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Exposure assessment

 
Encyclopedia of Public Health: Exposure Assessment
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The field of exposure assessment has its origins in industrial hygiene. It has evolved rapidly since 1980, and is now used to examine personal contacts with toxicants found in the personal or community environments. The accurate characterizations of exposure to toxic agents can lead to identification of the distribution of exposure and determination of the mean and high-end values, all of which are central to effective risk assessment. The science to conduct exposure analyses and assessments can be placed within a continuum that follows the movement of a toxicant from its source, through to an ultimate health effect. Information on human exposure provides a firm scientific link between the information obtained by traditional environmental sciences, which focus on effluents, and health effects that can result from contact with environmental toxicants. Understanding and interdicting specific pathways for contact are essential in protecting the public health and the environment.

An exposure is defined as "an event consisting of contact at a boundary between a human and the environment at a specific contaminant concentration for a specified interval of time" (National Research Council). Understanding total exposure requires that its fundamental variables of concentration and time are summed over all possible microenvironments where people spend their time.

The data collected to complete an exposure assessment requires both indirect and direct measurements techniques. Direct measurements assess a person's exposure, using monitors attached to the individual, or through the sampling of biological media such as blood or urine. Indirect measurements involve collecting information about where, when, and how people spend their time, and about the concentrations of a contaminant associated with a medium that contributes to important routes of exposure. The data from indirect measurements are used to estimate a person's exposure using both simple and complex models. Detailed exposure and dose analyses frequently use a combination of direct measurements.

Theoretical advances for simulating exposure occurred in the early 1990s, contributing to an understanding of integrated multimedia and multiroute exposures and helping provide initial estimates of exposure within the general population and high-exposure subgroups. All modeling activities, however, must be validated by measurements.

The construction of models of individual or population exposures to contaminants is essential, since it is nearly impossible to measure all exposures experienced by an individual or by the general population. Therefore, statistically representative groups are selected from the population, and measurements from these groups are used to estimate the exposure of the population, using deterministic or fundamental models.

Exposure assessment is particularly pertinent to understanding risks associated with environmental hazards. Risk is a function of both hazard, which is intrinsic to the chemical or physical agent, and the intensity of a person's or population's exposure. Failure to pay attention to specific conditions within a community (e.g., consumption of homegrown vegetables) can lead to either underestimating risk. Community studies focus on measuring or estimating exposure of susceptible populations and of individuals or populations with the highest levels of exposure.

(SEE ALSO: Pollution; Risk Assessment, Risk Management; Safety Assessment; Uncertainty Analysis)

Bibliography

Georgopoulos, P., and Lioy, P. J. (1994). "Conceptual and Theoretical Aspect of Exposure and Dose Assessment." Journal of Exposure Analysis and Environmental Epidemiology 4:253–285.

Lioy, P. J. (1999). "Exposure Analysis: Reflection on Its Growth and Aspirations for Its Future." Journal of Exposure Analysis and Environmental Epidemiology 91:273–281.

Lioy, P. J; Detels, R.; Holland, W. W.; McEwen, J.; and Omenn, G. S. (1997). "The Analysis of Human Exposure to Contaminants in the Environment." In Oxford Textbook of Public Health, 3rd edition, ed. R. Detels et al. Oxford, UK: Oxford Medical Publications.

National Research Council (1994). Human Exposure Assessment for Airborne Pollutants: Advances and Opportunities. Washington, DC: National Academy Press.

Ott, W. R. (1995). "Human Exposure Assessment: The Birth of a New Science." Journal of Exposure Analysis and Environmental Epidemiology 5:449–472.

Zartarian, V. G.; Ott, W.; and Duan, N. (1997). "A Quantitative Definition of Exposure and Related Concepts." Journal of Exposure Analysis and Environmental Epidemiology 7:411–438.

— PAUL J. LIOY


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Wikipedia: Exposure assessment
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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.

Contents

Definition

Exposure Assessment is the process of estimating or measuring the magnitude, frequency and duration of exposure to an agent, along 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.

Applications

Quantitative measures of exposure are used:

  • In Risk assessment, together with inputs from Toxicology, to determine risk from substances released to the environment
  • To establish protective standards
  • In Epidemiology, to distinguish between exposed and control groups
  • To protect workers from some occupational hazards

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
E=\int_{t_1}^{t_2} C(t)\, dt

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:

  • Inhalation, 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
    • Point of contact - Continuous measure of the contaminant reaching the target through all routes.
    • Biomarkers of Exposure Assessment - Measure of the contaminant or other proportionally related variable in the body
  • Indirect approach - Measure of the contaminant in all media encountered by the target and during all activities multiplied by the amount of time spent in each location, or the contact rate with each media.

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 actitity/location diaries. Mathematical exposure models 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, high-end and bounding estimates. For example, to calculate the lifetime average daily dose one would use the equation below:

LADD = (ContaminantConcentration)(IntakeRate)(ExposureDuration) / (BodyWeight)(AverageLifetime)

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:

Four Strategies for Confronting Variability [1] 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 [1] Description
Classical statistical methods (descriptive statistics and inferential statistics) Estimating the population exposure distribution directly, based on measured values from a representative sample
Sensitivity analysis Changing one input variable at a time while leaving others constant, to examine effect on output
Propagation of uncertainty 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

References

  • A Strategy for Assessing and Managing Occupational Exposures, Third Edition Edited by Joselito S. Ignacio and William H. Bullock AIHA Press
  • Lioy, Paul (1990). "Assessing total human exposure to contaminants'". Environmental Science & Technology 24 (7): 938–945. doi:10.1021/es00077a001. 

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Encyclopedia of Public Health. Encyclopedia of Public Health. Copyright © 2002 by The Gale Group, Inc. All rights reserved.  Read more
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