FILTRATION | HEPA An appropriately specified Hepa filter can be used in mechanically ventilated spaces and recirculating systems to practically remove small airborne particulates. CIBSE Journal technical editor Tim Dwyer looks at what must be considered when integrating them into HVAC systems UNDERSTANDING HEPA FILTERS A s the world returns to work, the loading of particulates in the outdoor air is rising to pre-pandemic levels and, in many areas, is reported as already above 2019 levels. The range of particle sizes is vast (see The filtering challenge panel). Larger dust particles may be readily removed from an airstream with low cost, low air pressure drop, panel and bag filters specified to standard EN ISO 16890:2016 Air filters for general ventilation. However, as particles become smaller, it is more challenging to capture them while maintaining a workable pressure drop and reasonable filter longevity. It is these smaller particles particularly those categorised as PM 2.5, PM 1.0 and ultrafine that have been identified as having significant detrimental impacts on health. An appropriately specified and installed high efficiency particulate air (Hepa) filter may be used in mechanically ventilated spaces and in recirculating systems to practically remove these smaller, airborne particulates. Hepa filters, such as the simplified example in Figure 2, are used routinely in laboratories and operating theatres to protect against infection from airborne virus and bacteria and are capable of filtering ambient particulates from vehicles and combustion processes (such as wood burning). In CIBSEs current Covid-19 guidance, it is noted that for poorly ventilated spaces with a high occupancy and where it is difficult to increase outdoor ventilation rates it may be appropriate to consider using air cleaning and disinfection devices known as (recirculating) room air cleaners. To be effective, air cleaners need to have at least Hepa filter efficiency (and/or potentially use germicidal UV (GUV) radiation) and to have a substantial part of room air pass through them.1, 2 Standards There are several national standards for Hepa filters that have evolved over the past 80 years, however, ISO 29463 High efficiency filters and filter media for removing particles from air derived from EN 1822 High efficiency air filters (EPA, Hepa and Ulpa) provides the appropriate benchmark for all global applications by defining 13 different filter classes ranging from ISO 15 E to ISO 75. (The ISO was designed to accommodate the practices of other national standards including those from the US and Japan.) In Europe, ISO 29463 co-exists with a revised EN 1822 standard that maintains its own equivalent classification system for air filters so retaining the designations of Hepa filter classes that are familiar to many H13 and H14 and having test methods that are in accordance with ISO 29463 Parts 2-5. Hepa is designated in ISO 29463 in the range ISO 35 H-ISO 45 H and sits between efficient particulate air (EPA) and ultra-low penetration air (Ulpa) filters. A key difference between ISO 29463 and various national standards is that, instead of testing mass relationships or total concentrations, the assessment of filter efficiency is based on particle counting at the most penetrating 44 September 2020 www.cibsejournal.com CIBSE September 2020 pp44-46 HEPA filters.indd 44 21/08/2020 15:05