Abstract
Introduction: Positive pressure breathing air-fed protective suits from three vendors are commonly used in biosafety level 4 (BSL-4) laboratories: they are Dover Chemturion suits (ILC Dover, DE), Delta suits (Honeywell Safety, NC), and HVO suits (HVO-ISSI-Deutschland GmbH, Germany). To address the potential risk of infectious agents being introduced through the supplied breathing air stream, some suit manufacturers incorporate protective filters on the suits themselves. However, these integrated filters are not amenable to in situ testing for efficacy verification. We have been using external filters from Matheson USA on the positive pressure suits since our BSL-4 laboratories were commissioned two decades ago. As part of our BSL-4 protective suit management program, we test these filters before them being put into use, and annually thereafter. In the past few years, we have observed these filters failing at a higher rate, as high as two out of three of the new filters tested at one point.
Objective: The purpose of this study was to procure personal protective filters from other sources and validate their efficacy long-term.
Methods: Filters from Respirex, HVO, and Honeywell were validated for filter integrity and filter loading.
Results: Respirex filters performed well during the initial testing and periodic testing thereafter. Regular testing of the Respirex filters has now been ongoing for 30 months with continued successful performance.
Conclusion: Filters from Respirex are a suitable option to protect personnel wearing positive pressure suits in BSL-4 laboratories.
Keywords: biosafety level 4, containment level 4, biocontainment, personal protective equipment, positive pressure suit, personal protective filter
Introduction
The Containment Level 4 or Biosafety Level 4 (BSL-4) laboratory is the maximum biocontainment laboratory,1 where work with dangerous and exotic risk group 4 pathogens is conducted.2 There are two models for BSL-4 laboratories: a cabinet laboratory, where handling of pathogens is performed in a line of Class III biosafety cabinets, and a suit laboratory, where personnel wear positive pressure protective suits.3 Cabinet laboratories can be ergonomically challenging for certain tasks and are not generally suitable for large animal work.4 As a result, the majority of BSL-4 laboratories are suit laboratories,5 where the workers enjoy enhanced range of motion and visibility compared with those working in cabinet laboratories.
Extensive training6 and a range of complex equipment and facilities are used by those working at BSL-4 to protect operators from exposure to infectious agents.4 As a form of personal protective equipment, a positive pressure suit completely isolates the worker in a safe microenvironment from the potentially hazardous laboratory environment.7 The suits offer protection in two ways: a hermetically sealed physical barrier created by the impermeable suit material and the positive pressure provided by the breathing air, which forces clean air outward from the suit through one way valves and keeps potentially contaminated laboratory air from being drawn into the suit even during a suit breach scenario.8
According to WHO's Laboratory Biosafety Manual (LBM) and the U.S. Department of Health and Human Services' Biosafety in Microbiological and Biomedical Laboratories (BMBL), the breathing air supplied to the suits must be high-efficiency particulate air (HEPA) filtered.2,9 The WHO's LBM says, “Personnel who enter the suit area are required to don a one-piece, positively pressurized, HEPA filter-supplied air suit” (p. 26). The BMBL states, “Personnel who enter this area must wear a positive pressure suit supplied with HEPA filtered breathing air” (p. 55). The breathing air systems must also meet other requirements such as redundant compressors, emergency backup, and alarms in case of a failure.1,2,9 However, it is unclear at what location the HEPA filter should be on the breathing air system. In addition to the particulate filters already in place on our breathing air system, we have been using “HEPA certified” external Matheson filters, Model No. 6134-P8FF (Matheson, PA) on our BSL-4 suits (Figure 1). The Matheson filter has a stainless steel body in which a cylindrical filter element is housed; one end of the element is sealed with a glued-on endcap, which prevents the unfiltered air going straight through (Figure 2).
Figure 1.
A Matheson personal protective filter (circled) connects the breathing airdrop from the laboratory ceiling and the suit's breathing air inlet such that the air passes through the filter before entering the suit.
Figure 2.
Cross-sectional view of a Matheson filter. The stainless steel housing is cut open to show the cylindrical filter element along with the glued-on endcap.
Our filter testing involves an overall particle penetration test to assess the integrity of the filter, and an airflow resistance test to see whether the filter will allow the minimum airflow required by the positive pressure suits. We test each new filter before putting it to use and annually thereafter. Matheson filters have served our BSL-4 program reliably for many years. They have failed occasionally after prolonged use, primarily due to reduced airflow from filter loading. In 2015 however, we started noticing increased failures, especially among the newer filters, which failed on integrity testing rather than airflow. These filter integrity failures were usually associated with the endcap coming off the filter element; a filter that rattled when shaken indicated the endcap had already come loose, a sign of filter failure. For several years, our BSL-4 program had to adopt a protocol wherein each user had to shake the filter for rattling to ensure that it was good before use. Even though the filter supplier could not determine why the glue–endcap bond was failing, upon closer examination, we noticed the newer filters appeared to have used less glue. In the older filters, the dried-on glue alone was sufficient to seal the entire end of the filter even if the endcap had gone off the filter. Unlike these older filters, the newer filters' dried-on glue seal had holes when the endcap was removed, which then failed on integrity testing (Figure 3). Since personal filters were critical to our BSL-4 laboratory operations, we started looking for filters from other sources. Filters from Respirex International (United Kingdom), HVO, and Honeywell were used for this validation study.
Figure 3.
Cut open view of a newer Matheson personal protective filter. Endcap came off the glue, revealing breaches in the dried glue on that end of the filter element.
Materials and Methods
Filters
Filters from HVO, Honeywell, and Respirex were tested for comparison against the Matheson filters (Figure 4). The HVO and Honeywell filters were attached to the suits by the manufacturer, and were removed from their respective suits for this study. The Honeywell filter was attached to the breathing air inlet on the inside of the suit. Like the Matheson filter, the Honeywell filter was also a sealed unit, which could not be disassembled; it had a plastic housing encasing the filter element on the inside. The HVO filter, in contrast, was attached to the outside of the suit on the breathing air inlet; it also had a plastic housing, which could be disassembled to reveal a cylindrical cartridge attached to the housing using a screw-on mechanism. The Respirex filter could also be disassembled, where a cylindrical filter cartridge was encased inside a plastic housing and the endcap attached to the cylindrical cartridge using a screw-on mechanism as opposed to glue. The metal Matheson filter was sturdy and relatively heavier (475 g), whereas the others were lighter (HVO 280 g, Respirex 160 g, Honeywell 22 g) and smaller in comparison.
Figure 4.
Personal protective filters from four sources were validated in this study. Left to right: HVO, Respirex, Honeywell, and Matheson.
Filter Testing
We have been using two parameters for the filter efficacy determination: airflow resistance and filter integrity. Airflow resistance testing was performed to see whether a filter could meet the minimum airflow required by the suits, 140 L per minute (lpm). This was accomplished using an in-house assembled system (Figure 5) consisting of a pressure gauge and an airflow meter (Dwyer, IN). Shop air, pressure regulated at 27 pound per square inch, our breathing air system's delivery pressure, was passed through the airflow meter that was serially connected to a test filer. If the measured airflow was <140 lpm, the filter was considered unsuitable and removed from use.
Figure 5.
Airflow resistance test system, which consists of an airflow meter (1) and a pressure gauge (2). A filter (3) connected to the system and ready to be tested is shown.
An overall penetration test using poly alfa olefin aerosol (0.3 μm average size) was used to determine the integrity of a filter; a leakage of >0.005% was considered a failure. We modified our existing HEPA filter test bench by adding polypropylene fittings and valves to accomplish this (Figure 6). The overall penetration test is a comparison of the upstream aerosol challenge (100%) to the downstream aerosol challenge (leakage in percentage of upstream challenge) to determine the amount of total leakage.
Figure 6.
Filter integrity testing system. A HEPA filter test bench was modified to perform the overall penetration test, a filter being tested is also shown. HEPA, high-efficiency particulate air.
Results
We tested only one of each HVO and Honeywell filters removed from two old suits, both of them passed on airflow resistance testing (>140 lpm); however, they failed on overall aerosol penetration testing. The HVO filter failed with 0.47% leakage and Honeywell with 0.007%. We did not pursue validating more of these filters as they come attached to the BSL-4 suits and cannot be easily tested in situ. None of the new Respirex filters failed on airflow testing; such failure has never been an issue in the past with new filters. We had occasional airflow failures on filters after prolonged use, likely due to the disinfectant getting into the filters in the chemical decontamination shower while exiting BSL-4. All new Respirex filters (N = 20) also passed on the overall aerosol penetration testing (Table 1). This is in contrast to 66% of new (N = 15) Matheson filters failing on integrity testing in January 2018 (leakage range: 0.015%–5.26%).
Table 1.
Initial and long-term periodic efficacy testing data of 20 Respirex filters
| Date put in service | Test dates |
|||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| February 18 | May 18 | July 18 | October 18 | June 19 | July 19 | November 19 | January 20 | June 20 | August 20 | |
| February 18 | Pass | Pass | Pass | Pass | Pass | |||||
| February 18 | Pass | Pass | Pass | Pass | Pass | |||||
| February 18 | Pass | Pass | Pass | Pass | ||||||
| February 18 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Faila | Passb | |||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| July 19 | Pass | Pass | Pass | Pass | Pass | |||||
| July 19 | Pass | Pass | Pass | Pass | ||||||
| June 20 | Pass | Pass | Pass | |||||||
Failed on airflow resistance only; filter integrity test result was acceptable.
Same filter with the new replaceable filter cartridge.
Respirex filters also maintained their performance on periodic filter integrity testing after long-term use; however, one filter failed on airflow testing (130 lpm) at 29 months. Upon closer examination, we noticed that the filter cartridge was discolored, possibly by loading from moisture and/or disinfectant in the BSL-4 chemical decontamination shower (Figure 7). Our BSL-4 protocol requires the employees exiting containment to disconnect the filter from the breathing airdrop and decontaminate the exposed ends briefly in the chemical shower. We were able to purchase replaceable filter elements (#100-12-AQ; Parker Hannifin Corporation, NY) to swap out the failed cartridge; the newly assembled filter easily passed our initial testing (0.000% leakage and >140 lpm).
Figure 7.
Replacing failed cartridge in a Respirex filter. (1) Disassembled filter showing the failed cartridge beside its plastic housing after prolonged use, (2) new filter cartridge purchased to replace the failed filter, and (3) the filter with the new cartridge ready to be assembled for testing.
Discussion
Although some BSL-4 suit manufacturers incorporate protective filters onto their suits, they are not amenable to in situ testing for efficacy verification. We have been using external filters on our BSL-4 positive pressure suits for personal protection; external filters such as Matheson and Respirex are preferred as they can be easily tested before use and periodically thereafter. One end of the external filter is connected with the suit's breathing air inlet and the other to the breathing airdrop using quick connects (Parker 60 Series Quick couplings), such that breathing air has to pass through the filter before entering a suit. Staff members working in BSL-4 often have to disconnect and reconnect their suits from the breathing airdrop for moving about while performing their duties. This is done by disengaging/reengaging the quick connect at the filter–breathing airdrop junction. Such activity could pose a risk of introducing infectious agents into the breathing airstream if the person's hands are contaminated, especially while working in an animal cubicle, where animals are infected with RG4 agents. A properly functioning protective filter would protect the employees from such risks; thus, the consequences of not wearing a protective filter could be fatal. Therefore, it is critical to undertake rigorous and periodic in-house testing of personal protective filters to ensure that they provide and continue to provide the protection intended for BSL-4 personnel wearing positive pressure suits.
Even though the Matheson filters have served us well for many years, they started failing in recent years. In contrast, all of the newly procured Respirex filters passed both integrity and airflow resistance testing; they were also lighter, smaller, and less expensive in comparison (Matheson: USD 476, Respirex: USD 278). Moreover, when they do fail after prolonged use, the filter element cartridges for Respirex filters can be readily procured and easily replaced at low cost (USD 28/filter); reuse of such refurbished and efficacy tested filters is also good for the environment. Only one each of HVO and Honeywell filters was validated for this study, which is the major limitation of this study.
Our BSL-4 users now prefer the small and light Respirex filters in comparison with the heavy metal Matheson filters. However, their plastic housing could be damaged after prolonged and/or rough use, and such breaches could raise fresh safety concerns for the BSL-4 employees. Continued vigilance is required to determine how long the plastic filter housing will last before beginning to fail.
Conclusion
Rigorously tested personal protective filters offer protection for BSL-4 personnel wearing positive pressure suits. Our study shows filters from Respirex International are a suitable option to accomplish this.
Authors' Note
The authors do not have any connections to or vested interest in any of the companies or products cited/used in this study.
Author Disclosure Statement
No financial/personal interest/belief to disclose that could have influenced the objectivity of the work reported in this paper.
Funding Information
This work has been funded by the Government of Canada, through the Public Health Agency of Canada.
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