Modern Building Services

22 MODERN BUILDING SERVICES JULY 2022 FEATURE INDOOR AIR QUALITY O utdoor air pollution in both cities and rural areas was estimated to have caused 4.2 million premature deaths worldwide per year based on 2016 data; with this mortality rate being attributed to exposure to fine particulate matter of 2.5 microns or less in diameter (PM2.5), which causes cardiovascular and respiratory disease and cancers. It is often quoted that indoor air can be many times more polluted than outdoor air, but it isn’t always that way. We cannot simply assume that bringing outdoor air into a building without air cleaning that same air will always improve the situation. History Particle filters selected for air handling units during the design phase of a building have typically only served the purpose of protecting the HVAC system fromaccumulating dust, to minimise energy use, and reducing failures of components. For the last two decades, the typical rule of thumb for filter specification for a standard air handling unit was a G4 panel prefilter followed by an F7 bag filter according to the previous filter standard EN 779. Increased sophistication of air handling units has meant they are more sensitive to the accumulation of dust with systems such as those incorporating desiccant thermal wheels needing higher grade of filters upstream of them to keep them operating efficiently. As there has been little legislation to mandate it, the selection of HVAC particulate filter efficiencies has rarely been driven by outside air quality conditions despite there having been guidance in place for some time such as BS EN 13779 superseded by BS EN 16798-3:2017. The same filter grades were historically selected for both locations in rural areas as well as for central city sites. New legislation The recent update to Part F Building Regulations - Approved Document F: Ventilation, that came into force in June calls for, amongst other pollutants, to have reduced levels of fine particulate matter drawn into buildings and references WHO figures as targets. Unfortunately for building occupants, the figures used are not the updated 2021 figures, but the much higher outdated 2010 figures. Filter selection The now obsolete although still often referenced EN 779 was replaced in 2016 by a new global standard ISO 16890. This standard allows engineers to select filters according to their efficiency for removing a known range of particle sizes those being PM10, PM2.5 and PM1 across all types of filters. As referenced above, if the ODA, required IDA or pollution source is known, the appropriate filters can be selected to address these specific particle sizes at known efficiencies. Most air handling units will have two stages of filtration; a coarse grade prefilter followed by a final secondary filter. ISO 16890 suggests that combinations of final filtration can be used to further increase efficiency. Conventional multi-pocket secondary stage bag filters with their deep pockets are designed to maximise the cross-sectional area at higher efficiencies when compared with shallower depth pleated panel pre-filters but it doesn’t mean that the surface area can’t be increased further without a detrimental increase in overall pressure drop across the filter. As designers and operators of building HVAC systems we have a duty of care to make sure indoor spaces are safeguarded against the dangers posed by what is often referred to as “fresh air” says AdamTaylor , CEO of ARM Environments. Reducing airborne particulates is for life, not just for covid