INTRODUCTION
What is an Exposure Assessment?
How is Exposure
Different from Dose?
How is Exposure
Different from Risk?
Who Performs
Exposure Assessments?
What
is an exposure assessment?
Exposure characterization versus exposure
assessment
An exposure characterization is viewed as a qualitative description
of the exposure potential from the chemical product. A characterization
would provide a general explanation as to how the product
is used, in what functions, and under what applications;
it would not necessarily include measured or calculated
values.
Some examples of qualitative exposure
reports are provided in the "Examples
of Qualitative Exposure Assessment" section of
the website.
An exposure assessment determines the
level, duration and extent of contact with a particular
chemical. In performing an exposure assessment, the following
questions should be addressed:
What
is the source of the potential exposure?
Examples: industrial operations, pesticide use, consumer
products
What potential media, or
point of exposure, does the chemical end up in?
Examples: air (indoor, outdoor), water (drinking
water, bathing water, river water), food, soil, sediment,
plants
Is the chemical changed before
it gets to these media?
By what route of exposure
does the chemical get to a receptor?
Examples: ingestion, dermal (skin) contact, inhalation
Who is the ultimate receptor
that is exposed?
Examples: worker, consumer, community resident,
ecological receptor
|
The outcome of an exposure assessment
is an exposure dose, i.e., an estimate of the concentration
of the emitted chemical that is being contacted. Exposure
units are expressed as:
- Mg/liter (mg/l) for liquids
- Mg/gram (mg/g) for solids
- Mg/cubic meter (mg/m3) for air/vapor/gas
How
is exposure different from dose?
The literature is not consistent in its definition of exposure.
Although most people agree that exposure means contact with
a chemical or agent, universal agreement has not been reached
as to whether that contact takes place at a person's exterior
or whether the the exposure means the chemical has penetrated
the boundaries and been absorbed through the skin, lungs,
or gastrointestinal tract. The following definitions were
set forth by EPA's National Center for Environmental Assessment
in their Final Guidelines for Exposure Assessment.
"The process of a chemical entering
the body can be described in two steps: contact (exposure),
followed by actual entry (crossing the boundary). Absorption...leads
to the availability of an amount of the chemical to biologically
significant sites within the body (internal dose or absorbed
dose)."
The definition of terms becomes increasingly
murky in the arena of "exposure models". Some developers
have viewed "exposure models" as models that determine environmental
concentrations that a person or other receptor may come
in contact with (external contact). Other developers of
"exposure models" are actually referring to models that
calculate a dose that the receptor has ingested, inhaled,
or has absorbed through the skin. This Web site does its
best to distinguish between the two families of models by
categorizing them as:
- FATE AND TRANSPORT MODELS that determine
environmental exposures (external contact) to ecological
receptors
- EXPOSURE models that determine predicted
doses to humans.
How
is exposure different from risk?
Exposure is a component of risk. Exposure occurs through
contact with a chemical, either by drinking, inhaling or
touching it. The amount of a chemical to which a receptor
is exposed does not indicate the risk; the hazard associated
with the chemical must also be known. A chemical's hazard
describes its potential to cause adverse effects to human,
animal or plant life. The risk of a chemical is a function
of both the level of exposure and the level of hazard. Often,
this is expressed as a "Margin of Exposure" (MOE). The MOE
is typically calculated from a threshold concentration for
an adverse effect, divided by the estimated exposure. Thus,
a very small exposure compared to the adverse effect level
results in an MOE > 1 and may represent a low risk situation.
Conversely, an MOE < 1 indicates that adverse effects
could occur.
Who
Performs Exposure Assessments?
Human exposure assessments are developed by toxicologists,
industrial hygienists, and others to help assess potential
risks associated with exposures to chemicals in consumer
products and via other sources. The "science" of exposure
assessment has advanced a lot over the past several years,
and continues to advance. Key "players" have included U.S.
EPA scientists, academicians, experts in consulting companies,
and exposure assessors in companies (often via the former
Chemical Manufacturers Association, now the American Chemistry
Council). Key professional societies include the International
Society of Exposure Analysis (ISEA: www.iseaweb.org/)
and the Society for Risk Analysis (SRA: www.sra.org/).
