Sunday, August 3, 2014

8 hour OELs versus Acute Bolus Exposures

Relying on full shift integrated sampling for comparison to an 8 hour occupational exposure limit could be problematic under some circumstances. 

Consider a hypothetical chemical with an 8 hour ACGIH TLV of 100 ppm (no STEL and no Ceiling TLV). Let’s also conditionally accept that we are okay with the AIHA exposure strategies criteria that a measured exposure of less than 10 percent of this OEL is considered an exposure that is not unacceptable.

Now let’s consider two very different exposure scenarios.   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 ppmv after repeated 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 the vast majority of the 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 ppmv. 

Obviously, these represent two very different exposures that are measured as the same average over a one work-day time interval.   Depending on the chemical they could have resulted in very different effects on the worker being exposed.   For the sake of this example, let’s assume that the maximum instantaneous exposure was 10 ppmv in Scenario 1 with a minimum around 3 ppmv.  In other words, it occurred at a fairly constant rate of exposure with an average of 7 and a maximum instantaneous breathing zone concentration of 10 ppmv.   In Scenario 2 the average concentration over the 2 minute period of bolus exposure would be calculated as follows:

7 ppmv * (480/2)  = 1680 ppmv

Another way of checking this is to use the standard time-weighted averaging equation:

8 hour TWA  =  (C1* t1  + C2*t2  + … Cn*tn)/(480 minutes)

If the above initial equation is correct, then C1  In the second scenario is equal to 1680 ppmv, t1 = 2 min,  t2 = tn = 0 and total time = 480 minutes.    It checks, the 8 hour TWA comes out to be 7 ppmv.

The graph below illustrates this dramatic difference in maximum average breathing zone concentration for the different time periods for the two scenarios:

The right-hand bar would be 840 ppmv if it occurred over 4 minutes, 420 ppmv over 8 minutes and so on.

Supposed that the threshold mechanisms that protect the body by detoxifying or otherwise handling the chemical can be overcome with short-term, high concentration exposure; clearly, exposure Scenario 2 could result in a much stronger toxicological effect than Scenario 1.

As such, acute bolus exposure is of concern and the ACGIH addresses such situations with the following approach:

“Excursions in worker exposure levels may exceed 3 time the TLV-TWA for no more than a total of 30 minutes during a work day, and under no circumstances should they exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded”  [emphasis added]

Indeed, if we could anticipate the occurrence of any bolus exposure in a specific workplace location and time then it could be identified and addressed separately as its own scenario.    We may not always have this luxury however and may sometime be “stuck” with an integrated sample that indicates an acceptable exposure but an employee who has experienced a relatively high exposure and an untoward health effect.  One potential answer is to use real time monitoring of the employee’s breathing zone with a concentration-set alarm when the possibility of a bolus exposure exists. 

Questions for Discussion within the LinkedIn Groups: 

How would you handle a situation in which you had high bolus exposures occurring infrequently and somewhat unpredictably?  

Easy answer perfect world answer:  You monitor everyone everyday with real time monitors with alarms.   Real world question:    What might you do with hundreds of potentially exposed workers and bolus occurrences which are, at least at this point, unpredictable and are relatively infrequent?   


  1. Clear example Mike. As always, there is no one real world answer. As I am a risk assessor and not an industrial hygienist I take the risk of a stupid answer. But anyway, it depends on what is known about the chemical in terms of hazard (what effect, is it a very severe effect, is there anything known about possible differences acute versus chronic exposure). Assuming this is all not known, I think you need to think of some kind of monitoring of complaints systems with a short feedback loop. If health complaints, find out whether high bolus exposure might have occured (Good Manufacturing Practice?), if yes, take action to prevent same kind of high bolus exposure happening again and report (estimation of bolus exposure level in combination with health complaints) so that a wider community could learn as well. Longer term outcome up might be setting of a STEL or ceiling TLV for that particular chemical.

  2. I would highly recommend putting fixed sensors in the area with alarm levels set for the appropriate levels. For exsample: We have fixed alarms for HCN which goes off visualy at 4ppm and the audible alarm goes off at 10ppm. However, there are some many variable unknown to make a accurate conclusion. I like the descusion though :)

  3. Hi Mike,
    A very long time ago, I refer to 1979, the Chaiman of the German MAK-committee published valuable recommendations. He suggested to use five classes of compounds with different excusion factors from 2-10, varying duration of short-term exposure (5-60 min) and frequency per 8 h (1-8 times) based on the kinetics and dynamics of the toxicity of the compound under study. I have used his recommendation several times.
    Paper: Henschler D. Begrenzung von Konzentrationspitzen der Exposition... Arbeitsmed Sozialmed Preaventivmed 14 (1979) 191-193.

  4. Understanding the tasks being performed when establishing similar exposure groups can help avoid such an oversight. Batch operations and infrequent tasks with high exposures are why STELs, ceiling limits and excursion limits are important. 8 hour exposure limits assume a certain level of variability but can be misleading without a proper task profile.

  5. Always limited by what we don't know, but I will take a stab. In a relative sense, the process associated with Scenario 1 is in control and the one for Scenario 2 is not. Therefore, in addition to the IH/health risk assessment centric suggestions already provided, we should meet with the industrial engineer for this process and identify his/her indicators of the process being out of control. Assuming there are none yet existing, this would be an excellent opportunity to leverage IH skills for something other than only exposure assessment. Thinking in terms of engineering/operations/quality control/cost control while developing exposure assessment strategies and vice versa enhance the occupational hygienist's value.

  6. Thanks to all for the great answers above. This is a wonderful medium and your thoughtful responses are all greatly appreciated. Mike Jayjock