by Israel Biran, PhD., Executive Vice President & Co-Founder, enVerid Systems
While it has yet to be definitively demonstrated that aerosolized SARS-CoV-2, the virus behind COVID-19, can be transmitted through HVAC systems, ASHRAE has stated that the “Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled.” The latest commercial guide from ASHRAE’s Epidemic Task Force recommends several strategies to mitigate any airborne transmission of SARS-CoV-2 including the use of mechanical filtration, ultraviolet germicidal irradiation (UVGI), humidity control, and dilution through increased outdoor air (OA) ventilation. Some of these strategies lead to the same outcome but at widely varying costs.
HVAC engineers need to design cost-effective HVAC systems that reduce the potential airborne transmission of SARS-CoV-2 in new commercial buildings, so enVerid has reviewed the most recent research and has developed a cost/benefit model for assessing the relative efficacy and cost of various OA ventilation and filtration approaches in the age of COVID-19. The model confirms that increasing filtration is preferable to increasing ventilation based on relative risk of infection and cost.
High-efficiency filters were found to be as effective at preventing the spread of bioaerosols, minuscule airborne droplets that carry the virus, as increased ventilation, and could do so with a far less energy-intensive impact than ventilation. Upgrading current buildings with high-efficiency HVAC filtration to address virus transmission is a simple and immediate solution. For new build and major retrofits, enVerid’s models show that when advanced filtration is combined with ASHRAE’s Indoor Air Quality Procedure (IAQP) and air scrubbing technology, substantial savings can be realized. For example, in an analysis of a 100,0000 ft2 commercial building in New York City using this approach, a savings of $500,000 was projected for combined first cost and 20-year operating cost.
The full white paper discussing the model and its outputs is available here. Based on our research and calculations, we have learned the following:
- For commercial buildings, improved filtration significantly reduces the risk of airborne transmission of SARS-CoV-2.
The airborne transmission of viruses can occur through bioaerosols, which are particles smaller than 5 microns that contain viruses. When an infected person coughs or sneezes, small drops of mucus containing the viruses are formed. Most of these droplets are bigger than 5 microns and settle on surfaces as fomites, remaining viable for up to 2-3 days. However, smaller droplets (< 5 microns), also called bioaerosols, can stay airborne and potentially could travel through HVAC systems.
There are two primary approaches to removing bioaerosols from indoor air: filtration and ventilation. In the case of filtration, the higher the efficiency of the filter, the higher the percentage of bioaerosol removal from filtered air. This filtration efficiency is defined by MERV (Minimum Efficiency Reporting Value), which is an ASHRAE-created standard that evaluates filter efficiency. MERV 7 filters (along with MERV 8, commonly used in commercial buildings) achieve a 42.2% average efficiency in filtering bioaerosols, while MERV 13 filters achieve an 85.9% efficiency for particles the size of bioaerosols.
- Increasing OA ventilation adds material cost without significantly reducing the risk of airborne transmission. Furthermore, it makes it harder to control humidity, and increases the intake of particulate matter from polluted outside air.
Bioaerosols can also be removed from the air through increased ventilation, which is to say by diluting indoor air with outside air (OA). While some have advocated for this approach, we find it to be inefficient, costly, and deleterious to occupant comfort. In examining ventilation, we know that the rate of bioaerosol dilution is correlated with the percentage of OA relative to recirculated air.
For example, with 15% OA (typical under ASHRAE 62.1 Ventilation Rate Procedure (VRP)), about 15% of the bioaerosols will be removed from the indoor air. If OA is increased by 30% (VRP + 30%), then approximately 19.5% of the bioaerosols will be removed. This is a very small relative efficiency gain compared to filtration, and increasing OA comes at a much higher cost in terms of upsizing HVAC equipment and consuming more energy during building operation.
The industry is beginning to recognize the negative effects of increasing OA ventilation. For example, Taylor Engineering’s recent COVID-19 White Paper advises against it:
Given the very low estimated infection rate at the code minimum ventilation rate even assuming airborne infection occurs, and the 100% certainty that increasing minimum setpoints will significantly increase energy use and associated negative environmental impacts, we do not recommend doing so.
At the end of the day, building owners and HVAC engineers need to balance risks with costs. Often this trade-off requires spending more money to reduce risk. Fortunately, in the case of choosing ventilation vs. filtration to reduce the risk of airborne transmission of viruses, such a tradeoff is not necessary.
We calculated the cost-benefit of OA ventilation using VRP vs. IAQP + sorbent-based air cleaning with different levels of filtration efficiency. The modeling results for a 100,000 ft2 commercial building in New York City are displayed in the chart below, which shows that VRP ventilation (blue line) increases lifecycle costs significantly compared to IAQP + sorbent-based air cleaning (green line) without delivering a meaningful reduction in the relative risk of airborne transmission.
This is because implementing higher levels of filtration is much less costly than upsizing HVAC systems and increasing energy consumption to accommodate additional OA. The combined first cost and 20-year operating costs of using filtration and IAQP for OA ventilation are almost $500,000 less than the equivalent (in terms of bioaerosol removal) level of VRP OA ventilation.
Over and above the increased cost of using ventilation to reduce the spread of COVID-19, increasing OA ventilation carries additional shortcomings.
- Humidity control: Many studies have shown that relative humidity (RH) of 40-60% inhibits the spread of viruses. Reducing OA using ASHRAE 62.1 IAQP can improve humidity control in buildings by reducing the introduction of humid air in the summer and dry air in the winter. This preserves occupant comfort while reducing the risk of viral infection in the building.
- Pollution control: Increased OA ventilation means increased pollution in buildings in cities, near highways, and in other high-smog settings. A growing number of pre-print studies are examining a possible link between pollution and both the spread and impact of respiratory viruses on people who are infected. Based on these studies, building owners and HVAC engineers should consider pollution levels before increasing ventilation.
While scientists continue to investigate the possibility that SARS-CoV-2 can be transmitted through HVAC systems, building owners and HVAC engineers should design new mechanical systems with robust filtration (MERV 13 or higher for commercial buildings) to minimize the risk of airborne transmission of SARS-CoV-2. Increasing ventilation in addition to improved filtration does not materially reduce the risk of airborne transmission of viruses and adds significant cost. In fact, increasing ventilation may actually be counterproductive to reducing the risk of airborne transmission of viruses such as SARS-CoV-2 due to challenges controlling humidity and outside pollution with higher OA. The best strategy is to design for improved filtration and use ASHRAE’s Indoor Air Quality Procedure (IAQP) with sorbent-based air cleaning to reduce OA requirements and offset the cost of improved filtration.
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