Exposure Assessment - Measurement of Exposure

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 Monitoring - 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 Assessment - 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 may also be used to explore hypothetical situations of exposure.