Exposure
limits (OELs) seem to be on everyone’s mind and for good reason; as exposure
assessors we find it very hard to live without them! A prime example comes from Hila Wright, CIH,
MPH who sent me the following after I requested ideas for future blogs.
“Would you talk about your approach to how you handle compounds of concern that do not have published exposure limits, but do have a body of available tox and epi data? Would you do your own risk assessment and try to come up with your own exposure limit to compare any exposure assessment results against? What would be your approach (especially since some of the multiplicative factors used in risk assessment seem somewhat haphazard)? How do you distinguish the good research papers from the bad (this is an area that I have trouble with). Or do you stick to a more qualitative approach, such as control banding?”
Hila, I am going to try and answer your questions directly
and succinctly; thus, some of the details will not be included but will form the
subject of future blogs. As such, my
answers are going to be somewhat shallow but hopefully still
helpful.
I have never been shy about setting working exposure limits
when none are published and there is some available toxicology data, especially
repeat dose (chronic or sub-chronic) toxicology data on the chemical of
interest. If you
would like to do this I would suggest first getting a copy of the Documentation
of TLVs from the ACGIH. It is not cheap
but it has all the details of the TLV committee’s deliberations on the
hundreds of chemicals for which they have set OELs. If you read enough of these you start to see
how they are doing it. There are, of course, a lot of details and
folks may chide me for making it too simple but I frankly do not see it as
being very complicated for most substances. From my perspective, their approach breaks
down to taking a NOEL (No Observed Effect Level) or LOEL (Low Observed Effect
Level) from a toxicological study and dividing it by a safety factor.
I can understand how you would see the “multiplicative factors”
in this process “[as]… somewhat haphazard”.
In fact, I have not found any written rules as to how large the safety
factors need to be for OELs. Indeed, this
appears to come under the almost magical category of “expert judgment”; however, if you
read enough examples in the TLV documentation for similar chemicals to the one
you are interested in, you will get the idea of how large a factor to divide
the NOEL or LOEL by. You may even disagree
with their assignment of any particular safety factor as I had done on occasion.
The next step I would suggest, especially when you are early in
the process, is to seek out the advice and counsel of a toxicologist and
explain to him or her exactly what you are trying to do and your thinking about
what the OEL should be given the data.
Companies do this all the time for their unique chemicals. They typically have a Committee to set OELs. The committee usually has Occupational
Doctors, Epidemiologists, Toxicolgists and Industrial Hygienists. You would be
a committee of one, or two if you have a toxicologist. If you want to engage the company that made
the chemical – just share your documentation with them and ask for their review
and advice on your work.
Whatever you do, I would advise that you put down all your deliberations in writing; that is, document your process and decision for that
particular working OEL.
Deciding good toxicological data from bad is an important
issue. Modern studies are often good
because they are typically done under modern requirements for GLP (good laboratory
practices). Studies done by and for large
companies or the government are often pretty good. Again, seek the advice of a toxicologist partner as
to the value of any particular study.
It used to be that studies from Eastern Europe were suspect
but I am not sure that is still the case.
Clearly old studies from Eastern Europe or the old USSR might be considered
suspect. It all breaks down into having
an understanding of what was done and how it was done. If that information in not available in the
report and the laboratory not well known then using the data could be
problematic. Again, consulting with a
toxicologist could be very valuable.
I consider control banding something to be done when you do
not have enough data to set a working OEL as described above. That is, its use is strictly from hunger for
data and requires a very conservative approach of trading conservatism for this
lack of data. That is, the bands should
be appropriately and consciously set to be considerably biased and more overestimating of risk
than if you had the data.
I hope readers will find the above quickie explanation
helpful. In any event, it has generated
the following topics to be explored in a future blog; namely,
- Meaning of NOEL and LOEL
- Sizing a Safety Factor
- Safety Factor Approach versus Estimating Risk Levels at the OEL
Well done. Suggest you contact companies such as duPont, ExxonMobil and others who have internal committees with broad composition who develop both the OEL and an analytical method to monitor the agent and provide the type of documentation one sees with ACGIH TLVs.
ReplyDeleteYou may also consider the aid of toxicology consulting firms as a source for a toxicologist.
ReplyDelete