Module 8 Comments QuestionsETC

This was a "Participation Homework" that had many questions. This represents the "high end" of this type of homework. Questions (Q) by student; Answers (A) by instructor.

The text was most unclear when HQ was being discussed. It read that if the value was greater than 1 and less than one was unsafe. Is this right? Then only a value of 1 would be safe.

There was a gross typographical error, less than 1.0 is safe, not unsafe.

Q. Are there any know combined effects that are synergistic? That seems to be something that should be noted whenever possible since it could be very dangerous. If there was a synergistic combined effect, how would that be noted? Or wouldn’t it?

A. Excellent question. Many synergistic effects are known. Carbon monoxide and methylene chloride is one that comes to mind. Most of these are noted at higher doses. I don’t know of any that have been verified at environmentally relevant doses. You would have to suspect that two chemicals were a problem, and then do a web search with the two plus synergy, “combined effects” and such. I don’t remember seeing any of these in IRIS or such. There may be a database of suspected synergies, but I not aware of it.

How do you determine the ambient concentration of a contaminant? Would this be done by modeling or monitoring? Is there a better method than another?

A. Monitoring will give you the exact answer for the concentration at the monitoring site on the day you do the sampling. Anywhere, anytime other than that, you have to model or make assumptions.

Why is the dermal route lacking relevant RfD? Is this because of the difficulty of tracking how much is absorbed through the skin, etc?

Ideally, the research work is done in the order of priorities, the most common and most hazardous first. Dermal is not nearly as common in environmental settings. For workplace hazards, where it is known, it is possible to shield the workers with gloves and so on. In general small lipophilic molecules are absorbed through the skin, but most of these are volatile, hence low dermal exposure in the environment. Unlike laboratory experiments where the chemical is placed directly on the skin, environmental exposures involves mass transfer between the contaminant from a soil grain or organic center in the soil, into the skin. This is difficult to model.

I got question 10 correct (1 in 10,000), but to be totally honest, I am not exactly sure HOW I got it correct. Would you please explain the answer? Actually, it would be helpful if you explained all the answers on this homework assignment (it would be a good way to reinforce my methods and check for potential mistakes).

OK, I’ll review the test and post answers to the most common.

Q. Can a chemical have both an RfC and an RfD? If they can, would the number be different or the same? Why would they be different?

A. Yes, yes. If the target organ were different than the route of entry, both RfC and RfD would depend the amount that is absorbed into the blood. Regarding amount absorbed, the RfC has imbedded assumptions about the amount the subject breaths and the amount absorbed from the air (a portion of the contaminant may be exhaled). The RfD has an assumption about the amount absorbed from the GI tract. These assumptions are a combination of the result of animal exposures and the use of PBPK models.

Just double checking—the inhalation and ingestion routes would both use the RfC to determine the overall HI?

A. ingestion uses RfD.

Q. Would there be multiple RfD than for different target organs?

A. Not target organs, but there could be two, one for ingestion and one for dermal. Also, there can be more than one for ingestion, if there is some known great difference between short term and chronic exposures.

In IRIS, what is the Drinking Water unit risk used for? How can the concentrations and specified risk levels be used effectively?

Just a quick way of getting an answer, also using the slope factor, or presenting it to the public, involves integrating the slope factor with your assumptions regarding inhalation and drinking habits. With the unit risk, those assumptions are hidden.