Methods of Exposure Quantification

There are three approaches of estimating exposure quantitatively. These three approaches are illustrated below (WHO, 2000; EPA, 1992):

• Point-of-contact measurement
• Scenario evaluation
• Reconstruction

Point-of-Contact Measurement

The exposure is measured at the outer boundary of the human body (point of contact) while it is taking place (Baldauf, et al., 2006). This approach measures both the time of contact and exposure concentration and integrating them.

A typical example of the point-of-contact measurement is the radiation dosimeter (EPA, 1992). The small badge-like device measures exposure to radiation as the exposure occurs. The device provides an integrated estimate of exposure for the period of time over which the measurement has been taken (WHO, 2000). Another example is the carbon monoxide (CO) point-of-contact measurement studies where subjects carry a small CO monitor for several days (EPA, 1992). The other examples are dermal patch studies and duplicate meal studies (Hoppin et al., 2006). Table 1-2 below shows examples of estimating exposure using the point-of-contact approach.

Table 1-2. Examples of Exposure Using the Point-of-Contact Approach

Type of measurement	Characterization	Needed information	Examples
Air pump/vapors and particles	Exposure via air medium	Duration of sample, representativeness of the sampled population	Baldauf, et al., 2006
Passive samplers for vapors	Same as above	Same as above	Lebret et al., 2000
Split food/water samples	Exposure via ingested media	Amount of media ingested, and representativeness of the population	Kathryn et al., 2006
Skin patch samples	Exposure via dermal contact	Duration of sample, skin penetration, and the population representativeness	Hoppin et al., 2006

The above table was developed based on EPA Guidelines for Exposure Assessment (EPA, 1992).

The key point in all of the above examples is that the measurements are taken at the interface between the person and environment while exposure is occurring (WHO, 2000). The advantage of this approach is that it measures exposure directly and is likely to give the most accurate exposure value for the period of time over which the measurement was taken (Lioy PJ, 1990; Wallance LA, 1995). The limitation is that it is often expensive and monitors and techniques do not currently exist for many of agents (EPA, 1992). In addition, this approach may require assumptions to be made concerning the relationship between short-term sampling and long-term exposures (EPA, 1992).

Scenario Evaluation

The scenario evaluation determines the exposure concentration of an agent in a medium or location and links this data with the time (exposure duration) that individual or population contact (Laden et al., 2006; Lebret et al., 2000). The exposure scenario is defined as the set of assumptions about how this contact takes place (EPA, 1992). In evaluating exposure scenarios, the investigator usually characterizes the agent concentration and the time of contact separately (Morgan et al., 2005; Backer et al., 2005).  

To characterize an agent concentration is to develop estimates of exposure concentration (Lebret et al., 2000). The estimate is typically done indirectly by measuring, modeling, or using existing data on the agent concentration in the bulk media (e.g., water, air, and soil), rather than at the point of contact (Laden et al., 2006). The assumption that the concentration in the bulk medium is the same as the exposure concentration at the point of contact is a clear source of potential error in the exposure estimate and must be explored and discussed in the uncertainty analysis (Laden et al., 2006; EPA, 1992; WHO, 2000). Generally, the closer the medium can be measured to the point of contact in both space and time, the less uncertainty there is in the characterization of exposure concentration (EPA, 1992).

To characterize time of contact for an individual or populations is to develop estimates of the frequency and duration of exposure (WHO, 2000). Like an agent concentration characterization, the estimates are usually obtained indirectly by use of activity diaries data, demographic data, questionnaire survey data, behavior observation data (Morgan et al., 2005; Backer et al., 2005). In the absence of the above-mentioned data, the investigator has to make assumption about behavior of an individual or population under study (Laden et al., 2006).

After obtaining both exposure duration and exposure time data, the investigator ultimately combines them in an exposure scenario under some assumptions (Morgan et al., 2005; Backer et al., 2005). The advantage of this approach is that it is an inexpensive method (Laden et al., 2006). Also, it is particularly suited to analysis of the heath effects of proposed action. It is both strength and a weakness of scenario development that evaluation can be performed with little or no data; it is a technique that is best used when some knowledge exists about the soundness, validity, and uncertainty of the underlying assumptions (EPA, 1992). Table 1-3 below shows examples of estimating exposure using the scenario evaluation approach.

Table 1-3: Examples of Exposure Using the Scenario Evaluation Approach

Type of measurement	Characterization	Needed information	Examples
Fixed location monitoring	Media of air, water, soil, samples used to characterize trend and status	Human activity pattern, relationship between the monitoring location and population	Laden et al., 2006
Source monitoring	Release rates in the environment	Exposure pathway information, human activity, temporal release	Stack sampling, pollution control sampling
Food samples	Concentration distribution in food supply	Dietary habits of study populations, relationships between concentration in uncooked versus prepared food	Cooked-food diet sample such as a hamburg
Drinking water samples	Concentration distribution in drinking water	Fate and distribution of an agent from point of sample to point of consumption, people  consumption rate, human activity pattern	Tap water
Microenvironmental samples	Ambient concentration in a defined location	Human activity pattern and the population representativeness	Morgan et al., 2005
Breathing zone samples	Air concentration where people breath	Human activity pattern and the population representativeness	Backer et al., 2005

The above table was developed based on EPA Guidelines for Exposure Assessment (EPA, 1992).

Reconstruction

The exposure is estimated from a dose, which in turn can be reconstructed through internal indicators (biomarkers, body burden, excretion levels, etc.) after the exposure has taken place (EPA, 1992; WHO, 2000). Table 1-4 below shows examples of estimating exposure using the reconstruction approach.

The advantage of this approach is that it generates a good estimate of past exposure (Gosselin et al., 2006; Gordon et al., 2006). If a total dose is known and information about intake and uptake rates is available, an investigator can estimate an average past exposure (Xu and Weisel, 2005). The limitation is that the methodology is not available for every agent due to interferences or the reactive nature of the agent (Mucha et al., 2006; Aronson et al., 2000).

Table 1-4. Examples of Exposure Using the Reconstruction Approach

Type of measurement	Characterization	Needed information	Examples
Blood	Internal dose of an agent or amount of metabolites	Pharmacokinetics, variability in population, storage in body, relationship to body burden, time frame since exposure	Gosselin, et al., 2006
Breath	Internal dose of vapors and amount of metabolites	Pharmacokinetics, variability in population, storage in body, relationship to blood, time frame since exposure	Gordon et al., 2006; Xu and Weisel, 2005
Urine	Internal dose or amount of metabolized	Same as breath	Mucha et al., 2006
Adipose tissues	Internal dose of long-lived and storage in human body	Same as breath	Aronson et al., 2000

The above table was developed based on EPA Guidelines for Exposure Assessment (EPA, 1992).

Summary

Each of the above approaches is independent since the three approaches use different data to quantify exposure exposure (EPA, 1992; WHO, 2000). The independence of these three approaches is important concept in verifying or validating results. Each of the three has strengths and weaknesses; using them in combination can considerably strengthen the credibility of an exposure assessment (EPA, 1992).