• Overview of the subject and problems
• EIA2007 Asbestos Roundtable discussion
• Results of an early study
• Possible approach for ASTM air monitoring standard
• Response and input

Overview of negative air exhaust monitoring (1)

A common practice at abatement sites is to take air samples in the exhaust duct from a negative air machine or just outside the exit of the duct. Figure 1 shows a typical placement of a filter cassette for this application.

Figure 1

These samples, which are frequently required by specifications and regulations, have two distinct problems associated with them. Samples inside the duct or at the immediate exit lack validity unless taken isokinetically. That means the velocity of the air entering the filter cassette must match the free-stream velocity in the duct. If it doesn’t, the particulate collected on the filter doesn’t represent the size distribution in the duct. The velocity into a 25mm cassette in a 12" duct is nowhere near isokinetic, as shown in Figure 2.

Figure 2

To avoid this problem, some project monitors place the cassette outside and well downstream of the exit of the duct. This approach actually samples a mixture of exhaust and room or ambient air (Figure 3), which is what a passerby would breathe. Practical problems such as tampering can arise with ground-level locations while the very necessity of monitoring upper-floor exhaust is questionable.

Figure 3

The supposed reason for taking these samples is to evaluate the performance of the negative air machine. No data has ever been published relating fiber counts in the exhaust to those inside the enclosure. As best, qualitative results such as discoloration of the filter have been proffered to reflect leakage around or through the HEPA filter, but that condition can be inferred from observing the pressure drop across the filter.

EIA2007 Asbestos Roundtable discussion ---------------------------------------------------------------------Top

The above points were raised during the Asbestos Roundtable at the EIA2007 conference. My recollection of the discussion is summarized below.

• None of the attendees claimed to have taken isokinetic air samples in the exhaust duct of a negative air machine. A study was mentioned where this was done, but DOP was used as the test agent; air samples for fiber counting were not collected.
• The consensus regarding samples taken in the exhaust duct was that the results are qualitative at best.
• There was discussion about the problems of measuring the free-stream velocity in the duct, let alone matching the velocity through the cassette to the free-stream velocity.
• Sampling air outside the duct exit seemed to be more favorably received, with discussion on how far downstream to position the cassette to minimize the effects of turbulence. Practical considerations such as exhausting from upper-floors and tampering with pumps left outside a building were discussed.
• There was discussion about leaving the cap on the filter cassette (plug removed) and adjusting the flow rate to achieve isokinetic velocity through the hole in the cap. This creates a possible problem of uneven distribution of fibers on the filter and a laboratory manager reported having seen discoloration in the center of such a filter. No one could recall seeing a published article on uneven fiber distribution due to sampling with the cap on for 37mm or 25mm cassettes.

Results of an early study --

Soon after this page was initially posted, a study (2) was brought to my attention where isokinetic sampling was done in ducts attached to the inlet and outlet of negative air machines and fiber counting, not DOP testing, was performed. To increase the air velocity into the cassettes, the caps were left on and a tube inserted through the hole. Filters were analyzed by TEM using an indirect preparation method to overcome the problem of uneven fiber distribution. The results showed that several of the HEPA units fell far short of their performance specifications.

While this study supports the case for leak-testing negative air machines, whether air samples in the exhaust duct are a practical way to do it remains in doubt.

Possible approach for ASTM air monitoring standard -------------------------------------------------------------Top

The ASTM air monitoring standard is intended to provide guidance to the project monitor who takes the samples, project designers who write the specifications, contractors whose equipment is being monitored and regulators who impose the requirement in their states. The following approach is intended to provide information useful for project management purposes with emphasis on determining possible exposure to airborne fibers in the vicinity of the exhaust.

Based on the views expressed at the Roundtable and otherwise exploring the topic, the following approach is suggested in the standard:

• Acknowledge that sampling negative air exhaust is required by some regulations and specifications, usually with little if any guidance or criteria.
• Explain that there is no demonstrable relationship between fiber counts from negative air exhaust samples and fiber levels inside the enclosure, regardless of where and how they are taken.
• Explain that while elevated fiber levels in the vicinity of the exhaust may be due to a malfunction of the negative air machine, they may also be the result of breaches in the containment or releases of fibers from other ACM or non-ACM in the area.
• Discuss isokinetic sampling inside the duct and the practical impossibility of doing it under field conditions. Discourage sampling inside the duct and give reasons why quantitative results without isokinetic sampling are meaningless.
• Explain that samples give qualitative results at best, such as discoloration of the filter, and that the performance of the negative air machine is better done by inspecting the unit, monitoring the pressure gauge, smoke testing for leaks and measuring the air velocity into the filter.
• Encourage air sampling in a room inside the building where the exhaust discharges in cases where it is not possible to discharge outside the building. Explain that samples should be taken at least eight duct diameters downstream of the exit and that the sample will represent a mixture of exhausted air and room air.
• Discourage sampling outside the exit of a duct exhausting outside an upper-floor window, or outside a building at ground level unless the pump and filter can be protected against damage and tampering.
• Recommend that sampling be rotated among multiple exhaust ducts on larger enclosures.

Response and input ------------------------------ --- ----- ----- --------- ---------------- -Top

Your response to this approach and suggested language for the standard is solicited. In particular, if you have done isokinetic sampling in a negative air machine exhaust and have a procedure we could use in the standard, that would be much appreciated. Send your reply to Tom directly at tom.laubenthal@atcassociates.com or to me at andyobe@aol.com. If you want to be added to distribution for review drafts of the ASTM standard please let me know.

Thank you for your interest and cooperation.

Andy Oberta
The Environmental Consultancy
www.asbestosguru-oberta.com
(512) 266-1368

(1) Also see Chapter 4, Sidebar 4 of the ASTM Manual on Asbestos Control: Surveys, Removal and Management – Second Edition (Manual 23), Andrew F. Oberta, available from www.astm.org
(2) Quantitative Evaluation of HEPA Filtration Systems at Asbestos Abatement Sites, Roger C. Wilmoth et al. Environmental Choices Technical Supplement, Vol. 2, No. 1, Fall 1993. Environmental Information Association, Chevy Chase, MD.