Exposure assessment

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

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 synonims 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 $$\int_{t_1}^{t_2} C(t)\, dt$$

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 - 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.

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 informationon 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.