardous waste:
Potentially harmful substances that have been released or discarded into the environment.

Health consultation:
A review of available information or collection of new data to respond to a specific health question or request for information about a potential environmental hazard. Health consultations are focused on a specific exposure issue. Health consultations are therefore more limited than a public health assessment, which reviews the exposure potential of each pathway and chemical [compare with public health assessment].

Health education:
Programs designed with a community to help it know about health risks and how to reduce these risks.

Health investigation:
The collection and evaluation of information about the health of community residents. This information is used to describe or count the occurrence of a disease, symptom, or clinical measure and to estimate the possible association between the occurrence and exposure to hazardous substances.

Health promotion:
The process of enabling people to increase control over, and to improve, their health.

Health statistics review:
The analysis of existing health information (i.e., from death certificates, birth defects registries, and cancer registries) to determine if there is excess disease in a specific population, geographic area, and time period. A health statistics review is a descriptive epidemiologic study.

Indeterminate public health hazard:
The category used in ATSDR's public health assessment documents when a professionaljudgment about the level of health hazard cannot be made because information critical to such a decision is lacking.

The number of new cases of disease in a defined population over a specific time period [contrast with prevalence].

The act of swallowing something through eating, drinking, or mouthing objects. A hazardous substance can enter the body this way [see route of exposure].

The act of breathing. A hazardous substance can enter the body this way [see route of exposure].

Intermediate duration exposure:
Contact with a substance that occurs for more than 14 days and less than a year [compare with acute exposure and chronic exposure].

In vitro:
In an artificial environment outside a living organism or body. For example, some toxicity testing is done on cell cultures or slices of tissue grown in the laboratory, rather than on a living animal [compare with in vivo].

In vivo:
Within a living organism or body. For example, some toxicity testing is done on whole animals, such as rats or mice [compare with in vitro].

Lowest-observed-adverse-effect level (LOAEL):
The lowest tested dose of a substance that has been reported to cause harmful (adverse) health effects in people or animals.

Medical monitoring:
A set of medical tests and physical exams specifically designed to evaluate whether anindividual's exposure could negatively affect that person's health.

The conversion or breakdown of a substance from one form to another by a living organism.

Any product of metabolism.

Milligram per kilogram.

Milligram per square centimeter (of a surface).

Milligram per cubic meter; a measure of the concentration of a chemical in a known volume (a cubic meter) of air, soil, or water.

Moving from one location to another.

Minimal risk level (MRL):
An ATSDR estimate of daily human exposure to a hazardous substance at or below which that substance is unlikely to pose a measurable risk of harmful (adverse), noncancerous effects. MRLs are calculated for a route of exposure (inhalation or oral) over a specified time period (acute, intermediate, or chronic). MRLs should not be used as predictors of harmful (adverse) health effects [see reference dose].

State of being ill or diseased. Morbidity is the occurrence of a disease or condition that alters health and quality of life.

Death. Usually the cause (a specific disease, condition, or injury) is stated.

A substance that causes mutations (genetic damage).

A change (damage) to the DNA, genes, or chromosomes of living organisms.

National Priorities List for Uncontrolled Hazardous Waste Sites (National Priorities List or NPL):
EPA's list of the most serious uncontrolled or abandoned hazardous waste sites in the United States. The NPL is updated on a regular basis.

No apparent public health hazard:
A category used in ATSDR's public health assessments for sites where human exposure tocontaminated media might be occurring, might have occurred in the past, or might occur in the future, but where the exposure is not expected to cause any harmful health effects.

No-observed-adverse-effect level (NOAEL):
The highest tested dose of a substance that has been reported to have no harmful (adverse) health effects on people or animals.

No public health hazard:
A category used in ATSDR's public health assessment documents for sites where people have never and will never come into contact with harmful amounts of site-related substances.

[see National Priorities List for Uncontrolled Hazardous Waste Sites]

Physiologically based pharmacokinetic model (PBPK model):
A computer model that describes what happens to a chemical in the body. This model describes how the chemical gets into the body, where it goes in the body, how it is changed by the body, and how it leaves the body.

A craving to eat nonfood items, such as dirt, paint chips, and clay. Some children exhibit pica-related behavior.

A volume of a substance that moves from its source to places farther away from the source. Plumes can be described by the volume of air or water they occupy and the direction they move. For example, a plume can be a column of smoke from a chimney or a substance moving with groundwater.

Point of exposure:
The place where someone can come into contact with a substance present in the environment [see exposure pathway].

A group or number of people living within a specified area or sharing similar characteristics (such as occupation or age).

Potentially responsible party (PRP):
A company, government, or person legally responsible for cleaning up the pollution at a hazardous waste site under Superfund. There may be more than one PRP for a particular site.

Parts per billion.

Parts per million.

Prevalence :
The number of existing disease cases in a defined population during a specific time period [contrast with incidence].

Prevalence survey:
The measure of the current level of disease(s) or symptoms and exposures through aquestionnaire that collects self-reported information from a defined population.

Actions that reduce exposure or other risks, keep people from getting sick, or keep disease from getting worse.

Public comment period:
An opportunity for the public to comment on agency findings or proposed activities contained in draft reports or documents. The public comment period is a limited time period during which comments will be accepted.

Public availability session:
An informal, drop-by meeting at which community members can meet one-on-one with ATSDRstaff members to discuss health and site-related concerns.

Public health action:
A list of steps to protect public health.

Public health advisory:
A statement made by ATSDR to EPA or a state regulatory agency that a release of hazardous substances poses an immediate threat to human health. The advisory includes recommended measures to reduce exposure and reduce the threat to human health.

Public health assessment (PHA):
An ATSDR document that examines hazardous substances, health outcomes, and communityconcerns at a hazardous waste site to determine whether people could be harmed from coming into contact with those substances. The PHA also lists actions that need to be taken to protect public health [compare with health consultation].

Public health hazard:
A category used in ATSDR's public health assessments for sites that pose a public health hazard because of long-term exposures (greater than 1 year) to sufficiently high levels of hazardous substances or radionuclides that could result in harmful health effects.

Public health hazard categories:
Public health hazard categories are statements about whether people could be harmed byconditions present at the site in the past, present, or future. One or more hazard categories might be appropriate for each site. The five public health hazard categories are no public health hazard, no apparent public health hazard, indeterminate public health hazard, public health hazard, and urgent public health hazard.

Public health statement:
The first chapter of an ATSDR toxicological profile. The public health statement is a summary written in words that are easy to understand. The public health statement explains how people might be exposed to a specific substance and describes the known health effects of that substance.

Public meeting:
A public forum with community members for communication about a site.

An unstable or radioactive isotope (form) of an element that can change into another element by giving off radiation.

Any radioactive isotope (form) of any element.

[See Resource Conservation and Recovery Act (1976, 1984)]

Receptor population:
People who could come into contact with hazardous substances [see exposure pathway].

Reference dose (RfD):
An EPA estimate, with uncertainty or safety factors built in, of the daily lifetime dose of a substance that is unlikely to cause harm in humans.

Registry :
A systematic collection of information on persons exposed to a specific substance or having specific diseases [see exposure registry and disease registry].

Remedial Investigation:
The CERCLA process of determining the type and extent of hazardous material contamination at a site.

Resource Conservation and Recovery Act (1976, 1984) (RCRA):
This Act regulates management and disposal of hazardous wastes currently generated, treated, stored, disposed of, or distributed.

RCRA Facility Assessment. An assessment required by RCRA to identify potential and actual releases of hazardous chemicals.

See reference dose.

The probability that something will cause injury or harm.

Risk reduction:
Actions that can decrease the likelihood that individuals, groups, or communities will experience disease or other health conditions.

Risk communication:
The exchange of information to increase understanding of health risks.

Route of exposure:
The way people come into contact with a hazardous substance. Three routes of exposure are breathing [inhalation], eating or drinking [ingestion], or contact with the skin [dermal contact].

Safety factor:
[see uncertainty factor]

[see Superfund Amendments and Reauthorization Act]

A portion or piece of a whole. A selected subset of a population or subset of whatever is being studied. For example, in a study of people the sample is a number of people chosen from a larger population [see population]. An environmental sample (for example, a small amount of soil or water) might be collected to measure contamination in the environment at a specific location.

Sample size:
The number of units chosen from a population or environment.

A liquid capable of dissolving or dispersing another substance (for example, acetone or mineral spirits).

Source of contamination:
The place where a hazardous substance comes from, such as a landfill, waste pond, incinerator, storage tank, or drum. A source of contamination is the first part of an exposure pathway.

Special populations:
People who might be more sensitive or susceptible to exposure to hazardous substances because of factors such as age, occupation, sex, or behaviors (for example, cigarette smoking). Children, pregnant women, and older people are often considered special populations.

A person, group, or community who has an interest in activities at a hazardous waste site.

Statistics :
A branch of mathematics that deals with collecting, reviewing, summarizing, and interpreting data or information. Statistics are used to determine whether differences between study groups are meaningful.

A chemical.

Substance-specific applied research:
A program of research designed to fill important data needs for specific hazardous substances identified in ATSDR's toxicological profiles. Filling these data needs would allow more accurate assessment of human risks from specific substances contaminating the environment. This research might include human studies or laboratory experiments to determine health effects resulting from exposure to a given hazardous substance.

Superfund Amendments and Reauthorization Act (SARA):
In 1986, SARA amended CERCLA and expanded the health-related responsibilities of ATSDR. CERCLA and SARA direct ATSDR to look into the health effects from substance exposures at hazardous waste sites and to perform activities including health education, health studies, surveillance, health consultations, and toxicological profiles.

Surface water:
Water on the surface of the earth, such as in lakes, rivers, streams, ponds, and springs [compare with groundwater].

[see epidemiologic surveillance]

A systematic collection of information or data. A survey can be conducted to collectinformation from a group of people or from the environment. Surveys of a group of people can be conducted by telephone, by mail, or in person. Some surveys are done by interviewing a group of people [see prevalence survey].

Synergistic effect:
A biologic response to multiple substances where one substance worsens the effect of another substance. The combined effect of the substances acting together is greater than the sum of the effects of the substances acting by themselves [see additive effect and antagonistic effect].

Teratogen :
A substance that causes defects in development between conception and birth. A teratogen is a substance that causes a structural or functional birth defect.

Toxic agent:
Chemical or physical (for example, radiation, heat, cold, microwaves) agents which, under certain circumstances of exposure, can cause harmful effects to living organisms.

Toxicological profile:
An ATSDR document that examines, summarizes, and interprets information about a hazardous substance to determine harmful levels of exposure and associated health effects. A toxicological profile also identifies significant gaps in knowledge on the substance and describes areas where further research is needed.

The study of the harmful effects of substances on humans or animals.

An abnormal mass of tissue that results from excessive cell division that is uncontrolled and progressive. Tumors perform no useful body function. Tumors can be either benign (not cancer) or malignant (cancer).

Uncertainty factor:
Mathematical adjustments for reasons of safety when knowledge is incomplete. For example, factors used in the calculation of doses that are not harmful (adverse) to people. These factors are applied to the lowest-observed-adverse-effect-level (LOAEL) or the no-observed-adverse-effect-level (NOAEL) to derive a minimal risk level (MRL). Uncertainty factors are used to account for variations in people's sensitivity, for differences between animals and humans, and for differences between a LOAEL and a NOAEL. Scientists use uncertainty factors when they have some, but not all, the information from animal or human studies to decide whether an exposure will cause harm to people [also sometimes called a safety factor].

Urgent public health hazard:
A category used in ATSDR's public health assessments for sites where short-term exposures (less than 1 year) to hazardous substances or conditions could result in harmful health effects that require rapid intervention.

Volatile organic compounds (VOCs):
Organic compounds that evaporate readily into the air. VOCs include substances such asbenzene, toluene, methylene chloride, and methyl chloroform.

Other glossaries and dictionaries:

Environmental Protection Agency:
National Center for Environmental Health (CDC):
National Library of Medicine:


The following describes the ATSDR analysis of inhalation exposures to contaminants in aircraftemissions from Naval Air Station Fallon (NASF). This analysis was completed in multiplesteps, which are described below. The analysis is based largely on aircraft engine testing dataconducted by the Navy's Aircraft Environmental Support Office. These data were found to begenerally consistent with emission factors adopted by the Federal Aviation Administration(FAA) and used in ATSDR's previous evaluation of aircraft emissions from Kelly Air ForceBase.

The analysis of emissions from NASF estimated that ambient air concentrations for all pollutantsconsidered were either below health-based comparison values or reasonably consistent withlevels routinely measured in small communities and suburban locations across the United States. This analysis is based entirely on emissions and dispersion modeling and may understate oroverstate actual ambient air concentrations.

Published approaches for characterizing aircraft emissions. Aircraft emissions from anairport or military base are determined by many factors, such as the amount of aircraft operations,the aircraft engine types, the fuel burned, and the durations that aircraft operate in differentengine modes. Multiple approaches have been used to estimate aircraft emissions for differentscenarios. These approaches all focus on characterizing the aircraft emissions that occur whileaircraft operate on the ground and during takeoff and landing; they do not characterize emissionswhile aircraft operate aloft. Following is a brief summary of the candidate approaches identifiedto date:

  • FAA approach. FAA developed the Emissions and Dispersion Modeling System(EDMS) to characterize aircraft emissions from commercial airports and military bases. FAA released Version 4.0 of the EDMS software last year (CSSI 2001). EPA hasdesignated EDMS as a "preferred air quality model" for assessing impacts of aircraftemissions (EPA 1996). EPA specifically noted that EDMS's intended use is forpredicting the changes in ambient air concentrations that might result from changes inaircraft operations.

  • Navy data. Several studies conducted by the Navy on aircraft emissions (AESO 1998,1999a, 1999b, 2000a, 2000b) were utilized. The individual studies document measuredair emission rates for different combinations of aircraft engines, engine modes, and fueltypes. Many of the aircraft engines considered in these tests are used by aircraft at NASF.

  • ATSDR calculations. ATSDR utilized several spreadsheets for other militaryinstallations, particularly Kelly Air Force Base (Kaplan 2002) indicating how aircraftemissions have been estimated . The analyses in these spreadsheets draw from multipledata sources, including the Navy and documents in the open literature.

  • EPA approach. EPA quantified aircraft emissions when developing its National ToxicsInventory, a nationwide emissions inventory of hazardous air pollutants. Emissions fromcommercial aircraft were considered in the final inventory, but emissions from militaryaircraft were excluded due to the "limited availability of emission factor and speciationdata and appropriate activity data" (ERG 2000).

Based on this initial review, ATSDR considered the first three approaches documented abovewhen characterizing aircraft emissions at NASF.

Activity data for NASF. Because aircraft emissions are highly dependent on the number ofaircraft operations, types of aircrafts, and fuels used, ATSDR obtained information on aircraftactivity at NASF. Base personnel provided the following data on the number of aircraft "sorties"and "operations" per year (Rogers 2001):

Number of Sorties
Number of Operations

NASF considers the following different activities as individual "operations":  landing, takeoff,and entering into restricted air space. A "sortie," on the other hand, is essentially every time anaircraft leaves and returns to the base. Therefore, a plane that takes off from NASF, entersrestricted air spaces three separate times, and lands at NASF is considered a single sortie, withfive operations. Emissions estimates are based on the number of sorties, which best reflect theactivities that contribute to overall emissions.(2) Specifically, emissions are calculated assuming41,200 sorties occur per year--the highest aircraft activity rate from the available data. According to base personnel, data on the number of sorties from years prior to 1998 are notavailable (Rogers 2001).

Base personnel also communicated the percentage of aircraft types found at NASF (Rogers2001). The number of sorties per aircraft type was estimated by multiplying the percent of totalaircraft by the total number of sorties per year. According to this approach, the estimatednumbers of sorties per aircraft type follow:

Aircraft Type
Other tactical
Cargo planes
Percent of Total Aircraft
Estimated Number of Sorties

Finally, all aircraft at NASF operated on JP-5 jet fuel until October, 1993, when the baseswitched entirely to JP-8 fuel. Emissions estimates in this evaluation are based on currentconditions at NASF, that is, use of JP-8 fuel.

Emissions estimates. Multiple approaches were used for characterizing aircraft emissions atNASF, based on the activity data documented in the previous section. First ATSDR comparedemissions estimates for total hydrocarbons calculated using different approaches, and thenestimated emissions for specific contaminants using what was judged to be the mostrepresentative approach. The following paragraphs describe our evaluations:

  • Step 1: Compare hydrocarbon emissions estimates for different approaches. Initialanalyses focused on comparing results from four different emissions approaches. Thiswas done to determine whether the approaches involved considerable differences and alsoto understand what aircraft engine modes account for the greatest portion of estimatedemissions. For this initial evaluation, the following scenario was used: 41,200 landing-takeoff (LTO) cycles involving only F/A-18 aircraft operating on JP-5 or JP-8 fuel. Thisscenario was selected because four different approaches are available for estimatingemissions of total hydrocarbons (HC), which allows for direct comparisons betweenapproaches published by different parties.
  • Table C-1 compares the emissions estimation for total hydrocarbons for four differentapproaches. Footnotes to the table describe the model inputs that were used to estimateannual emission rates for the scenario. Two important observations are apparent fromthis initial evaluation. First, the emissions estimates across the four approaches arereasonably consistent, all falling approximately within a factor of two of each other. Thisconsistency is important, because it gives confidence that none of the approaches grosslyunderstates or overstates emission rates. Second, all four approaches predict that engineidle time accounts for the greatest proportion of hydrocarbon (HC) emissions. Therefore,estimated emission rates for any airport or military base will be very sensitive to the inputfor the average time that jet engines idle, which is determined primarily by start-up timeand taxi time.

    According to every emissions estimation approach examined, aircraft emissions duringidling engine modes account for the overwhelming majority (>95%) of total emissionsavailable for downwind transport. Consequently, site-specific emission rates are highlydependent on the average amount of time that aircraft idle. The aircraft idling time usedin this evaluation are default parameters based on emissions tests conducted at EdwardsAir Force Base (AESO 1998), which are assumed in this model to be similar toconditions at NAS Fallon. Also, some of the emissions estimation approaches reviewedinclude emissions data for "in-frame, maintenance testing" of aircraft engines. Theseapproaches estimate that typical emissions associated with maintenance activities for asingle aircraft are approximately equal to the emissions that the same aircraft releasesduring 30 landing-takeoff cycles (AESO 2000b). If the maintenance-related air emissionsoccurs at NAS Fallon, the actual emissions would increase.

  • Step 2: Estimate emissions of individual contaminants for the NASF evaluation. As notedpreviously, we compiled the data in Table C-1 to compare different emissions estimationapproaches, which we found to have comparable predictions. To characterize air qualityimpacts at NASF, we used what we considered to be the most representative approach: emission factors and aircraft engine mode data published by the Navy's AircraftEnvironmental Support Office (AESO 1998), or data presented in the final two columnsin Table C-1. The final emissions estimates for aircraft at NASF were based on emissionfactors and aircraft engine mode data published by the Navy's Aircraft EnvironmentalSupport Office (AESO 1998). This approach was selected because it is the only data setreceived that presents emission factors for the aircraft type and fuel most commonly usedat NASF: other references presented emissions factors for other aircraft engines not inuse at NASF and for JP-5 fuel. As Table C-1 shows, the HC emissions predicted by this approach are consistent with those predicted by other similar approaches.
  • The emission rates calculated are based on chemical-specific emission factors reportedfor different engine modes (AESO 1998), combined with data compiled in anotherreference on typical times that aircraft operate in specific engine modes (AESO 2000b, asshown in Table C-1). The evaluation is based exclusively on F/A-18 aircraft using JP-8fuel during 41,200 sorties, or landing-takeoff (LTO) cycles.(3) Specifically, for eachchemical, the time in each mode was multiplied by a chemical's emission rate in thatmode and then multiplied by the total LTO cycles. The resulting emission rate in tons peryear was converted to units of grams per second for purposes of the dispersion modeling.Table C-2 lists the emission rates that ATSDR calculated for every chemical. Elevenchemicals were identified as being hazardous air pollutants, combustion by-products, andmost often detected in aircraft engine exhaust (AESO 1998).

    The emissions estimates that we calculated are not equivalent to exposure. Rather,emissions will move through the air and gradually disperse as they move downwind. Adispersion model is needed to predict what ambient air concentrations would result fromthe estimated emission rates. In other words, emissions data in Table D-1 were used asan input to our dispersion modeling analysis, which is described below.

Dispersion modeling. Aircraft emissions from NASF, like those at virtually all commercialairports and military installations, are not continuous or stationary sources. Rather, aircraftemissions occur periodically during the year, and the release point is a mobile source. Modelingsuch emissions sources is a challenging task, given that the majority of regulatory air dispersionmodels have been developed for continuous, stationary sources.

The initial evaluation of the aircraft emissions used the SCREEN3 dispersion model. TheSCREEN3 model is a screening tool that was designed to assess worst-case air quality impactsfrom three general types of continuous emissions sources (point, area, and volume) (EPA 1995). Aircraft emissions at NASF were modeled as occurring from a volume source. This approachassumes that contaminants are continuously released from a fixed volume of air. Though thisapproach does not perfectly represent conditions at NASF, it allows for an initial evaluation ofpotential air quality impacts associated with the aircraft emissions.

Model inputs for the volume source were as follows: emission rate, 1.0 gram per second; sourceheight, 3.0 meters; initial lateral dimension, 525.0 meters; initial vertical height, 30.0 meters; andrural dispersion coefficients were assumed. Unit emission rates were used to calculate anormalized air concentration, which could then be applied to all contaminants. The height of thevolume source was assumed to be the approximate height of aircraft engines. The dimensions ofthe volume source were assumed to be the approximate dimensions of an aircraft runway, or5,000 meters by 1,000 meters. Per direction of the SCREEN3 user's guide, these values weredivided by 4.3 to calculate the initial lateral and vertical dimensions that are required to be inputto the model. Based on these inputs, SCREEN3 predicted the highest 1-hour average normalizedconcentration to be 12.14 (g/m3)/(g/s) at the receptor of concern, which was located 5kilometers (approximately 3 miles) downwind from the source. This is the distance that separatesthe residential areas in Fallon from the end of the runway at NASF. To translate this normalizedconcentration to an annual average time frame, the maximum 1-hour average value wasmultiplied by a factor of 0.1--consistent with procedures EPA published for screening analyses(EPA 1992). Therefore, the normalized annual average concentrations was 1.214 (g/m3)/(g/s).

The annual average concentrations of specific chemicals was estimated by multiplying theemission rates and the normalized concentration. Table C-2 lists the estimated annual averageconcentrations and their corresponding health-based comparison values. Of the 11 chemicalsconsidered, only three--acrolein, benzene, and formaldehyde--had estimated ambient air levelshigher than corresponding health-based comparison values. None of the estimatedconcentrations exceeded the comparison values by more than a factor of five. Closerexamination of the data reveals that the estimated ambient air concentrations are lower thanlevels, or fall within the range of levels, typically measured in rural, suburban, and urban areasthroughout the United States. Specifically, EPA's Urban Air Toxics Monitoring Program(UATMP) has recently reported the following ranges of average concentrations at 12 monitoringstations across the country(4): average acrolein levels in the 1997 UATMP ranged from 0.02 to0.08 ppb; average benzene levels ranged from 0.21 to 1.56 ppb; and average formaldehyde levelsranged from 1.83 to 8.68 ppb (ERG 1999). This observation suggests that other sources of airpollution found throughout the United States (e.g., mobile sources) also contribute to ambient airconcentrations of these three pollutants.

Regarding model sensitivity, it is noted that the screening analysis for this source varies withessentially three input parameters: the lateral dimension of the volume source, the verticaldimension of the volume source, and the emission rate. The sensitivity to the volume sourcedimensions was examined by running the SCREEN3 model with different source configurations,but the modeling outputs were not highly sensitive to the volume source dimensions. Specifically, decreasing the initial lateral dimension by a factor of two caused the predictedconcentrations to increase by 46%, and decreasing the initial vertical dimension by a factor oftwo caused the predicted concentrations to increase by only 10%. It is likely that the overallmodeling evaluation is far more sensitive to the estimated emission rates, as determined largelyby the idling time.

Conclusion. Table C-2 presents estimated highest annual average air concentrations that resultfrom aircraft emissions at NASF. Estimates were made for the 11 hazardous air pollutants mostfrequently detected in aircraft emissions, and are based entirely on emissions data for F/A-18aircraft operating on JP-8 fuel. This aircraft type accounts for the largest number of aircraft atthe base. Multiple data analyses show that the aircraft emissions are dominated by contributionsfrom aircraft idling.

Of the 11 chemicals considered, only acrolein, benzene, and formaldehyde had estimated airconcentrations higher than health-based comparison values, but by relatively small margins (afactor of 5 or less). It is important to note that ambient air concentrations of these threechemicals are consistently higher than the most conservative health-based comparison values atlocations throughout the United States, and the predicted levels for NASF fall within the range,or below the range, of levels routinely measured in small communities around the country.


AESO 1998. Aircraft Engine and Auxiliary Power Unit Emissions Testing Report. Prepared byEnvironmental Quality Management, Inc.; prepared for the United States Air Force. November30, 1998.

AESO 1999a. Aircraft Emission Estimates: F-5 Landing and Takeoff Cycle and In-Frame,Maintenance Testing Using JP-5. Aircraft Environmental Support Office. AESO MemorandumReport No. 9923. February 1999.

AESO 1999b. Toxic Organic Contaminants in the Exhaust of Gas Turbine Engines for JP-5 andJP-8 Fuel. Aircraft Environmental Support Office. AESO Report No. 12-90, Revision B. February 1999.

AESO 2000a. Aircraft Emission Estimates: F-14 Landing and Takeoff Cycle and In-Frame,Maintenance Testing Using JP-5. Aircraft Environmental Support Office. AESO MemorandumReport No. 9813, Revision F. April 2000.

AESO 2000b. Aircraft Emission Estimates: F/A-18 Landing and Takeoff Cycle and In-Frame,Maintenance Testing Using JP-5. Aircraft Environmental Support Office. AESO MemorandumReport No. 9815, Revision D. May 2000.

CSSI 2001. Emissions and Dispersion Modeling System (EDMS) Reference Manual. Preparedby CSSI, Inc.; prepared for the U.S. Department of Transportation, Federal AviationAdministration. Document number FAA-AEE-01-01. May 2001.

EPA 1992. Screening Procedures for Estimating the Air Quality Impact of Stationary Sources,Revised. U.S. Environmental Protection Agency. Document number EPA-454/R-92-019. October 1992.

EPA 1995. SCREEN3 Model User's Guide. U.S. Environmental Protection Agency. Documentnumber EPA-454/B-95-004. September 1995.

EPA 1996. Guideline on Air Quality Models. 40 CFR 51, Appendix W. U.S. EnvironmentalProtection Agency. August 12, 1996.

ERG 1999. 1997 Urban Air Toxics Monitoring Program (UATMP). Prepared by EasternResearch Group; prepared for the U.S. Environmental Protection Agency. January 1999.

ERG 2000. Documentation for the 1996 Base Year National Toxics Inventory for AircraftSources. Prepared by Eastern Research Group; prepared for U.S. Environmental ProtectionAgency. June 2, 2000.

Kaplan 2002. Electronic mail correspondence between Brian Kaplan (ATSDR) and JohnWilhelmi (ERG). July 3, 2002.

Rogers 2001. Electronic mail correspondence between Captain Roy Rogers (NASF) and MaryAnn Simmons (Navy Environmental Health Center). August 5, 2002.

Table C-1.

Assumed Duration of Engine Modes and Total Hydrocarbon Emissions: Based on 41,200 Landing-Takeoff Cycles for F/A-18 Aircraft Using JP-5 or JP-8
Engine Mode
(Power setting)
Data for Different Emissions Estimation Approaches
EDMS Evaluation
ATSDR Evaluation
Time in Mode HC Emissions Time in Mode HC Emissions Time in Mode HC Emissions Time in Mode HC Emissions
Idle/Taxi 40.4 minutes 1008.41 tons 21.8 minutes 543.45 tons 44 minutes 1,096.9 tons 44 minutes 777 tons
Unstick (75%) NA NA 0.3 minutes 5.83 tons 0.6 minutes 11.74 tons NA NA
Run-up (80%) NA NA NA NA 0.5 minutes 1.71 tons NA NA
Approach (85%) 3.22 minutes 5.07 tons NA NA 3.0 minutes 2.88 tons 4.1 minutes 0.073 tons
Climb-out (95%) 0.31 minutes 0.54 tons 1.1 minutes 1.57 tons 1.0 minutes 1.42 tons 1.0 minutes 0.024 tons
Take-off (100%) 0.94 minutes 2.39 tons 0.8 minutes 2.03 tons 0.5 minutes 1.28 tons 0.5 minutes 4.61 tons
Totals 44.8 minutes 1016.4 tons 24.0 minutes 552.9 tons 49.6 minutes 1,115.9 tons 49.5 minutes 782 tons
Percent from Idle 90% 99% 91% 98% 89% 98% 89% 99%


References: EDMS Evaluation (CSSI 2001); ATSDR Evaluation (Kaplan 2002); AESO JP-5 Data (AESO 2000b); AESO JP-8 Data (AESO 1998).

The various idle engine modes correspond to the time that the aircraft starts, warms up, and taxis on runways.

NA indicates that an engine mode was not part of the description from this reference.

The JP-8 study (AESO 1998) did not include times for the engine modes; instead, the other AESO report's data were used.

For these evaluations, a 100% engine mode for take-off indicates that emissions were measured while engine afterburners were engaged.

Regarding the EDMS evaluation, an idle time of 40.36 minutes was selected to be consistent with the AESO data, even though the EDMS default idle time for F/A-18 aircraft in this software is 0.36 minutes. All other times per engine mode are EDMS defaults.

Regarding the AESO data evaluation, the time in mode entries are taken from 1998 interviews with maintenance personnel.

Data from the ATSDR evaluation are based on times in mode and emission factors tabulated in a spreadsheet used to evaluate aircraft emissions at Kelly Air Force Base. The time in mode originate in a publication titled "Aircraft Engine Emissions Estimator."

Table C-2.

Estimated Emission Rates and Annual Average Ambient Air Concentrations
Contaminant Estimated Emission Rate (g/s) Estimated Annual Average Air Concentration Lowest Health-Based Comparison Value Type of Comparison Value
g/m3 ppb g/m3 ppb
Acetaldehyde 0.0198 0.024 0.013 0.07 0.04 CREG
Acrolein 0.0626 0.076 0.033 0.02 0.009 EMEG-intermediate
Benzene 0.183 0.222 0.070 0.1 0.03 CREG
1,3-Butadiene 0.545 0.660 0.300 0.004 0.002 CREG
Ethylbenzene 0.0264 0.032 0.007 4,350 1,000 EMEG-intermediate
Formaldehyde 0.347 0.421 0.343 0.08 0.07 CREG
Methyl ethyl ketone 0.00171 0.002 0.001 1,000 340 RfC
Naphthalene 0.0511 0.062 0.012 3 0.6 RfC
Styrene 0.0265 0.032 0.008 260 60 EMEG-chronic
Toluene 0.092 0.112 0.030 300 80 EMEG-chronic
m,p-Xylene 0.0579 0.070 0.016 440 100 EMEG-chronic
o-Xylene 0.0282 0.034 0.076 440 100 EMEG-chronic


Estimated annual average air concentration is based on multiplying the SCREEN3 output (highest 1-hour average concentration) by a factor of 0.1 (EPA-recommended value for estimating annual average impacts from a screening evaluation).

The table addresses the hazardous air pollutants that were most frequently detected in the engine testing (AESO 1998).

The source document reported methyl ethyl ketone and isobutyraldehyde as a single contaminant, presumably because the analytical equipment couldnot distinguish the two chemicals. This analysis assumes that the entire amount detected was methyl ethyl ketone--the chemical with morereadily available toxicity information.

The following abbreviations are used for health-based comparison values: Cancer Risk Evaluation Guide (CREG), Environmental Media EvaluationGuide (EMEG), and reference concentration (RfC). The lowest comparison value for all xylene isomers was applied to the last two rows.

2 This approach essentially assumes that "touch-and-go" operations do not contribute to the overall emissions. ATSDR has no data on how many of these operations occur during a year. However, the idle mode of aircraft engines is associated with the largest portion of aircraft emissions. Since "touch-and-go" operations presumably do not involve idle engine modes, neglecting these operations is expected to have only marginal impacts on the estimated emission rates.
3 To a first approximation, representing all aircraft at NASF appears to overstate emissions. EDMS model predictions, for example, indicate the following HC emissions for a single LTO: F/A-18, 49.34 pounds; F-14, 29.00 pounds; F-5, 13.40 pounds; E2C, 1.07 pounds; and EA6, 30.74 pounds. Therefore, assuming all aircraft sorties were conducted by F/A-18s does not appear to underestimate air quality impacts.
4 Though the name of EPA's monitoring program implies that sampling occurred only in urban environments, the monitoring locations included rural, suburban, and urban areas, including: five locations in Vermont, a remote location in Arkansas, industrial locations in Louisiana, and urban areas in New Jersey and Texas.

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