Exposure assessment
Encyclopedia
Exposure assessment is a branch of environmental science
that focuses on the processes that take place at the interface between the environment containing the contaminant(s) of interest and the organism(s) being considered. These are the final steps in the path to release an environmental contaminant, through transport to its effect in a biological system. It tries to measure how much of a contaminant can be absorbed by an exposed target organism, in what form, at what rate and how much of the absorbed amount is actually available to produce a biological effect. Although the same general concepts apply to other organisms, the overwhelming majority of applications of exposure assessment are concerned with human health, making it an important tool in public health
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with the number and characteristics of the population exposed. Ideally, it describes the sources, pathways, routes, and the uncertainties in the assessment.
Exposure Analysis is the science that describes how an individual or population comes in contact with a contaminant, including quantification of the amount of contact across space and time. 'Exposure Assessment' and 'Exposure Analysis' are often used as synonyms in many practical contexts.
The receptor-based approach can be opposed to the source-based approach. This approach begins by looking at different sources of contaminants such as industries and power plants. Then, it is important to find out if the contaminant of interest has reached a receptor (usually humans). With this approach, it is very hard to prove that a pollutant from a source has reached a target.
Mathematically, exposure is defined as
where E is exposure, C(t) is a concentration that varies with time between the beginning and end of exposure. It has dimensions of mass times time divided by volume. This quantity is related to the potential dose of contaminant by multiplying it by the relevant contact rate, such as breathing rate, food intake rate etc. The contact rate itself may be a function of time.
Exposure to a contaminant can and does occur through multiple routes, simultaneously or at different times. In many cases the main route of exposure is not obvious and needs to be investigated carefully. For example, exposure to byproducts of water chlorination can obviously occur by drinking, but also through the skin, while swimming or washing, and even through inhalation from droplets aerosolized during a shower. The relative proportion of exposure from these different routes cannot be determined a priori.
Direct approach
Indirect approach
In general, direct methods tend to be more accurate but more costly in terms of resources and demands placed on the subject being measured and may not always be feasible, especially for a population exposure study.
Examples of direct methods include air sampling though a personal portable pump, split food samples, hand rinses, breath samples or blood samples. Examples of indirect methods include environmental water, air, dust, soil or consumer product sampling coupled with information such as activity/location diaries. Mathematical exposure models
may also be used to explore hypothetical situations of exposure.
Exposure factor values can be used to obtain a range of exposure estimates such as average
, high-end and bounding estimates. For example, to calculate the lifetime average daily dose one would use the equation below:
All of the variables in the above equation, with the exception of contaminant concentration, are considered exposure factors. Each of the exposure factors involves humans, either in terms of their characteristics (e.g., body weight) or behaviors (e.g., amount of time spent in a specific location, which affects exposure duration). These characteristics and behaviors can carry a great deal of variability
and uncertainty
. In the case of lifetime average daily dose, variability pertains to the distribution and range of LADDs amongst individuals in the population. The uncertainty, on the other hand, refers to exposure analyst's lack of knowledge of the standard deviation, mean, and general shape when dealing with calculating LADD.
The U.S. Environmental Protection Agency's Exposure Factors Handbook provides solutions when confronting variability and reducing uncertainty. The general points are summarized below:
Environmental science
Environmental science is an interdisciplinary academic field that integrates physical and biological sciences, to the study of the environment, and the solution of environmental problems...
that focuses on the processes that take place at the interface between the environment containing the contaminant(s) of interest and the organism(s) being considered. These are the final steps in the path to release an environmental contaminant, through transport to its effect in a biological system. It tries to measure how much of a contaminant can be absorbed by an exposed target organism, in what form, at what rate and how much of the absorbed amount is actually available to produce a biological effect. Although the same general concepts apply to other organisms, the overwhelming majority of applications of exposure assessment are concerned with human health, making it an important tool in public health
Public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals" . It is concerned with threats to health based on population health...
.
Definition
Exposure Assessment is the process of estimating or measuring the magnitude, frequency and duration of exposure to an agent, alongwith the number and characteristics of the population exposed. Ideally, it describes the sources, pathways, routes, and the uncertainties in the assessment.
Exposure Analysis is the science that describes how an individual or population comes in contact with a contaminant, including quantification of the amount of contact across space and time. 'Exposure Assessment' and 'Exposure Analysis' are often used as synonyms in many practical contexts.
Applications
Quantitative measures of exposure are used:- In Risk assessmentRisk assessmentRisk assessment is a step in a risk management procedure. Risk assessment is the determination of quantitative or qualitative value of risk related to a concrete situation and a recognized threat...
, together with inputs from ToxicologyToxicologyToxicology is a branch of biology, chemistry, and medicine concerned with the study of the adverse effects of chemicals on living organisms...
, to determine risk from substances released to the environment - To establish protective standards
- In EpidemiologyEpidemiologyEpidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
, to distinguish between exposed and control groups - To protect workers from some occupational hazards
Receptor-based approach
The receptor-based approach is used in exposure science. It starts by looking at different contaminants and concentration that reach people. An exposure analyst can use direct or indirect measurements to determine if a person has been in contact with a specific contaminant.Once a contaminant has been proved to reach people, exposure analysts work backwards to determine its source. After the identification of the source, it is important to find out the most efficient way to reduce adverse health effects. If the contaminant reaches a person, it is very hard to reduce the associated adverse health effects. Therefore, it is very important to reduce exposure in order to diminish the risk of adverse health effects. It is highly important to use both regulatory and non-regulatory approaches in order to decrease people’s exposure to contaminants. In many cases, it is better to change people’s activities in order to reduce their exposures rather than regulating a source of contaminants.The receptor-based approach can be opposed to the source-based approach. This approach begins by looking at different sources of contaminants such as industries and power plants. Then, it is important to find out if the contaminant of interest has reached a receptor (usually humans). With this approach, it is very hard to prove that a pollutant from a source has reached a target.
Exposure
In this context exposure is defined as the contact between an agent and a target. Contact takes place at an exposure surface over an exposure period.Mathematically, exposure is defined as
where E is exposure, C(t) is a concentration that varies with time between the beginning and end of exposure. It has dimensions of mass times time divided by volume. This quantity is related to the potential dose of contaminant by multiplying it by the relevant contact rate, such as breathing rate, food intake rate etc. The contact rate itself may be a function of time.
Routes of exposure
Contact between a contaminant and an organism can occur through any route. The possible routes of exposure are:- InhalationInhalation exposureInhalation is a major route of exposure that occurs when an individual breathes in polluted air which enters the respiratory tract. Identification of the pollutant uptake by the respiratory system can determine how the resulting exposure contributes to the dose...
, if the contaminant is present in the air - Ingestion, through food, drinking or hand-to-mouth behavior
- Dermal absorption, if the contaminant can be absorbed through the skin
Exposure to a contaminant can and does occur through multiple routes, simultaneously or at different times. In many cases the main route of exposure is not obvious and needs to be investigated carefully. For example, exposure to byproducts of water chlorination can obviously occur by drinking, but also through the skin, while swimming or washing, and even through inhalation from droplets aerosolized during a shower. The relative proportion of exposure from these different routes cannot be determined a priori.
Measurement of exposure
To quantify the exposure of particular individuals or populations two approaches are used, primarily based on practical considerations:Direct approach
- The direct approach measures the exposures to pollutants by monitoring the pollutant concentrations reaching the respondents. The pollutant concentrations are directly monitored on or within the person through point of contact, biological monitoring, or biomarkers. The point of contact approach indicates the total concentration reaching the host, while biological monitoring and the use of biomarkers infer the dosage of the pollutant through the determination of the body burden. The respondents often record their daily activities and locations during the measurement of the pollutants to identify the potential sources, microenvironments, or human activities contributing the pollutant exposure. An advantage of the direct approach is that the exposures through multiple media (air, soil, water, food, etc.) are accounted for through one study technique. The disadvantages include the invasive nature of the data collection and associated costs.
- Point of contact - Continuous measure of the contaminant reaching the target through all routes.
- Biological MonitoringBiomonitoringAquatic biomonitoring is the science of inferring the ecological condition of rivers, lakes, streams, and wetlands by examining the organisms that live there...
- is another approach to measuring exposure. This measures the amount of a pollutant within the body in various tissue media such adipose tissue, bone, or urine. Biological monitoring measures the body burden of a pollutant but not the source from whence it came. The substance measured may be either the contaminant itself or a biomarker which is specific to and indicative of an exposure to the contaminant. - Biomarkers of Exposure AssessmentBiomarkers of exposure assessmentBiomarkers are very useful in the world of exposure assessment when dealing with direct measurement methods. The use of biomarkers in exposure studies is also referred to as biomonitoring. They are chemicals, metabolites, susceptibility characteristics, or changes in the body that relate to the...
- Measure of the contaminant or other proportionally related variable in the body.
Indirect approach
- The indirect approach measures the pollutant concentrations in various locations or during specific human activities to predict the exposure distributions within a population. The indirect approach focuses on the pollutant concentrations within microenvironments or activities rather than the concentrations directly reaching the respondents. The measured concentrations are correlated to large-scale activity pattern data, such as the National Human Activity Pattern Survey (NHAPS), to determine the predicted exposure by multiplying the pollutant concentrations by the time spent in each microenvironment or activity for by multiplying the pollutant concentrations b the contact rate with each media. The indirect approach or exposure modeling determines the estimated exposure distributions within a population rather than the direct exposure an individual has experienced. The advantage is that process is minimally invasive to the population and is associated with lower costs than the direct approach. A disadvantage of the approach is that the results were determined independently of any actual exposures, so the exposure distribution is open to errors from any inaccuracies in the assumptions made during the study, the time-activity data, or the measured pollutant concentrations.
In general, direct methods tend to be more accurate but more costly in terms of resources and demands placed on the subject being measured and may not always be feasible, especially for a population exposure study.
Examples of direct methods include air sampling though a personal portable pump, split food samples, hand rinses, breath samples or blood samples. Examples of indirect methods include environmental water, air, dust, soil or consumer product sampling coupled with information such as activity/location diaries. Mathematical exposure models
Mathematical exposure modeling
Mathematical exposure modeling is an indirect method of determining exposure, particularly for human exposure to environmental contaminants. It is useful when direct measurement of pollutant concentration is not feasible because direct measurement sometimes requires skilled professionals and...
may also be used to explore hypothetical situations of exposure.
Exposure factors
Especially when determining the exposure of a population rather than individuals, indirect methods can often make use of relevant statistics about the activities that can lead to an exposure. These statistics are called exposure factors. They are generally drawn from the scientific literature or governmental statistics. For example they may report informations such as amount of different food eaten by specific populations, divided by location or age, breathing rates, time spent for different modes of commuting, showering or vacuuming, as well as information on types of residences. Such information can be combined with contaminant concentrations from ad-hoc studies or monitoring network to produce estimates of the exposure in the population of interest. These are especially useful in establishing protective standards.Exposure factor values can be used to obtain a range of exposure estimates such as average
Average
In mathematics, an average, or central tendency of a data set is a measure of the "middle" value of the data set. Average is one form of central tendency. Not all central tendencies should be considered definitions of average....
, high-end and bounding estimates. For example, to calculate the lifetime average daily dose one would use the equation below:
All of the variables in the above equation, with the exception of contaminant concentration, are considered exposure factors. Each of the exposure factors involves humans, either in terms of their characteristics (e.g., body weight) or behaviors (e.g., amount of time spent in a specific location, which affects exposure duration). These characteristics and behaviors can carry a great deal of variability
Variability
The term variability, "the state or characteristic of being variable", describes how spread out or closely clustered a set of data is. This may be applied to many different subjects:*Climate variability...
and uncertainty
Uncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...
. In the case of lifetime average daily dose, variability pertains to the distribution and range of LADDs amongst individuals in the population. The uncertainty, on the other hand, refers to exposure analyst's lack of knowledge of the standard deviation, mean, and general shape when dealing with calculating LADD.
The U.S. Environmental Protection Agency's Exposure Factors Handbook provides solutions when confronting variability and reducing uncertainty. The general points are summarized below:
| Four Strategies for Confronting Variability | Examples |
|---|---|
| Disaggregate the variability | Develop distribution of body weight for subgroup |
| Ignore the variability | Assume all adults weigh 65 kg |
| Use a maximum or minimum value | Choose a high-end value for weight distribution |
| Use the average value | Use the mean body weight for all adults |
| Analyzing Uncertainty | Description |
|---|---|
| Classical statistical methods (descriptive statistics Descriptive statistics Descriptive statistics quantitatively describe the main features of a collection of data. Descriptive statistics are distinguished from inferential statistics , in that descriptive statistics aim to summarize a data set, rather than use the data to learn about the population that the data are... and inferential statistics) |
Estimating the population exposure distribution directly, based on measured values from a representative sample |
| Sensitivity analysis Sensitivity analysis Sensitivity analysis is the study of how the variation in the output of a statistical model can be attributed to different variations in the inputs of the model. Put another way, it is a technique for systematically changing variables in a model to determine the effects of such changes.In any... |
Changing one input variable at a time while leaving others constant, to examine effect on output |
| Propagation of uncertainty Propagation of uncertainty In statistics, propagation of error is the effect of variables' uncertainties on the uncertainty of a function based on them... |
Examining how uncertainty in individual parameters affects the overall uncertainty of the exposure assessment |
| Probabilistic analysis | Varying each of the input variables over various values of their respective probability distributions(i.e. Monte Carlo integration) |
External links
- AIHA EASC AIHA Exposure Assesssment Strategies Committee
- AIHA - American Industrial Hygiene Association (AIHA)
- ISES - International Society of Exposure Science
- Exposure Factors Handbook by the US EPA
- Software to Estimate Human Exposure to Chemicals by Creme Software
- A Strategy for Assessing and Managing Occupational Exposures, Third Edition Edited by Joselito S. Ignacio and William H. Bullock AIHA Press