Exposure modeling is no substitute for air monitoring. I didn't think I needed to say this in my May and June columns on modeling software. Unfortunately, I was surprised recently to hear an industrial hygienist mention that he modeled solvent exposures in a batch mixing operation because previous air monitoring results showed a wide range in employee exposures.
This hygienist somehow felt that modeling was more representative of worker exposure than actual air sampling results. This is a dangerous misapplication of modeling.
It's normal for air sampling exposure measurements to vary significantly. It's generally accepted that measured exposures can vary by two to three times, even in adequately controlled operations. Sample results in poorly controlled operations will vary even more.
This variability is the basis for the Threshold Limit Value Excursion Limit published by the American Conference of Governmental Industrial Hygienists. Very simply put, exposure variation more than three to five times indicates a poorly controlled operation and steps should be taken to improve or restore controls.
Models are simplified representations of the real world. Sure, computers enable the development and use of complex models. But even the best models available do not account for all variables. Worse yet, model approximations can be wildly wrong when misused by someone unfamiliar with the model's proper application and limitations.
The fact is that air sampling measures actual real-world exposures with a high level of accuracy and precision. Models, on the other hand, can only approximate exposure.
Modeling is no substitute for sampling. So why is so much time spent on developing and using exposure models? Because there are situations when we can't do air sampling.
Recently, I had the pleasure of discussing exposure modeling with Michael Flynn, Ph.D. Flynn is a professor in the Department of Environmental Sciences and Engineering at the University of North Carolina, where he conducts research in the use of computational fluid dynamics to model human exposure to toxic airborne contaminants (www.sph.unc.edu/envr/research/labs/cem.htm).
"Models help us estimate exposure when measurements are not possible or are unavailable," stated Flynn. "Often in hygiene, we are faced with estimating what an exposure was in the past (retrospective epidemiology studies, accident investigation, etc.,) or will be in the future (e.g., what will the exposure be if I install a ventilation system). However, I think the most important use of models is to help understand, in a clear quantitative way, how exposure depends upon various process parameters, work practices, ventilation flow rates, etc. This can then help the hygienist make more informed decisions about how to minimize exposure and reduce risk."
"Estimates of exposure based on models should not be considered a substitute for personal sampling." Flynn concluded. "The gravity of the decisions that hygienists make involving human health demands the most careful and thorough of investigation."
Models should only be used with an understanding of their proper application and limitations. The models discussed previously, CONTAMW and ChemSTEER, provide extensive documentation explaining the basis for the models, their application and limitations. Take the time to read and understand the documentation before using the models. Model operations you've sampled and compare your virtual model with the real world.
Formal training is also available. The American Industrial Hygiene Association offered two professional development courses at the 2003 American Industrial Hygiene Conference (AIHCE). One class, "Mathematical Models for Assessing Exposure to Indoor Air Contaminants," provides a broad overview of exposure models that can be run using a calculator or spreadsheet. Instruction includes how to perform an uncertainty analysis of model exposure estimates.
The second class, "Reconstructing Exposure and Dose: Utility for the Practicing Industrial Hygienist," explores methods for reconstructing either total lifetime dose or dose associated with specific exposure events. I expect both classes will be offered at the 2004 AIHCE.
Model With Care
Whenever possible, air sampling should be used to determine employee exposure to air contaminants. Air sampling is not difficult. John Rekus' excellent three-part series, "Yes, Even You Can Do Industrial Hygiene Air Sampling (www.occupationalhazards.com/full_story.php?WID=7113) is an excellent tutorial for those new to the sampling train.
Don't reject the results if air monitoring shows large variations. Take the time to understand why the results vary. Learn as much as possible about the operation. Talk to the employees and supervisor. What tasks contribute most to employee exposure? Do employees follow proper procedures and use the required controls? Are engineering controls adequate and properly maintained?
Understanding the operation and asking the right questions will invariably identify the reason for the varying measurements. And, more often than not, the reason will trigger ideas on how to improve controls and procedures to reduce exposure and better protect worker health and safety.
Now pull out your modeling tools to evaluate how your ideas to improve controls may reduce exposures.
Never put the modeling cart before the sampling horse. And before using a model, remember the supervisor who wrote in an employee's annual performance review, "He's a model employee a small and simplistic representation of the real thing."
A big "thank you" goes to Kim Gordon and the other members of the Atlantic Provinces Section of the American Industrial Hygiene Association. The section maintains a sweet Web site (www.aihaaps.ca/main/main.html) with a special focus on Personal Digital Assistants.
The site made my vivisimo.com Web search hit list last week and I visited the site to see what was new. My mouse instinctively flew to the link to the eye-catching article, "How to Read PDF Files on a PDA with iSilo" by Ricky Spears.
You may remember that I was disappointed with the PDA version of Adobe Acrobat Reader ("You Can Take it With You," www.occupationalhazards.com/full_story.php?WID=5306) because PDF file size increases when converted for the PDA. This is the wrong direction for memory-challenged PDAs.
What Ricky Spears discovered is a way to use iSiloX (www.isilo.com) to convert PDF files to html by accessing a tool on the Adobe Web site. The method is well documented in Spears' article.
The conversion is not pretty, but it works. A major drawback is that the conversion strips out all graphics. Another problem is the text formatting in the output file is less than ideal.
The major benefit is the HTML output file is so much smaller than the original PDF. For example, the PDF file of OSHA Publication 2202, "The Construction Industry Digest," grows from 632 KB to 710 KB when converted to Acrobat Reader for the PalmOS. The same file is just 62 KB when converted to HTML.
I don't know about you, but I can handle poor text formatting and no graphics to have a digest of OSHA's construction industry standards on my itsy bitsy PDA.
Contributing Editor Michael Blotzer, MS, CIH, CSP is an occupational hygiene and safety professional, writer and computer enthusiast who brakes for animals on the information superhighway. He can be reached by mail addressed to Occupational Hazards, by fax at 309- 273-5493, or by electronic mail at [email protected]