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Appropriate air filters should be used and maintained within HVAC systems, biological safety cabinets, horizontal laminar flow benches, and pathology workstations. Filters are selected based on the type of contaminants they are designed to trap. Filters within HVAC systems are usually designed to trap particulates. In spe­cial exhaust systems and fume hoods, filters may be installed to trap gases, vapors, and particulates. Filters within biological safety cabinets and horizontal laminar flow benches are designed to trap fumes, mists, and particulates. Filters in pathology workstations are usually designed to trap formaldehyde vapors.

Minimum filter efficiencies are specified in Table 6-4 of ASHRAE Standard 170-2013. Minimum efficiency reporting values (MERV) are based on the method of testing described in ANSI/ASHRAE Standard 52.2: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. MERV-A ratings are based on a test described in Appendix J of Standard 52.2. The MERV-A ratings more accurately predict filter efficiency over time so hospitals and design engineers should require MERV-A ratings when ordering or specifying filters.

Table 5-1  Minimum Filter Efficiencies

HEPA filters are designed to filter 99.97 percent of particles with a diameter of 0.3 microns. The size of 0.3 microns is the most evasive for a particulate filter. A HEPA filter is more efficient in trapping particles with larger or smaller diameters. When HEPA filters are installed within HVAC systems, they should be chal­lenged with a testing agent to ensure that they are meeting their designed efficiency. Dioctyl phthalate (DOP) is usually used as the challenging agent.  A DOP generator creates aerosols with a mass median diameter of 0.27 microns that are injected into the HVAC system’s airstream, upstream from the filter bank. A light-scattering photometer then determines the penetration of DOP aerosols that occurs through the filter and around its gasket.

HEPA filters are also placed within biological safety cabinets, hori­zontal laminar flow benches, and some exhaust ducts. One or two HEPA filters are placed within biological safety cabinets, depending on the type of cabinet. Biological safety cabinets should be tested and certified annually per the National Sanitation Foundation International Standard Number 49: Class II (Laminar Flow Biohazard Cabinetry (NSF 49). Certification of biological safety cabinets includes challenging HEPA filters with DOP aerosols. A biological safety cabinet may exhaust into its room or outdoors, depending on its design and the pathogens used within the unit.

Filtration performance may affect IAQ in several ways. Poorly maintained filters with inadequate seals and breaches in the filter media promote the passage of contaminants into occupied spaces. This seriously affects IAQ in critical areas where clean environ­ments are needed. Particulate filters with MERVs of 14 (90 to 95 percent dust spot efficiency) or HEPA filters may be needed for critical patient areas to protect patients from airborne pathogens that may cause infections.
Severely occluded particulate filters increase the resistance of air­flow through the filters, affecting air supply rates. When filters are not changed according to the manufacturer’s instructions, the per­formance of the HVAC system is compromised.

When HEPA filters are not properly maintained and tested regu­larly in biological safety cabinets, there is risk of contamination of the product within the unit or release of airborne pathogens into the occupied environments. Some types of biological safety cabi­nets are designed to supply HEPA filtered air over the work surface to protect the product on the work surface from contamination. HEPA filters are used within all biological safety cabinets to pre­vent airborne pathogens released at the work surface from enter­ing the occupied environment. When the HEPA filters within a biological safety cabinet are not performing as intended, products on the work surface may be contaminated, or pathogens used within the biological safety cabinet may be exhausted into the room.

For pathology workstations, permanganate filters are used for removal of formaldehyde vapors in air. This type of filter works dif­ferently than particulate filters. As the formaldehyde vapor–laden air passes through the filter, the vapors are absorbed onto the filter media. When the filters experience their full absorption capacity, formaldehyde vapors are not absorbed onto the media and pass through the filter. This results in formaldehyde vapors escaping into the occupied environment.
To ensure that appropriate filters are being used and maintained in accordance with the manufacturer’s instructions, maintenance and laboratory personnel should be trained on the features of filters. Personnel should know the capabilities of the filters and when they should be changed. Filter inspections and testing (HEPA filters) should be included in preventive maintenance plans.

Excerpt from: Mechanical Systems Handbook for Health Care Facilities
J. Robbin Barrick, PE, and Ronald G. Holdaway, PE
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