Below is one part of a series of white papers based on our Conversation Series: New Perspectives on Health and Well-Being where we discussed a range of mitigation strategies with our community of engineers, architects, specialists, and industry professionals. We break down all the latest and greatest (and not so great) methods of disease control through engineering, design, and building maintenance strategies by their effectiveness and considerations for sustainability.
Trapping particles with different filters
The first of two steps in using air filtration to mitigate COVID spread is to trap virus particles. Seen in figure 1, filtration media will reduce pass-through of particulate matter at different rates depending on the level of the filter’s efficiency. The current recommendation by ASHRAE is to utilize MERV 13, which many new buildings comply with to achieve green building ratings like LEED. Moving from MERV13 to MERV15 (see Figure 2) shows a notable increase in captured particulate matter, using a flu virus as an example in this case. This raises the following question: Can MERV13 filters easily be replaced with MERV15?
Answering this question requires a holistic consideration for the space within the system/equipment as well as concerns about the pressure drop that can be created by a higher MERV rating, as a higher pressure drop means that more fan power will be required to push the air through the filter. The extent of engineering and facilities management effort and cost is dependent on this holistic consideration.
In some cases, it is possible that MERV 15 filters will support a more efficient system, saving energy and keeping the system cleaner. However, there is also evidence to the contrary: proving MERV 13 and MERV 15 fan energy costs are equivalent in multiple applications.
Killing the trapped particles
The second of the two steps to mitigate COVID-19 spread is to inactivate the trapped virus particles. Photocatalytic oxidation uses UV-C light in panel form and causes a chemical reaction with a titanium dioxide coating on the filter that kills particles that come in contact with it. Effectiveness depends on intensity and time of exposure. An example of combining particle capture with particle inactivation might include a MERV13 or 15 filter combined with a UV-C panel slid in between the cooling coils of an air handling unit.
Another “kill it” method discussed somewhat frequently in light of COVID-19 is bipolar ionization. Positive and negative ions attach to bacteria and viruses and through a chemical reaction, turn into reactive hydroxyl radicals. These penetrate the virus membrane, destroying the structure of the virus, not allowing it to infect a host. It is noted in the industry that older versions of this technology would create potentially harmful amounts of ozone, and that the “needlepoint” version of the technology is meant to significantly reduce or eliminate ozone production following UL 2998 (Environmental Claim Validation Procedure (ECVP) for Zero Ozone Emissions from Air Cleaners), which requires a limit of 0.005 parts per million by volume of ozone.
Environmental impacts of mitigation efforts
Re-circulation of air through high-efficiency HVAC filters can help reduce energy costs associated with outdoor air ventilation. Fan energy costs for MERV 13 and MERV 15 have been proven similar in multiple applications, though this depends on the type of filter to avoid increasing pressure drop and therefore fan energy when moving from MERV 13 to MERV 15. A 2013 study on this topic showed variations in filtration costs differed with upfront cost and labor cost. In all applications, HEPA filters exhibit significantly high fan energy and filter costs, but labor costs remain consistent with MERV 15
When introducing UV-C light in close proximity with filters, the filter material needs to be carefully considered. Common filters are made of polypropylene and acrylic fibers. UV-C light can break down the fibers, sending new particles into the air stream. UV-resistant filters, such as those made of glass media, is a productive option to consider to reduce this risk.
The longevity of the filters is also a factor to be considered. The expected lifetime of a MERV 13 filter is around 4 months while MERV 15 and HEPA both last for around a year. The cost additions/savings of maintenance and replacement filters in comparison to a possible energy penalty or upfront cost of the MERV15 filter should be reviewed for a holistic decision-making process. Reduced maintenance costs are also possible with higher-grade filters by keeping heating and cooling coils cleaner for longer.
The current COVID-19 pandemic has helped to underscore the importance of air quality, and an investment in high-quality air filtration will support the long-term health and productivity of building occupants.
When reopening buildings within the context of the current global pandemic, there are a number of HVAC system variables to consider for improving air quality. Designing for a specific level of air filtration, in combination with other best practices, will support a healthier environment.
Air filtration efficiency depends on two main steps: “trapping” and “killing” of the particle or pathogen. Different filters show varying levels of efficiency and necessity.
MERV (Minimum Efficiency Reporting Value) filters are rated on a scale of 1-20, with 1 being the least filtration and 20 being the most filtration.
HEPA (High-Efficiency Particulate Air) filters are the most effective air filters and are often used in healthcare settings. HEPA filters are not necessary or recommended outside of healthcare facilities.
Electrostatic filters charge small particles to stick to each other, resulting in larger clusters that are more easily caught in filters. An example of an electrostatic filter is a non-ionizing, polarizing MERV15.
UV-C light panels can be used in filtration systems to inactivate viral particles.
Figure 1: Graph compares particle removal efficiency depending on filter type and particle size. (source: http://built-envi.com/publications/nafa_iit_wellsriley%20-%20FINAL.pdf)
Figure 2: Compares types of filters and the amount of flu particles trapped by each filter. (Source: Fresh Air presentation)
Figure 3: This chart identifies strategies, calls out sustainability factors and ranks the efficacy of COVID-19 / SARS-CoV2 mitigation and keys in a color and abbreviation linking to the larger, compiled strategy chart.
Figure 4: This image is a key, specifying the location of each solution on the compiled strategies chart.
ASHRAE Guidance for Building Operations During the COVID-19 Pandemic
Lists mitigation strategies including air filtration upgrades and supplements.
HVAC filtration and the Wells-Riley approach to assessing risks of infectious airborne diseases
National Air Filtration Associate report discusses more filter controls in regards to airborne disease mitigation.