Sunday, February 9, 2014

Making Your Own Exposure Limits (OELs)

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


  1. 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.

  2. You may also consider the aid of toxicology consulting firms as a source for a toxicologist.