Pre-Exposure Considerations

Although the goals or endpoints of SPMD studies vary widely, a number of questions should at least be considered prior to initiation of exposures. These questions include the following 1) are there threshold limit values (air) or water quality criteria for chemicals of concern, and if so, has the lowest environmental concentration of concern (Cc) been established for target compounds 2) will study 

Clearly, question 1 is not applicable when the project is a reconnaissance for unknown toxics. In regard to question 2, investigators can sometimes (assuming key parameters such as values are known or can be approximated and that uptake 

Generally, there is little data available on site conditions with the possible exceptions of seasonal average temperatures and river flow rates at USGS gaging stations (question 3). Thus, reconnaissance of study sites is often needed to assess exposure conditions and potential problems. A reconnaissance of study sites is also an important part of assessing the potential for vandalism and theft (question 6) 

The following equations can be used for predicting times that SPMD sampling will represent linear, curvilinear and equilibrium kinetics (question 7), assuming key parameters such as Rs values are known or can be approximated. 

When exposure time exceeds slightly more than four-fi/2S or tg for a chemical, the SPMD is essentially at equilibrium with the ambient environment. Therefore, the time period in between the fi/2 and tg values represents the curvilinear region of uptake. Because it is not possible to accurately predict in situ SPMD exchange rates (e.g., Rs) a priori, this exercise is for planning only and is no substitute for the use of PRCs as discussed in Chapter 3. 

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