Sub-Module 6A, Exposure Factors
Overview
You now have an idea about chemical fate and transport, from the source to the person. You know about partitioning and transport pathways, and use of a SCEM to display them and aid your analysis. Later we will use a computer model to compute some actual transport. From these you will be able to compute the concentration of the chemical of interest in the micro environment of the receptors, or "contacting" the receptor. ("Receptor" is the correct technical word, remember, it is a person, John, Mary or whoever, not a thing.) You also know from toxicology that we need the dose that is actually entering the receptor, typically in mg of chemical per kg of receptor weight per unit time, that crosses the boundary into the person. The boundary is the mouth, nose or skin. Entry via the mouth, is called ingestion; entry to the lungs via the nose (and mouth) is inhalation. All this lesson is about is how to translate the environmental concentration into the amount crossing those boundaries into the receptor.
Note that this is for entry via inhalation and ingestion, this entry is not the same as absorption that you learned about in toxicokinetics. For a toxic metal, for example, you may have a concentration of 100 mg of the metal per liter of your drinking water supply. In order to know how much you ingest, you need to know how much water you drink each day. If you drink 2 liters each day, you would be ingesting 200 mg per day. Not all of the 200 mg will be absorbed from your GI tract into your blood. Some of it might never leave your GI tract. Here we get a little bureaucratic. The assumption is that the toxicologists doing the dose-response evaluation have considered absorption when they computed the allowable doses. So the engineers and environmental scientists only need to compute the ingested dose and not worry about the absorption from the GI tract. Similar for inhalation.
How do people get exposed:
The term for the concentration to which a person is exposed is called the "Exposure Point Concentration" (EPC). For now, let's assume you have one number for this, and only one exposure route. (We'll get to statistics and multiple routes later.) With the EPC we can start the Intake Assessment. NEXT