EPA-Expo-Box (A Toolbox for Exposure Assessors)
Personal exposure monitoring directly measures an individual’s exposure as it occurs and can be used to measure exposure of an individual to contaminants in the breathing zone, in food and drink, and on the skin. Many sampling techniques for personal exposure monitoring are available and continue to be developed. Considerations for choosing a monitoring technique include:
- Feasibility – Are study subjects likely to comply with the study requirements for carrying equipment or recording activities?
- Accuracy – What level of detection is required for the scenario?
- Implementation – How many subjects are needed? Are there seasonal trends in exposure that should be considered?
- Expense – How much does the sampling equipment cost? Is sample analysis inexpensive?
When study participants keep an activity diary recording their locations and activities while wearing the monitor, the assessor has additional information to characterize potential emission sources and to better extrapolate from short-term measurements to long-term estimates. To ensure human safety, careful steps must also be taken to ensure monitoring methods do not subject individuals to any more exposure than necessary.
Passive and active monitors used to measure inhalation exposure are typically compact and located close to the breathing zone of the individual. Monitors that are comfortable and make little noise encourage use among study participants, but these constraints also limit the sophistication of the devices used and how much they can measure.
For both passive and active sampling, it is often necessary to extrapolate the absorbed dose from the measured concentration since the absorbed dose will vary depending on other factors including ventilation rate.
Passive sampling uses sorption or entrapment, driven largely by unassisted diffusion, to a diffusion tube, badge, detector tube, or similar device. Passive sampling is often more appropriate for measuring long-term exposure rather than acute exposure since the level of detection might be higher for a passive sampling device than an active one. For all passive sampling methods, the samples are analyzed using spectroscopy, gas chromatography, high performance liquid chromatography, or a similar method, depending on the chemicals of concern.
Diffusion badges currently are available for measurement of NO2, O3, SO2, CO, and formaldehyde. Organic vapors can be measured in passive devices using activated charcoal badges. Respirator pads inserted into respirators in place of the regular dust filters have been used to measure exposure concentrations in occupational settings.
Active sampling uses a small air pump to draw air through a filter, packed tube, or similar device. Unlike passive samplers, active sampling requires electricity and moving parts.
Active monitors are available to measure PM10 and PM2.5 using filters. The filter can be placed anywhere on the individual and the individual carries the pump and battery pack in a shoulder bag. Cyclone samplers are available to measure particulates. The sampler spins the particles in an air stream, forcing them to the sides of the device for collection. Impactor and denuder filter packs can be combined to measure aerosols and gases (e.g., SO2, NH3, and HNO3). A diffuser tube can be coated with different materials to measure different chemicals and gases.
Duplicate diet studies (or duplicate portion studies) are ways to measure concentrations of a chemical of concern in the diet. In these studies, individuals collect duplicate samples of all the foods they consume during a given period. Although this method can give an accurate estimate of exposures through ingestion of contaminated foods, it tends to be expensive to implement and requires that participants be literate and motivated to complete the study activities (i.e., collection of samples). Alterations in an individual’s diet and noncompliance with the study protocol can introduce bias into the intake estimates (Stockley, 1985).
Duplicate diet studies can provide direct measurements of chemical contaminants in food as well as the intake rate of various foods, typically normalized to the body weight of each participant. They can also help characterize the total amount of the chemical of concern in different food types, and, unlike other ingestion exposure assessment methods, the results account for changes in chemical concentration due to preparation such as reductions in surface residues due to washing or increases in chemicals due to cooking.
Patches, whole-body dosimeters, removal methods, and optical methods can be used to measure exposure to chemicals on the skin. Band-Aid, sticker-like patches, or gauze pads are placed on the body to collect the chemical of concern, while whole-body dosimeters are intended to measure exposure to the whole body.
Patches were first used approximately 30 years ago to investigate exposure to organophosphate pesticides (Durham and Wolfe, 1962) and have since been used for a variety of substances, including PAHs, copper oxide, and dusts (Soutar et al., 2000). Patches can collect personal measurements of dermal exposure to soil/sediment in a relatively easy and inexpensive manner; they can, however, miss critical areas of exposure depending on where on the body they are placed. Additionally, extrapolation from a relatively small patch to an entire body surface can introduce error.
Whole-body dosimeters are intended to measure exposure to the whole body. Examples include badges worn on the clothing, a coverall suit, and full-length cotton underwear (FIFRA SAP, 2007).
Removal methods include rinsing, wiping, and tape stripping to collect the contaminants of concern from the skin to be analyzed.
Optical methods involve treating the chemical of concern with a nontoxic fluorescent tracer and then using video imaging to identify and quantify the points where the chemical contacts the skin. For example, Fenske et al. (1987) used fluorescent tracers to measure tetrachlorophenol exposure of nine timber workers employed at a planing mill. These tracers have also been used to study dermal exposure to pesticide applicators (Fenske, 1990; Fenske et al., 1985). Portable x-ray fluorescence analyzers have been used to detect bromine concentrations resulting from polybrominated diphenyl ethers (PBDE) compounds emitted by consumer products from the homes of a cohort of individuals in the Great Lakes area (Imm et al., 2009).