Exposure Assessment is the aspect of risk assessment that determines the actual levels of exposure and absorbtion of toxicant among the population of exposed individuals. The levels of exposure are measured based on the frequency and duration of exposure as well as the levels of contaminant in the exposure media, such as soil, water, air, and food. Actual absorbtion is determined by toxicological studies.
The levels to which we may be exposed to contaminants depend upon the initial concentration at the source of contamination and its rate of distribution and dilution as it travels through air, water, soil, and food. The chemical reactions which occur in the exposure media may render the agent more or less toxic than the original compound. This is a very important consideration with regards to human contact and the estimated concentration of a toxicant upon exposure to the environmental media. Environmental fate studies provide information about the fate of chemicals in the environmental media and are used in exposure assessment for characterizing the exposure scenario.
Once concentration(s) in the environmental media at the point of contact with the target population has been established, the frequency and duration of exposure by members of the population are determined.
Exposure is calculated as:
x Intake x Duration x Frequency
Exposure for children is higher under the same conditions as adults due to differences in body weight. The units for exposure are milligram toxicant per kilogram body weight per day (mg/Kg/day).
The amount of toxicant we actually take in is called intake. An example of intake is the average amount of water an average adult drinks in one day. Absorbtion or uptake is the amount of substance actually absorbed into the body. Uptake is determined from the difference, for example between the amount of water we drink (uptake) and the amount which is eliminated from the body via urine, sweat, etc. The absorbtion factor is equivalent to the fraction, of water in this case, retained by the body.
Absorbed Dose is calculated as:
= Exposure x Absorbtion Factor
Uptake is determined by toxicokinetic studies. These toxicological studies observe the movement of substances in the body. There are four distinct phases of movement for a substance through the body: absorbtion, distribution, metabolism, and elimination. Rates of movement are determined by measuring the amount of chemical in various body fluids and tissues, such as blood and urine at timed intervals after a controlled exposure. This type of study is also used to determine therapeutic dosages for drugs.
There are three routes of exposure: dermal, oral and inhalation. Uptake from these routes of exposure differ based upon the chemical and physical properties of the toxicant as well as the permeability of the surface at the site of absorbtion; consider lung vs. skin vs. GI tract. The absorbed dose is an average of the daily intake value for one route. The absorbed dose for all three routes of exposure are added together for a total average absorbed daily dose (AADD).
Since intake rate per body weight for an individual changes over a lifetime the lifetime average daily dose (LADD) is calculated. The lifetime is split up into five periods: infant (1yr), child (1-6yrs), child (7-12 yrs), adolescents (13-18yrs), and adults (19-70yrs). The average absorbed daily dose (AADD) for these periods are added to get the LADD:
(1/70 x AADD infant) + (5/70 x AADD 1-6 ) + (6/70 x AADD 7-12 ) + (6/70 x AADD 13-18 ) + (52/70 x AADD 19-70 )
Sometimes the intake parameter for the adult is used in instead of the LADD. This is because the adult portion of the LADD is such a major fraction of a lifetime. But, this does not take into consideration the higher intake rates for children whom are not exposed over a lifetime. There is a growing public and agency concern over whether set regulatory standards are protective of children. Reviews of these standards are presently underway.
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This page was prepared by Theresa L. Pedersen, UCD EXTOXNET FAQ Team. September, 1997. I would like to thank Nu-may Ruby Reed, PhD for allowing me to draw freely from her lectures on Health Risk Assessment, which are the backbone of this page. Her lectures were conducted at the University of California, Davis in the fall quarter of 1996. Thanks again Ruby!