The Rule-of-10 in Occupational Exposure to VOCs

Some individuals are blessed with a rare combination of technical and managerial acumen along with a gift for seeing the big picture.  In the realm of Industrial Hygiene I can think of no one who personifies  these traits better than Mark Stenzl.   During Mark’s long and very productive career he developed the “Rule-of-10” for inhalation exposure to volatile organic substances.  The rule is presented below:

Level of Control	Fraction of Saturation Vapor Concentration (SVC)
Confined Space – Virtually no circulation	1/10th of Saturation
Poor – Limited Circulation	1/ 100th of Saturation
Good – General   ~ 6 air turnovers/hr.	1/1,000th of Saturation
Capture	1/10,000th of Saturation
Containment	1/100,000th of Saturation

You may remember that:

Saturation Concentration (ppm v/v) = 

(Vapor Pressure/Atmospheric Pressure)(1,000,000)

Below are some comments Mark sent to me when I asked him if I could write a blog about it.  After he said it was OK to write about it, he provided the following background:

“I have been using the Rule-of-10 for at least 35 years.  It was a basic component of our qualitative exposure assessment process at both Celanese and later at OxyChem.  We sent every exposure scenario through this qualitative process and predicted the likely exposure category (similar to AIHA’s except we had an extra category 0.1 to 0.25 times the OEL and then 0.25 to 0.5 time the OEL). We then established our sample strategy based on this assignment.  If we thought that exposures were above 0.5 times the OEL we collected at least 5 samples evenly over the year.  If our predicted exposure was between 0.25 and 0.50 we collected at least 4 samples and if the predicted exposures were between 0.1 and 0.25 times the OEL we collected 3 samples and for all exposures thought to be less than 0.1 times the OEL, we would randomly pick out 10 SEGs [Similar Exposure Groups] and collect 3 samples.  Usually, on an annual basis, we would analyze all of our data and prepare an interpretive statement for each facility. Part of the analysis was to evaluate how well our qualitative assessment performed.  We did consider several other determinants of exposure in our algorithm beyond the Rule- of-10.  They included the observed level of control, frequency and duration of the exposure scenario, the hazard vapor ratio and the particulate hazard index.  I found that the Rule-of-10 to be amazingly accurate considering how simple it is. I have found the rule to be very beneficial in other applications such as the chemical approval process related to the introduction of new chemicals into the workplace or the change of existing processes; in auditing IH programs; in conducting due diligence related to potential acquisitions; and in inspections.” 

Further information on the implementation of Mark’s Rule-of-10 along with a check-list approach, designed to provide a more detailed analysis, developed by Susan Arnold will be included in the upcoming 4^th Edition of the AIHA Exposure Strategies Book.   My sense is that these additions alone will be worth the price of this basic reference that every Industrial Hygienist should have.

In another note to me, Mark makes the following points: 

“In the examples of the application of the Rule-of-10 [in the 4^th Edition] …, the IH may only be in the workplace a few minutes and likely does not have access to a “good” basic characterization.  Even in these cases, if the IH only knows what chemicals are present (and approximate composition), have an OEL or at least an estimate of an OEL and know the vapor pressure (information that is readily available) they can use the Rule of 10 as a screen.  That is, they know what type of controls and configurations they should be observing and match that up with what they are observing.  So the end point is not to classify the exposure into the correct exposure category, but rather to use it as a tool to raise “red flags”; where they should ask more questions and where they need to do a more formal exposure assessment.”

The 4^th Edition of the Exposure Strategies Book should be out this year.  I heartily recommend you get it.