Overview
According to the EPA, an exposure assessment is the "process of measuring or estimating the magnitude, frequency, and duration of human exposure to an agent in the environment, or estimating future exposures for an agent that has not yet been released." Exposure assessments attempt to address some of the following questions.
- Who or what is exposed?
- Does the exposure occur through breathing air, drinking water, skin contact, or any other route?
- How much exposure occurs?
- How often and for how long does exposure occur?
Exposure assessments are generally used to characterize occupational exposures in the workplace and environmental exposures to the general population, such as emissions from industrial processes, contaminated food or water, consumer products containing hazardous chemicals, etc. The table below highlights the different applications for an exposure assessment.
Applications for an Exposure Assessment
- Epidemiological study
- In an epidemiological study, investigators are assessing whether exposure to an environmental contaminant or agent is associated with a given health outcome. The goal is ultimately to determine if a causal relationship takes place between exposure and disease.
- Occupational health
- Occupational health studies often attempt to characterize exposure and risk of exposure in the context of work-related activities. The goal is to both identify and control risks that result from physical, chemical, and other workplace hazards.
- Risk assessment
- With the goal of a risk assessment being to characterize the nature and magnitude of health risks from chemical contaminants and other stressors, the exposure assessment is the component that attempts to characterize who is exposed and how much are they exposed to.
- Routine surveillance
- Routine surveillance may be required by various types of industry to determine whether they meet regulatory standards put forth by agencies such as the EPA and OSHA. For example, the EPA's Great Lakes Fish Monitoring and Surveillance Program collects fish from the Great Lakes annually and analyzes them for contaminants.
- Other applications for exposure assessments include evaluating the effectiveness of an intervention strategy.
What is exposure?
Exposure is referring to when individuals come into contact with a substance or factor affecting human health, either adversely or beneficially. According to IUPAC glossary, exposure is defined as the "concentration, amount or intensity of a particular physical or chemical agent or environmental agent that reaches the target population, organism, organ, tissue or cell, usually expressed in numerical terms of concentration, duration, and frequency (for chemical agents and micro-organisms) or intensity (for physical agents)." Although this definition mentions contact with internal components of the body, exposure takes place externally on the body and does not automatically lead to an internal dose. There is a distinction between the external dose one is exposed to and the internal dose that is absorbed since they are not always the same. The units used to express exposure are concentration times time.
Often times when assessing exposure to a given agent, investigators are looking at adverse health outcomes. However, exposures to some agents can be beneficial as well. What are some examples of exposures that can be both harmful and beneficial? What are some factors that influence a given agent's affect on health?
Causal Agent
In order for exposure to occur, an individual has to be exposed to an agent. Identifying and understanding the behavior of the causal agent is paramount to understanding how an individual became exposed and any subsequent health effects. The physical and chemical properties of an agent will influence how it behaves in the environment (fate and transport) as well as in tissues (absorption, distribution, metabolism, and excretion).
Exposure Assessment Components
While the methods involved in any given exposure assessment will vary depending on the specific objective(s) of the study, the principles and basic components of an exposure assessment remain the same. The activity below highlights some important factors involved in conducting an exposure assessment. While these considerations will be explained in greater detail throughout the module, you may scroll over each item below for a brief overview.
Likelihood
Likelihood of exposure addresses the probability that contact between the agent and a human will ocur. To determine likelihood, it is important to identify how the contaminant of interest behaves in the environment and subsequently the potential ways in which contact may occur.
Some agents will be released into the ambient air and then remain suspended in the air while other agents will tend to stick to or bind to solid particles. Some agents may have high solubility, which tends to results in high mobility in aqueous environments. Other agents are highly volatile and will evaporate whereas their low volatility counterparts tend to be more persistent.
Magnitude
Magnitude of exposure addresses how much of the agent people are exposed to. This refers to the level, or dose, of exposure. This is particularly important in environmental health since in most cases there is no populatio that is completely unexposed to a given agent.
Whether one is simply exposed or not is often not enough to establish an association between exposure and an outcome of interest. The magnitude of exposure helps identify how much exposure must occur to have an effect.
Timing
Knowing the dose or magnitude of effect is only useful in the context of the timing of exposure, meaning the frequency (how many times) and duration of exposure (how long) to the agent of interest.
Duration and frequency will help determine the likely effects of exposure. Acute exposure is generally exposure that occurs over a shorter period of time but with greater magnitude and/or frequency whereas chronic exposures tend to occur over a longer period of time at smaller magnitudes.
Route
How are people being exposed? Are they inhaling the agent? Ingesting it? Or absorbing it through their skin? At this stage, one has to consider how the human body comes into contact with the contaminant of concern. While there is some debate about the most common route of exposure to contaminants, inhalation tends to be the quickest and easiest means of exposure to toxic substances, which are readily absorbed by the digestive tract.
For the purposes of this module, when referring to routes of exposure, we refer to ingestion, dermal, and inhalation. However, there are other routes of exposure such as injections and exposure via the eye.
Population
Who are the exposed? It is important to define and characterize the exposed population. Occupational and environmentally-mediated exposure are the most common types of exposure encountered in public health. This often involves having or developing an in-depth understanding of the populatino you wish to assess.
A poorly defined or characterized population may skew the results of your assessment or not properly characterize exoposure within the population.
