Answer to Question #3156 Submitted to "Ask the Experts"

Your question is not completely clear, in that you seem to be mixing the issues of filter testing and monitoring of the effluent.

The testing of high-efficiency particulate air (HEPA) filters depends on many factors. When first installed, one needs to determine if they are mounted correctly, such that the air flow is thru the filter and not leaking around the seals on the filter housing. This is done by using a specialized nontoxic aerosol that is easily detected downstream if there is leakage, but should be captured by the HEPA.

In that they are expensive filters, it is always best to have a rough prefilter, and as the HEPA is used (as a portable filter, or on a stack), one needs to measure particulate loading. This can be done thru a differential pressure drop across the filter.

If one has an airborne radiological contaminant, it is a good practice to measure the effluent being discharged. Depending on the nature of the radioactive material, this will drive the sampling method. If a HEPA is used, any release would no doubt be a fine particulate, thus, an equally efficient filter medium would be required to capture the particles. Such sampling should be done under isokinetic sampling conditions, such that there is no bias in particle size distribution collected, regardless of location of the effluent (that is, an indoor work location or stack effluent).

Lastly, air flow balance in a work area or building with ventilation on process equipment can be an important factor with worker protection and airborne inhalation hazards. A competent authority should be consulted if there is such a concern in your facility.

There are many reference materials and standards related to HEPA filter testing. DOE has a website on this subject and a draft Nuclear Air Cleaning Handbook that may be of help.

David J. Allard, CHP