Sunday, July 19, 2015

To Measure (or Estimate) Skin Exposure or Not

Those of you who read this blog know that I have featured the work of Chris Packham from the UK and his tireless work on stemming the risk from dermal exposure.

I recently got an email from Chris commenting on a resent “If the only tool you have is a hammer…” blog in which I again highlighted the need for better tools to assess dermal exposure and risk.   Chris’ correspondence goes on at some length recounting the limitations of the current methodologies and in general concluding that we should not even try to measure  (or estimate with models) dermal exposure at this point.  In short, Chris suggests that we put that effort into managing the risk by choking down the exposure rather than assessing it.

I am reproducing the last paragraph of his correspondence below with the essence of his message.

“To measure, or not to measure?
Hygienists, in their training, tend to concentrate on measurement. I suggest that this can lead to a form of ‘tunnel vision’, the view being that if we cannot measure then it is not important for the hygienist. When, admittedly some years ago, I participated in some of the AIHA conferences, it appeared to me that the hygienists’ concern was to be able to demonstrate compliance. The view seemed to be that: “We cannot measure and there are no regulatory limits, so as with skin we cannot demonstrate to the client/employer that he or she is compliant, let us just ignore this.
My opinion, for what it is worth, is that attempting to measure skin exposure takes time away from work to reduce exposure and thus, whilst from an academic viewpoint is, perhaps, time well spent, it does little to reduce the incidence and prevalence of damage to health due to workplace skin exposure. I believe that risk assessment for skin exposure will remain subjective for years to come. What is important is that we attempt to introduce a measure of consistency so that we rate the risks of each task assessment using the same criteria. We can then rank the risks such that we tackle them in order of declining severity. “

I would be happy to send Chris’ entire note to those who request it at; however, I am going to have to respectfully disagree with Chris on this issue. 

My position is that we can always make some quantitative estimate of dermal exposure using available models and information from sources like the EPA Exposure Factors Handbook and some worst case assumptions.   This process then bounds the quantitative upper level of exposure and is amiable to refinement with more data.

I have always found it very difficult to effectively manage any risk which has not been reasonably assessed.  The tools may be blunt but, in my opinion, they are better than not estimating the exposure potential and the risk from that potential at all.  Rather than being an academic exercise, I see these efforts as a concerted and rational effort to quantitatively gauge the level dermal exposure potential as a critical step in the assessment and management of risk.

Although significantly over-estimating the potential exposure, the “on the skin – in the body assumption” used as a worst case by some has its value in quantitatively bounding a worst case.  More important, it points the way to the value of experimental data to lower the uncertainty.   My sense is that the use of relatively poor tools is preferable to not attempting to use estimating tools at all. I believe that how we learn and progress is in the trying; that is, we do the best we can with the tools at hand and report the uncertainties associated with those efforts.

A prime example of this occurred many years ago when I was working for a large chemical company.  We had a product with a carcinogenic residual monomer and the business asked me what level of residual would render a virtually safe dose as was defined for this carcinogen at the time.   My analysis used worst case assumption including the “on the skin – in the body assumption” which, when all was done, meant that the monomer had to be reduced during manufacture to relatively low levels.  This assessment and the documented and rational risk management that came from it stood for a significant period of time until it was agreed that more data would help to refine the risk assessment.   With the addition of good experimental data, we determined that only a relatively small fraction of the amount on the skin could ever get into the systemic circulation and this provided considerable relief relative to how much residual monomer would be allowed within the product.

In this case we were required to present a quantitative estimate of dermal exposure which drove our estimation of risk.  We did so with the best information available to us which ultimately caused the gathering and expense of more data and a successful resolution to the issue.   This could not have been done if we simply abandon any estimation of exposure and instead tried to choke off the exposure without an assessment.

This is not to say that you cannot manage a risk without assessing it, only that it is quite difficult and can potentially be very inefficient to do so and present an impossible situation in a practical sense.

I look forward to continuing this dialog with Chris and other readers of this blog.


  1. I don’t understand Chris' bleak view on the utility of models. This is an area of active research and models are already available that can reliably predict systemic exposure from skin deposition for a wide variety of chemicals. This reminds me of the view some folks had toward PBPK models a few years back. The models were mathematically complex by the standards of the day, required large numbers of parameters, and the feeling was that if good values were not available for the parameters, the models were a waste of time. This essentially amounts to saying “no information is better than some information.” And this ignores the fact that even incomplete or partially parameterized models can be used to quantify uncertainty, explore sensitivity of model outputs to inputs, inform us of critical data needs, or predict extreme cases.

  2. Perhaps what we need to recognise is that models and exposure measurement techniques have their uses, but that from a practical perspective they could end up being rather lengthy processes if they were used for each and every chemical found in most workplaces and the varied uses they can be put to. There are situations where they are invaluable tools, but there value is only equal to our understanding of their limitations. If those using these tools do not understand their limitations, action can be taken that makes the situation worse rather than better. Prof Ronald Marks, a very eminent dermatologist and founder/expert in the field of skin measurements, once told me that "a fool with a tool is still a fool". It should also be mentioned that whilst uptake through the skin of chemicals that can cause systemic damage is without a doubt important we must not forget the significance of direct irritant damage to the skin both in the effect on the skin and the individual through irritant contact dermatitis but also in the affect this can have both on the ability of chemicals to penetrate the skin and their ability to cause type 4 sensitisation. The variability of individuals in their reaction to irritants is highly variable. In short models and measurement techniques have their uses and we should not stop trying to find more effective tools/techniques, but if that is what we spend most of our time focusing on rather than eliminating or reducing exposure then perhaps we should not be so surprised when the effects of skin exposure do not reduce as we would like them to.