Submodule 6A, page 6
Here are some questions about Intake Assessment that you might have asked:
1. What if the concentration of the chemical in medium changes? Aren't these concentration constantly changing?
2. Where did you get the contact rate with the medium? Some people drink wine or coffee all day but never drink tap water.
3. Is the contact duration the same a lifetime?
4. My body weight is much more / much less than 70 kg.
5. I don't understand averaging time.
After presenting the generic intake formula, you saw the intake formula for ingestion of tap water, which is essentially identical. Then you saw the formula for ingestion of food, which had an exposure frequency of 30 days per year. You should have been able to follow to logic to that point. From that point the evaluation of intake can get very complex and involved. Let's proceed as follows. First I'll mention some of those complexities, which we will be wrestling with in this course, then you'll get some references and insight into other complexities, and finally for homework we will explore some websites that offer some models and other goodies.
Exposure Point Concentration. Usually you are assessing a community of receptors spread over a location. You could approach this several ways. You could divide the community up into smaller and smaller receptor groups, each group becoming more uniform as you divide into smaller and smaller groups. (This is great if you are working under a reimbursable contract with a very large budget.) or you could try to determine the Reasonable Maximum Exposure, that is, just take the highest exposure in the community (This is great if you have not time or money.) Often you do not have "a number" but lots of numbers. Then you will need some statistics. Which we will get to in while. Yes the concentration will change with time. If the chemical or its effects are cumulative, you will account for this mathematically. If the effects are not cumulative, you can just work with the maximum.
Contact rate. How many mg of dirt do you eat each day? Go to: EPA, Exposure Factors Handbook, Chapter 4 once the pdf document loads go to page 18 of 27 (which is 4-17 of the text), read the table. Bookmark or save this document for your homework. This handbook has many such useful numbers. Useful in the sense we can plug them into an equations and get an answer. We will learn some statistics about dealing with the these numbers.
For body weight, we typically use 70 kg for adults and 15 kg for children and ignore this obvious source of variability. However if needed, we can approach this statistically as well.
Duration of contact. This might be seasonal, as in swimming in the summer. Often you deal with workers that have a different exposure time that residents.
Averaging time. For many chemicals that are not carcinogens, their effects, if any, are limited to approximatley the time the receptor is exposed. Therefore the duration of contact and the averaging time are the same. There are however, non-carcinogens that have cumulative effects. Typically chemicals that are poorly excreted. For these, the averaging time that you use in your intake assessment must be coordinated with the number that you are using for dose-response from the toxicologists. We will discuss some of these in our "special chemicals" module. For carcinogens we assume the the averaging time is a lifetime, a strange assumption I will explain in Sub-module 6B.