Dermal Exposure: The Almost Forgotten Route of Exposure for Workers

One of my favorite sayings is:  “If you only have a hammer then you tend to view all of your problems as nails.”   I cannot help but think of the applicability of this saying to the past and, to an extent, the current practice of Industrial Hygiene.    It speaks to the need of our profession to broaden our perspective and available tools.

I admit that my training as an IH was many years ago but during that period I received essentially no education in the recognition, evaluation and control of exposure and risk from the dermal route of exposure.   I would love to hear from those of you who have had more recent training as to how it is being addressed these days.

Those of you reading this blog for any period know that I have put in a lot of time into talking about Occupational Exposure Limits (OELs).   Of course, I am doing this because OELs represent half of the story of RISK in the equation that Risk = Exposure x 1/OEL.    When the resulting Risk ratio is greater than 1 the risk is unacceptable.  If the ratio is less than 1 it is OK or at least NOT unacceptable.   The workplace exposures that we measure with great skill and care as IHs and exposure assessors have NO contextual meaning without some comparison to a toxicological benchmark.   As Industrial Hygienists our primary source of toxicological benchmarks are the OELs.

Unfortunately, almost all the OELs that we deal with are one dimensional.  In a practical sense the only OELs of any significance that most of us use are expressions of AIRBORNE concentrations of the chemical of interest in the breathing zone of the person receiving the exposure.   This focuses us on the inhalation route of exposure while we are left somewhat clueless as to how we might handle and similarly evaluate other routes.

From my experience the most important “other route” of exposure in the work place is dermal.  This route of exposure is often dominant in industrial and commercial scenarios for chemical compounds with relatively low volatility.    For example, I recall a study in which workers sorting lumber freshly treated and wet with pentachlorophenol got well over 80% of their total exposure via the dermal route as evidenced by the comparison of air and biological monitoring.   

One can roughly convert an airborne OEL to a dermal OEL using the following assumptions:

•         100% systemic absorption via inhalation
•         10m³ inhaled air per 8 hours
•         70 kg worker

Let us take toluene at  8 hour OEL of 50 ppmv as a working example.   That is (92.1/24.4)(50ppmv) = 189 mg/m³

(189 mg/m³)(10 m³/day)/70 kg  = 27 mg/((kg)(day))

This is our dermal (actually an overall systemic) OEL expressed as a traditional toxicological dose metric.

Now all we need to do is figure out two things:

1. How much of the stuff goes onto your skin
2. How much then goes through your skin to the systemic circulation (blood and lymph).  

For the first task we can use work done at the EPA years ago that indicates when you splash or immerse your skin in water or aqueous solutions it is retained at about 3-4 mg/cm2 per splash or dip.   If you do it with light oil it goes up to about 15 mg/cm2.   The EPA’s Exposure Factors Handbook (http://www.epa.gov/ncea/efh/pdfs/efh-complete.pdf ) goes into this in more detail with better documentation.  This reference also shows you the surface area of various body parts.

Just as an aside, it should be no surprise that the vast majority of dermal exposure from chemicals that are “handled” goes to the hands and lower arms.   We measured this value at about 90% a few years ago when we tested wall painters and this is irrespective of whether they painted via brush, roller or spray.

Next you need to figure out how much of the applied dose goes through the skin into the systemic circulation.  You can assume the worst case that all of it goes through instantaneously and indeed, some regulatory protocols have done this; however, this is a fairly dramatic overestimation of exposure.  Indeed, it probably over estimates exposure by anywhere from 2-100 fold.

A better methodology is to use physical-chemical modeling (what else?).  The best models, in my estimation, have been put together in a user friendly form by Dr. Wil tenBerge and members of the AIHA Exposure Strategies Committee.  https://www.aiha.org/get-involved/VolunteerGroups/Documents/EXPASSVG-IHSkinPerm.pdf

I will discuss some of the details in IHSkinPerm in a future blog but I wanted to make you aware of its existence. 

Another aspect of dermal exposure worth mentioning here is the real possibility of dermal exposure from exposure to vapors in AIR. The practical upstart of this type of exposure is that a respirator might not be fully protective or even sufficient in some scenarios.  As far as I know, only Dr. tenBerge has tackled this estimate and it is also part of IHSkinPerm discussed above.

Given the above, I suggest that it is time for you to consider adding more tools so we can view and deal with our exposure assessment challenges more comprehensively and  more effectively.