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Absorption barrier: It is any one of the exchange barriers of the body that allow differential
diffusion of various substances across a boundary. Skin, lung tissue, and gastrointestinal tract
wall are some of the examples.

Accuracy: It is the measure of correctness of data, as given by the difference between the
measured value and the true or standard value.

Acid Aerosol: Acidic liquid or solid particles small enough to become airborne. High
concentrations can irritate the lungs and have been associated with respiratory diseases like

Acid Deposition: A complex chemical and atmospheric phenomenon that occurs when
emissions of sulfur and nitrogen compounds and other substances are transformed by chemical
processes in the atmosphere, often far from the original sources, and then deposited on earth in
either wet or dry form. The wet forms, popularly called "acid rain," can fall to earth as rain, snow,
or fog. The dry forms are acidic gases or particulates.

Acute Toxicity: It is sudden damage resulting from single exposure to a large concentration of a

Adsorption: The attachment of the molecules of a liquid or gaseous substance to the surface of
a solid.

Advection: Process of transport of an atmospheric property, or substance within the atmosphere,
solely by the mass motion of the atmosphere.

Aerosol: Particles of solid or liquid matter that can remain suspended in air from a few minutes to
many months depending on the particle size and weight.

Agent: It is a chemical, physical, mineralogical, or biological entity that may cause deleterious
effects in an organism after the organism is exposed to it.

Air Pollutant: Any substance in air that could, in high enough concentration, harm a person,
other animals, vegetation, or material. Pollutants may include almost any natural or artificial
composition of airborne matter capable of being airborne.

Air Pollution: It is the degradation of air quality resulting from unwanted chemicals or other
materials occurring in the air.

Air Quality Index (AQI): A numerical index used for reporting severity of air pollution levels to the
public. It replaces the formerly used Pollutant Standards Index (PSI).

Air Quality Standard (AQS): The prescribed level of a pollutant in the ambient air that should not
be exceeded during a specific time period to protect public health and welfare.

Air sampling: The collection and analysis of air samples for detection or measurement of
radioactive substances, particulate matter, or chemical pollutants.
Ambient air: The air occurring at a particular time and place outside of structures. Often used
interchangeably with "outdoor air."

Ambient level: The level (of a pollutant) in the general environment as characterized by an
average over a suitably long time and large volume.

Ambient measurement: It is a measurement which is usually of the concentration of a chemical
or pollutant taken in an ambient medium, normally with the intent of relating the measurement
value to the exposure of an organism that contacts the medium.

Ambient medium: It is one of the basic categories of material surrounding or contacting an
organism, e.g., outdoor air, indoor air, water, or soil, through which chemicals or pollutants can
move and reach the organism.

Arithmetic mean: The sum of all the measurements in a data set divided by the number of
measurements in the data set.

Atmospheric Stability: It is the ability of the atmosphere to enhance or resist atmospheric

Attributable risk: The rate of a disease in exposed individuals that can be attributed to the
exposure. This measure is derived by subtracting the rate (usually incidence or mortality) of the
disease among nonexposed persons from the corresponding rate among exposed individuals.

Background level (environmental): The concentration of a substance in a defined control area
during a fixed period of time before, during, or after a data-gathering operation.

Backwashing: Reversing the flow of water back through the filter media to remove entrapped

Bacteria: (Singular: bacterium) Microscopic living organisms that can aid in pollution control by
metabolizing organic matter in sewage, oil spills or other pollutants. However, bacteria in soil,
water or air can also cause human, animal and plant health problems.

Baghouse: An air pollution control device that traps particulates by forcing gas streams through
large permeable bags usually made of glass fibers.

Baghouse Filter: Large fabric bag, usually made of glass fibers, used to eliminate intermediate
and large (greater than 20 PM in diameter) particles. This device operates like the bag of an
electric vacuum cleaner, passing the air and smaller particles while entrapping the larger ones.

Beaufort Scale: This helps in identifying magnitude of wind speed from real life observations.

Bias: A systematic error inherent in a method or caused by some feature of the measurement

Biological half-life: The time required for a biological system (such as a human or animal) to
eliminate, by natural processes, half the amount of a substance (such as a radioactive material)
that has been absorbed into that system.
Biological medium: One of the major categories of material within an organism, e.g., blood,
adipose tissue, or breath, through which chemicals can move, be stored, or be biologically,
physically, or chemically transformed.

Biologically effective dose: The amount of a deposited or absorbed chemical that reaches the
cells or target site where an adverse effect occurs, or where that chemical interacts with a
membrane surface.

Blank (blank sample): An unexposed sampling medium, or an aliquot of the reagents used in an
analytical procedure, in the absence of added analyte. The measured value of a blank sample is
the blank value.

Body burden: The amount of particular chemical stored in the body at a particular time,
especially a potentially toxic chemical in the body as a result of exposure. It can be the result of
long-term or short-term shortage.

Bounding estimate: An estimate of exposure, dose, or risk that is higher than that incurred by
the person in population with the highest exposure, dose or risk. These are useful in developing
statements that exposures, doses, or risks are “not greater than” the estimated value.

Breathing zone: A zone of air in the vicinity of an organism from which respired air is drawn is
termed as breathing zone. Personal monitors are often used to measure pollutants in the
breathing zone.

Buoyancy effect: It is the plume rise due to the temperature difference between stack plume and
ambient air.

Carcinogen: A substance or agent that produces or incites cancerous growth.

Carcinogenic potency: The gradient of the dose-response curve for a carcinogen.

Chemicals of Commerce: The intermediary materials in the production of finished products are
called as chemicals of commerce.

Chronic Toxicity: It is the accumulated damage from the repeated exposure to small
concentrations of a toxin over long periods of time.

Common Laws: These laws are grown out of tradition and usage.

Comparability: The ability to describe likenesses and differences in the quality and relevance of
two or more data sets.

Comparative Risk Assessment: Process that generally uses the judgment of experts to predict
effects and set priorities among a wide range of environmental problems.

Continuous Plume: A plume resulting from releases of effluents from a source on a continuous
basis. The release and the sampling times are long compared with the travel time for such a

Cyclone: A device that uses centrifugal force to remove large particles from polluted air.
Data Quality Objectives (DQO): Qualitative and quantitative statements of the overall level of
uncertainty that a decision maker is willing to accept in results or decisions derived from
environmental data. They provide the statistical framework for planning and managing
environmental data operations consistent with the data user’s needs.

Dispersion: A suspension of particles in a medium; the opposite of flocculation; a scattering

Dose rate: In exposure assessment, dose per time unit (e.g., mg/day), sometimes also called

Dose response: Shifts in toxicological responses of an individual (such as alterations in severity)
or populations (such as alterations in incidence) that are related to changes in the dose of any
given substance.

Dose response curve: Graphical representation of the relationship between the dose of a
stressor and the biological response thereto.

Dose-Response Assessment: 1. Estimating the potency of a chemical. 2. In exposure
assessment, the process of determining the relationship between the dose of a stressor and a
specific biological response. 3. Evaluating the quantitative relationship between dose and
toxicological responses.

Dosimeter: Instrument to measure dose; many so-called dosimeters actually measure exposure
rather than dose.

Dropout boxes: These are enlarged portions of a duct system in which the gas velocity is low
and large particles settle out. These are also referred to as gravimetric settling devices.

Dust: Fine grain particles light enough to be suspended in air.

Ecological exposure: Exposure of non-human receptor or organism to a chemical, or
radiological or biological agent.

Effluent: Waste materials begin discharged into the environment, either treated or untreated.
Effluent generally is used to describe water discharges to the environment, although it can refer
to stack emissions or other material flowing into the environment.

Electrostatic Precipitator (ESP): A device that removes particles from a gas stream (smoke)
after combustion occurs. The ESP imparts an electrical charge to the particles, causing them to
adhere to metal plates inside the precipitator. Rapping on the plates causes the particles to fall
into a hopper for disposal.

Emission Factor: The relationship between the amount of pollution produced and the amount of
raw material processed. For example, an emission factor for a blast furnace making iron would be
the number of pounds of particulates per ton of raw materials.

Emission Inventory: A listing, by source, of the amount of air pollutants discharged into the
atmosphere of a community; used to establish emission standards.

Emission Standard: The maximum amount of air polluting discharge legally allowed from a
single source, mobile or stationary.
Emulsifier: It is a chemical that aids in suspending one liquid in another. It is usually an organic
chemical in an aqueous solution.

Encapsulation: The treatment of asbestos-containing material with a liquid that covers the
surface with a protective coating or embeds fibers in an adhesive matrix to prevent their release
into the air.

Environmental fate model: In the context of exposure assessment, any mathematical
abstraction of a physical system used to predict the concentration of specific chemicals as a
function of space and time subject to transport, intermediate transfer, storage, and degradation in
the environment.

Environmental medium: One of the major categories of the chemical found in the physical
environment that surrounds or contacts organisms, e.g., surface water, ground water, soil, or air,
and through which chemicals or pollutants can move and reach the organisms.

Epidemiology: The study of the distribution and dynamics of diseases and injuries in human
populations (or) Specifically, the investigation of the possible causes of a disease and its

Exposure concentration: The concentration of a chemical in its transport or carrier medium to
the point of contact.

Exposure pathway: The physical course of a chemical or pollutant takes from the sources to the
organism exposed.

Exposure route: The way a chemical or pollutant takes from the source to the organism

Exposure scenario: A set of facts, assumptions and inferences about how exposure takes place
that aids the exposure assessor in evaluating, estimating, or quantifying exposures.

Extrapolation: In risk assessment, this process entails postulating a biologic reality based on
observable responses and developing a mathematical model to describe this reality. The model
may then be used to extrapolate to response levels which cannot be directly observed.

Fabric Filter: A cloth device that catches dust particles from industrial emissions.

Facultative Bacteria: Bacteria that can live under aerobic or anaerobic conditions.

Fixed-location monitoring: Sampling of an environmental or ambient medium for pollutant
concentration at one location continuously or repeatedly over some length of time.

Gaussian distribution model: A commonly used assumption about the distribution of values for
a parameter, also called the normal distribution. For example, a Gaussian air dispersion model is
one in which the pollutant is assumed to spread in air according to such a distribution and
described by two parameters, the mean and standard deviation of the normal distribution.
[Modified by S. L. Brown]

Geometric mean: It is the nth root of the product of n values.
Guidelines: Principles and procedures to set basic requirements for general limits of
acceptability for assessments.

Greenhouse effect: It is the anticipated warming of the earth produced by discharging increasing
amounts of energy, pollutants, and combustion products to the atmosphere.

Hazard: A condition or physical situation with a potential for an undesirable consequence, such
as harm to life or limb.

Hazard assessment: An analysis and evaluation of the physical, chemical and biological
properties of the hazard.

Haze (Hazy): A phenomenon that results in reduced visibility due to the scattering of light caused
by aerosols. Haze is caused in large part by man-made air pollutants.

Health effect assessment: The component of risk assessment which determines the probability
of a health effect given a particular level or range of exposure to a hazard.

Heavy metals: Metallic elements like mercury, chromium, cadmium, arsenic, and lead, with high
molecular weights. They can damage living things at low concentrations and tend to accumulate
in the food chain.

High-end exposure (dose) estimate: A plausible estimate of individual exposure or dose for
those persons at the upper end of an exposure or dose distribution, conceptually above the 90
percentile, but not higher than the individual population who has the highest exposure or dose.

High-end Risk Descriptor: It is a plausible estimate of the individual risk for those persons at the
upper end of the risk distribution, conceptually above the 90 percentile but not higher than the
individual in the population with the highest risk.

IDLH (immediately dangerous to life and health): Concentration representing the maximum
level of a pollutant from which an individual could escape within 30 minutes without escape-
impairing symptoms or irreversible health effects.

Instantaneous Plume (Puff Diffusion): The release time or sampling time is short when
compared with the travel time.

Intake: It is a process by which a substance crosses the outer boundary of an organism without
passing an absorption barrier. It may occur through ingestion or inhalation

Internal dose: It is the amount of the substance penetrating across the absorption barriers of an
organism via either physical or biological processes.

Inversion: An atmospheric condition caused by a layer of warm air preventing the rise of
relatively cool air trapped beneath it. This holds down pollutants that might otherwise be
dispersed, and can cause an air pollution episode.

Involuntary risk: These risks are imposed because of circumstances beyond our control.

Isopleth: Lines on a graph connecting points of constant value; e.g., isopleths of visibility are
lines of equal visibility.
Lapse Rate: The negative of the temperature gradient is called the lapse rate and represents the
negative of the rate of change of temperature with altitude.

Lethal concentration fifty (LC50): A calculated concentration [in air] which when administered
by the respiratory route is expected to kill 50% of a population of experimental animals during an
exposure of four hours. Ambient concentration is expressed in milligrams per liter.

Lethal dose fifty (LD50): A calculated dose of a chemical substance which is expected to kill
50% of a population of experimental animals exposed through a route other than respiration.
Dose is expressed in milligrams per kilogram of body weight.

LOAEL (lowest-observed-adverse-effect-level): It is the lowest dose of a chemical in a study or
group of studies that produce statistically or biologically significant increase in frequency between
the exposed population and its appropriate control.

Local Toxicity: This toxicity is limited to particular organs.

Material Safety Data Sheet (MSDS): A compilation of information required under the OSHA
Communication Standard on the identity of hazardous chemicals, health, and physical hazards,
exposure limits, and precautions. Section 311 of SARA requires facilities to submit MSDSs under
certain circumstances.

Mixing Height: It is the average thickness of the layer within which pollutants can be mixed to a
particular geographic region over the course of a year.

Momentum Rise: It is a plume rise due to the exit velocity of the effluents (emissions).

No-Observed-Adverse-Effect-Level (NOAEL): A term used in risk assessment. An exposure
level at which there are no statistically or biologically significant increases in the frequency or
severity of adverse effects between an exposed population and a comparable non-exposed

Noise: Product-level or product-volume changes occurring during a test that are not related to a
leak but may be mistaken for one.

No-Observed-Effect-Level (NOEL): A term used in risk assessment. An exposure level at which
there are no statistically or biologically significant difference or severity of ant effect between an
exposed population and a comparable non-exposed population.

Opacity: It is the percent of light that is attenuated by the plume.

Particulate Matter (PM): Any material, except pure water, that exists in the solid or liquid state in
the atmosphere. The size of particulate matter can vary from coarse, wind-blown dust particles to
fine particle combustion products.

Pasquill Curves: These curves are based upon smoke plume elevation and angular spread.

PM2.5: Includes tiny particles with an aerodynamic diameter less than or equal to a nominal 2.5
microns. This fraction of particulate matter penetrates most deeply into the lungs.

PM10 (Particulate Matter): A criteria air pollutant consisting of small particles with an
aerodynamic diameter less than or equal to a nominal 10 microns (about 1/7 the diameter of a
single human hair). Their small size allows them to make their way to the air sacs deep within the
lungs where they may be deposited and result in adverse health effects

Perfusion: The movement of blood through the myriad of blood vessels in the alveolar
membrane is called perfusion.

Personal Protective Equipment: Clothing and equipment worn by pesticide mixers, loaders and
applicators and re-entry workers, hazmat emergency responders, workers cleaning up Superfund
sites, et. al., which is worn to reduce their exposure to potentially hazardous chemicals and other

PEL (Permissible Exposure Level): PELs represent conditions to which all workers may be
repeatedly exposed day after day without adverse health effects.

Pharmacokinetics: It is the science that relates the rate processes of absorption, distribution,
metabolism, and excretion of chemical substances in a biological system.

Plume: 1. A visible or measurable discharge of a contaminant from a given point of origin. Can
be visible or thermal in water, or visible in the air as, for example, a plume of smoke. 2 The area
of radiation leaking from a damaged reactor. 3. Area downwind within which a release could be
dangerous for those exposed to leaking fumes.

Point Source: A stationary location or fixed facility from which pollutants are discharged; any
single identifiable source of pollution; e.g. a pipe, ditch, ship, ore pit, factory smokestack.

Primary Effect: An effect where the stressor acts directly on the ecological component of
interest, not on other parts of the ecosystem.

Primary Standards: National ambient air quality standards designed to protect human health
with an adequate margin for safety.

Pollutant Standards Index (PSI): A numerical index formerly used for reporting severity of air
pollution levels to the general public. The PSI incorporated the five criteria pollutants -- ozone,
PM10, carbon monoxide, sulfur dioxide, and nitrogen dioxide -- into one single index.

Potential Temperature: It is the hypothetical temperature one would achieve if air at an actual
temperature and pressure is compressed in an isentropic fashion to the ground-level pressure.

Particulates: 1. Fine liquid or solid particles such as dust, smoke, mist, fumes, or smog, found in
air or emissions. 2. Very small solids suspended in water; they can vary in size, shape, density
and electrical charge and can be gathered together by coagulation and flocculation.

Radon: A colorless naturally occurring, radioactive, inert gas formed by radioactive decay of
radium atoms in soil or rocks.

Receptors: Receptors are humans, animals, plants, or materials that are adversely affected by

Reference Dose (RfD): An estimate delivered by the U.S. EPA (with uncertainty spanning
perhaps an order of magnitude) of the daily exposure to the human population, (including
sensitive subpopulations) that is likely to be without deleterious effects during a lifetime. The RfD
is reported in units of mg of substance/kg body weight/day for oral exposures.
Reference Exposure Concentration (RfC): An estimate, derived by the U.S. EPA with an
uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human
population, (including sensitive subgroups) that is likely to be without appreciable risk of
deleterious effects during a lifetime of exposure. The RfC is derived from a no or lowest observed
adverse effect level from human or animal exposures, to which uncertainty or "safety" factors are

Reference Exposure Level (REL): A term used in risk assessment. It is the concentration at or
below which no adverse health effects are anticipated for a specified exposure period.

Response: It is the measurable physiological changes produced by the toxin.

Risk: A measure of the probability that damage to life, health, property, and/or the environment
will occur as a result of a given hazard.

Risk Assessment: Qualitative and quantitative evaluation of the risk posed to human health
and/or the environment by the actual or potential presence and/or use of specific pollutants.

Risk Characterization: The last phase of the risk assessment process that estimates the
potential for adverse health or ecological effects to occur from exposure to a stressor and
evaluates the uncertainty involved.

Risk Management vs. Risk Assessment: Risk management is the goal of personal and
government policy whereas risk assessment is an activity that estimates the spectrum and
frequency of accidents and other negative events.

Sink: Place in the environment where a compound or material collects.

Scrubber: An air pollution control device that uses a high energy liquid spray to remove aerosol
and gaseous pollutants from an air stream. The gases are removed either by absorption or
chemical reaction.

Secondary Particle: Particles that are formed in the atmosphere. Secondary particles are
products of the chemical reactions between gases, such as nitrates, sulfur oxides, ammonia, and
organic products.

Secondary Standards: The purpose is to protect environment from known and anticipated
adverse effects of pollutants. The time table for achieving secondary standards will be determined
by state and local governments.

Source: Any place or object from which air pollutants are released. Sources that are fixed in
space are stationary sources and sources that move are mobile sources.

Standard Mortality Ratio (SMR): It is the ratio of observed deaths to the expected deaths.

Statistical significance: An inference that the probability is low that the observed difference in
quantities being measured

Statute Laws: These laws are passed by legislatures and are recorded in formal documents. The
right to sue is generally retained in statute laws.
STEL (short-term exposure limit): 15-min time-weighted-average exposure that should not be
exceeded at any time during a workday even if the 8-h time-weighted average is within the
threshold limit value.

Systemic Toxicity: This toxicity is distributed throughout the body.

Thermosphere: The outermost layer of the Earth's atmosphere extending from about 60 miles to
several hundred miles. The temperature of this layer varies from many hundreds to thousands of
degrees Celsius.

TLV: TLVs represent conditions to which all workers may be repeatedly exposed day after day
without adverse health effects.

Total Suspended Particulate (TSP): Particles of solid or liquid matter -- such as soot, dust,
aerosols, fumes, and mist -- up to approximately 30 microns in size.

Toxic Dose: The dose level at which a substance produces a toxic effect.

Toxic Pollutants: Materials that cause death, disease, or birth defects in organisms that ingest
or absorb them. The quantities and exposures necessary to cause these effects can vary widely.

Toxic Release Inventory: Database of toxic releases in the United States compiled from SARA
Title III Section 313 reports.

Troposphere: The layer of the Earth's atmosphere nearest to the surface of the Earth. The
troposphere extends outward about 5 miles at the poles and about 10 miles at the equator.

TWA (time-weighted average): An allowable exposure concentration averaged over a normal 8-
h workday or a 40-h workweek.

Uptake: The process by which a substance crosses an absorption barrier and is absorbed into
the body.

Voluntary risk: These risks are taken by people of their own free will.

Wind Rose: This diagram provides the graphical summary of the frequency distribution of wind
direction and wind speed over an extended period of time.

Worst case: A semi quantitative term referring to the maximum possible exposure, dose, or risk
that can conceivably occur, whether or not this exposure, dose, or risk actually occurs in a
specific population.



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