Sunday, August 17, 2014

Using IH MOD to Visualize Bolus Exposures

Two-weeks ago this blog  discussed the potentially very significant difference in toxic effect that could occur when what might be considered a relatively steady time-integrated exposure actually occurred as a bolus.     In the context of this discussion, we define time-integrated exposure as an exposure that happens at a relatively constant rate over a time period of hours.   A bolus exposure is defined in this discussion as a very high level exposure that happens over a period of seconds to minutes.   The point of the previous blog on this subject was that on those occasions that a bolus exposure occurred, but was measured as a time-integrated sample, that the average concentration might appear very tame compared to an 8 hour OEL while the actual exposure experienced by the worker could have been very high.    

My colleague Matt Le asked if we might be able to portray this difference in time graphically and if so what that portrayal might teach us.   I immediately thought of IH MOD as a tool that can readily help to visualize the time elements of inhalation exposure.  As such I set about to essentially reproduce last week’s Scenario 1 and Scenario 2.   In the first situation (Scenario 1), the breathing zone concentration of this compound is relatively constant throughout the 8 hour shift and results in a measured value 7 ppm after repeated daily sampling as an 8 hour time-weighted average (TWA).    In the second scenario (Scenario 2) the worker receives, for whatever reason, essentially no exposure for most of the sampling period but a bolus exposure to the compound that lasts only 2 minutes and also results in a measured integrated 8 hour TWA exposure of 7 ppm.

After firing up IH MOD it a pretty simple matter to simulate these two scenarios.

Scenario 1 is show below:
 Assuming it was indeed a constant source, 7 ppm was used as the starting concentration Co in the above model.   As you can see it is pretty much a flat line for both point-in-time and time-weighted average concentration out to the limit of the simulations which was 60 minutes.   The source as constant for the entire 60 minutes.

Scenario 2 used the same inputs (for room volume (V) and ventilation rate (Q)) as Scenario 1 but without a starting concentration.  The time of generation was set to 2 minutes and the generation rate was boosted to render a peak concentration slightly higher than 1600 ppm.  Indeed, the results would have been about the same if I had included 7 ppm as Co, it simply would have been washed out by the bolus.

Scenario 2 is below and shows the dramatic exposure potential for the 2 minutes in which the source was turned on.  Of course, the source was shut off after 2 minutes and the simulation allowed to run out for 60 minutes.  The time weighted average after 60 minutes was 50.1 ppm.   Running the model out for 480 minutes renders a calculated time-weighted average exposure of 6.8 ppm.

Other than providing a clear graphic picture of the differences in these two scenarios, that both represented 7 ppm as an 8 hour time-weighted average exposure, what did IH MOD tell us?   Well, the bolus dose (Scenario 2) required an intense short term (2 min) rate of contaminant input to the model that was over 230 fold higher that of Scenario 1.  The total amount of contaminant emitted during the bolus was about 8 fold higher than the total for Scenario 1 for one hour.  These ratios will vary with the modeling inputs selected but will always been a much larger value for the bolus exposure. 

Clearly bolus exposures represent unique and potentially critical events relative to worker exposure.

Questions for LinkedIn Group Discussion:
Do you have the potetial for bolus exposure in your workplace?  

If so how to you address them?

What do you think about having C or ceiling OELs for these possibilities especially for respiratory irritants or do you think the excursion limit of 5xOEL would be sufficient?

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