Sunday, February 16, 2014

15 min STEL in IH MOD

The blog before last I discussed the subject of point-in-time vs time-weighted average airborne concentration that might occur in an exposure scenario.   I went over how the current version of IH MOD makes life easier in estimating the point-in-time concentration at any time for any model.   Of course, we are typically not that interested in point-in-time concentrations except in situations where we have a Ceiling TLV or perhaps we are concerned about the possibility of obtaining lower explosive limit concentrations.

Because we are often working with 8 hour time-weighted average (8 hr TWA) TLV exposure limits or 15 minute short term time-weighted average (15 min STEL) TLV exposure limits, our primary focus is on the time-weighted average for these time periods.    Just to recap, the TWA is the integrated area under the C,t curve divided by the time interval.  IH MOD comes to the rescue here as well for 8 hour TWA concentrations because it calculates the TWA from the start of the scenario out to any time including 8 hours.  Thus, the 8 hr (480 min) TWA concentration is readily presented by IH MOD.

As I mentioned two week ago, the situation becomes a bit more complicated with you are looking for the maximum 15 min time-weighted average STEL during the exposure scenario.    Indeed, I went over some manual calculations that were necessary to get at that number.    I mentioned the possibility that this might “one day” be included in IH MOD.   Here is the quote from that blog:

“Perhaps someday we can get IH MOD enhanced to provide this calculation more directly.  My guess is that it would not be a trivial task but Daniel Drolet continues to surprise me.  He is a truly gifted programmer and I have no doubt that he can do it.  Indeed, what would be difficult for me could be relatively easy for him.”

Well Daniel took me up on this and did it!   He is truly a programming wizard and we are most fortunate to have him on our side.   In essence, the spreadsheet enables us to take any point in time along the scenario and calculate an approximate 15 minute TWA concentration forward  from that point.    See the screenshot below:

Of course, this allows us to pick the 15 minute interval that provides the maximum TWA concentration.

In this example it shows the maximum 15 minute STEL exposure occurring in the period 6.6-21.6 minutes into the scenario.   One  might (including me) think that it should occur in the period from 5 to 20 minutes.  The truth of it is that the 1.6 minutes after 20 minutes contributes more to the area under the curve than the 1.6 minutes after 5 minutes.  This slight shift was not intuitively obvious to me.   Daniel pointed this out to me when I questioned it.  It is another example of how these models provide insight.   

Daniel is a perfectionist and is working on making it more "elegant".  Future versions are inevitable; however, from my perspective is it good to go right now.    If you want to play around with this relatively untested BETA spreadsheet, please let me know (  and I will send you a copy.    I am sure that Daniel will appreciate any comments you have on it.  

Because of Daniel's tireless efforts we have what has been and continues to be the single best inhalation exposure assessment tool that I know of.

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