Sunday, September 28, 2014

Why Model Exposures when you can Measure Them?

The blog this week is a reworded excerpt from what I believe may be the best book an IH can own; that is, Mathematical Models for Estimating Occupational Exposure to Chemicals, 2nd Ed, AIHA Press.

Many Industrial Hygienists are faced with at least scores of exposure scenarios in which workers face exposures to many different compounds in a lot of different circumstances.   It is probably safe to say that under the current operating system the exposure associated with a majority of tasks are never monitored because the industrial hygienist judges them to be safe.   Indeed, John Mulhausen has taken the lead in discussing, disclosing and popularizing the fact that the most common number of samples collected to make this determination is zero.   Indeed, most hygienists are not required to present a formalized or systematic analysis to support these decisions.  That is, he or she has observed the universe of scenarios and has, for the most part and probably correctly, concluded that the exposure limit is not exceeded.  When asked how that determination was arrived at, the typical answer is that he or she applied expert judgment. That is, the occupational hygienist uses his or her combined experience to make this decision. When pressed further, the hygienist might say that it is because the system or scenario under consideration is relatively closed, that the vapor pressure is low, the exposure limit is relatively high, etc.  This combination of factors tell an experienced occupational hygienist that overexposure will not occur.  When professional judgment leads the hygienist to believe exposures may approach or exceed the exposure limit, a monitoring plan is put into action. The results of that monitoring determine whether controls are implemented.
I believe that much of exposure assessment in general and industrial hygiene in particular, has been practiced using the reactive, reflective, qualitative, and relatively undefined expert judgment as outlined above.  Within the IH world, this manner and technique of working has generally protected many workers from overexposure and subsequent adverse health effects.  However, it has a number of serious flaws, including:
(1)  It is difficult or impossible to explain objectively.
(2)  It is typically not supported by explicit quantified facts relating specific cause and effect.
(3)  It is not amenable to technology transfer (i.e., those new to the field find it hard to learn).
(4)  It is often insufficient to provide convincing evidence to affected workers or to defend against litigation or other legal challenges.
Thus, I believe that the standard method of only using direct measurement is clearly not the best we can do as industrial hygienists.  In fact, sometimes, measurements cannot be taken.  Consider the following situations:
  •          You want to monitor exposures, but there is NO method is available
  •          You cannot measure exposures “right now” when they are occurring
  •          You cannot measure exposures because you cannot be present, such as when they happen at another location, they happened previously (retrospective), or they have not happened yet (prospective)
  •          A small sample size of exposure monitoring events leads to a heavy bias toward concluding unacceptable exposures are acceptable
  •          The financial burden associated with technician’s time to collect sample and analytical fees are real-world challenges that restrict monitoring efforts

 As mentioned above the typical IH applies expert judgment relative to the decision to monitor or not.  I submit that they are actually using a subliminal model to make the call that the scenario requires monitoring or it does not.    The entire point of this aforementioned discussion within the book is that rather than simply invoking a claim of poorly or unsubstantiated professional judgment relative to their decisions, the use of explicit and transparent models enables the industrial hygienist to understand, display and defend the scientific rationale behind those decisions.

The book goes into a lot more detail relative to this argument but more important, it provides a wealth of technical information about specific models and modeling techniques.   I and my fellow authors receive no money from the sale of this book but I tell you honestly that I believe that no IH should be without it.

Questions for Discussion:
  1. Of those of you who have it, what do you think of this book and how could we make it better?
  2. What modeling problems have you tried to apply the material in the book to solve and how did that work for you?


  1. I received an email from Chris Packham in the UK in response to this blog. Chris' focus for many years has been on dermal exposure and I believe he makes some good points. Indeed, when we are dealing with high MW, low vapor pressure chemicals my sense is that dermal exposure can predominate in the work place. I could not put his comments here because they exceed the 4096 character allowed. If you would like to see them I will forward the email to you if you ask for it at

  2. Mr. Jayjock:

    Thank you for your post. I am a toxicologist and environmental engineer working mostly in the environmental rather than the occupational field, but the issue of monitoring vs. modeling is germane to exposures outside the workplace as well as inside.

    Like most things, there are pluses and minuses to each of these exposure assessment methods. Outdoor "air dispersion" computer models have been around for many years. If enough information is known about the emission sources and their strengths, these models (which incorporate local meteorological data) can produce fairly valid data on which to estimate a person's exposure to air pollutants.

    The advantages of these models, much like the models used for estimating indoor air concentrations in the workplace, are the ability to 1) estimate exposures at many different places at many different times and 2) estimate the maximum likely exposure levels for different averaging times. In addition, these models can predict the path of and the chemical concentrations in the "plume" of pollutants as it moves downwind. These models can provide a "movie" of the emissions over time and space. Most of the disadvantages have to do with the reliability of the assumptions that go into the model ("garbage in = garbage out" caveat).

    In my line of work, outdoor air monitoring is felt to provide the more representative data, but monitoring is limited (mostly by costs and other practical considerations) to specific times and locations. Monitoring provides a series of "snapshots" rather than a movie of chemical exposures.

    It seems to me that indoor air modeling would have additional challenges compared to outdoor air modeling. Air flows are not usually uniform indoors and depend on local and area ventilation, obstacles to air flow, and even worker movement within the work space.

    In contrast, worker monitoring is somewhat easier than outdoor monitoring since workers can wear portable "personal" monitors throughout the work day. Compared to people in the community, workers are generally less likely to move long distances during the day, although certainly they may move within the workplace to areas of higher or lower chemical exposures. There would typically be less variation in worker exposure levels during a work day and their exposures would not usually change as much from day to day when compared with people outside in the general environment.

    These are meant to be general comments only and each workplace, just as each community, need to be evaluated using site-specific data as much as possible.

    In summary, monitoring and modeling have a place in the assessment of exposure to potentially-toxic chemicals. Each has its own advantages and disadvantages. In the outdoor air, modeling is somewhat superior to monitoring. In my opinion, however, the reverse will hold true in most workplaces.

    I welcome any comments, questions, or criticisms you may have.

  3. The discussion has been most interesting and I would think that modelling is a must have for the hygienists toolkit. One thing I would say in favour of monitoring is that technology will likely see it become cheaper and easier to conduct on larger scales. An old example is the introduction of the passive monitors some 30 or so years ago.

  4. Dr. Dydek,

    Indeed, I see modeling and monitoring as complimentary with each having an important part to play. If is just that until relatively recently modeling was typically not considered or done..