Risk Characterization

Risk Characterization is the final step in risk assessment. It combines the exposure assessment and the toxicity evaluation (dose-response) into a statement of the probability and severity of some harm. Here we also express the uncertainties in our analysis. In this sub-module we will deal with the combining and put off the uncertainty analysis for few weeks.

As in the toxicity evaluation, we will divide the risk into cancer risks and non-cancer risks. Many chemicals have both types of risks; these require a separate statement for each type of risk.

Here is the SCEM for the PCP site. Note kids could be exposed in several different exposure scenarios (options): the situation as it exists now, the situation if the gravel pit is reactivated, and the situation if there are homes built over the old contaminated site. Presumably these two future scenarios would increase the risk to kids. You have to do risk assessments for these two cases because the possibility of those future land uses was clearly identified. If any or all of these show some significant risk (we will define significant in a moment) the project will likely do some more investigation and feasibility studies and identify likely cleanup options. The situation that would follow each of these cleanup options will also need a risk assessment. The original risk assessment of the current situation is then known as the "baseline" risk assessment. In the following, we will only consider one option, the baseline.

Cancer Risk

For cancer risk, all the risks are summed for all chemicals and pathways for each class of receptor for each scenario. So from the SCEM: The classes of receptors are kids (if we assume the kids who play at the site are the same ones who live in Birch Acres), adults, workers in cemetery, workers in farm, and people who eat blueberries. You could eliminate workers in the cemetery, if you could assume that workers in the farm are exposed much more often. (Although if the farm workers computed out to high risk, you would then need to check the cemetery workers as well.)

Let's just consider kids:
Earlier you computed a daily average intake for each of these routes. If PCP were a carcinogen, you would now get the slope factor, which has the following units: increase in cancer deaths per daily average exposure, and multiply each intake by the slope factor. (There are some notes on a following page about this.) This yields the increase in cancer deaths for that class of receptor.


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