6 Frequently Asked Questions about Airborne Transmission of COVID-19

Wells-Riley equation for evaluating the COVID-19 risk of infection in a 10,000 SF building

Bioaerosols, High Efficiency Filtration & Outside Air Ventilation

1. What are the current recommendations on airborne transmission of COVID-19?

There is a growing consensus in the scientific community that transmission of COVID-19 via airborne bioaerosols is a concerning likelihood. Approximately 250 experts wrote an open letter in early July to WHO encouraging the organization to broaden its guidance on airborne transmission of the COVID-19 virus. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (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.”

 2. What are bioaerosols? 

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. Microns are 1 millionth of a meter, and as a point of comparison, pollen particles are often 10 microns or bigger and bacteria are often about 1 micron.

Smaller versions of these droplets are known as bioaerosols. They are airborne particles smaller than 5 microns that can carry viruses, and are produced for example, when people talk. Bioaerosols are estimated at 0.3-5 microns. The COVID-19 virus is approximately 0.06 to 1.4 microns.  Viruses such as SARS-CoV-2 essentially hitch a ride within a bioaerosol, staying airborne and potentially traveling through HVAC systems.

3. I’ve heard people recommend more outside air ventilation. Seems like a natural way to deal with virus concerns.

Yes and no. While early recommendations focused on increasing outside air (OA) ventilation, like many issues with COVID-19, guidance and recommendations are evolving. There are concerns about the enormous expense that increased outside air ventilation imposes, and a new focus on the importance of high-efficiency filtration.

In settings that allow it, opening windows and allowing fresh air to circulate seems like a sensible, low tech response. However, in commercial buildings where windows typically do not open, ventilation occurs via the HVAC system. Conditioning outside air (bringing it up to the right temperature and adjusting humidity) is one of the most expensive components of building operation. Conditioning more OA will increase energy usage, driving up energy bills. In new buildings or retrofits, planning to condition more OA will require larger HVAC equipment, adding more first costs. Increased energy usage also impacts a building’s carbon footprint, increasing it just as many cities are introducing low carbon goals and, in some cases, imposing fees to prompt energy savings and a reduction in emissions. 

In many existing buildings it may not be feasible to significantly increase OA ventilation, particularly in the summer months. HVAC systems are carefully designed for specific parameters. While a system may be able to increase its normal intake of OA by 10-20 percent, due to limitations around cooling capacity because of the size of its chillers for example, it may not be able to add substantially more OA. Without a substantial increase in OA, a ventilation strategy then becomes insufficient to address COVID-19 mitigation.

Another concern about adding more ventilation is indoor air quality (IAQ). Many commercial buildings are in cities or are located near highways or industrial sites where pollution is a concern. More outside air means more health-damaging pollution particulates are brought into an office setting. Careful consideration needs to be given to maintain high levels of indoor air quality. 

4. Why is high efficiency filtration better?

 Multiple studies have confirmed the superiority of filtration over increased ventilation at reducing transmission risk of airborne viruses. A June 18, 2020 webinar presentation by the Chair of ASHRAEs Epidemic Task Force on Fundamentals of COVID-19 Risk Management stated that filtration can be a lower energy way to reduce aerosol/airborne infection risk” and filtration has benefits other than infection control.” ASHRAE recommends high efficiency filtration, with a minimum rating of MERV 13 for office buildings (see the next question for an explanation of MERV ratings). Upgrading current buildings with high-efficiency HVAC filtration is a simple and immediate solution to mitigating airborne transmission of the COVID-19 virus.

While high-rated filters do add an expense, they are not as expensive as the many costs associated with increased outside air ventilation. 

For those buildings that are in the design phase or are going through a major retrofit it is important to note that there are filtration strategies that can not only lower costs, but can actually offer savings in both first equipment costs and long term energy bill reduction. For new build and HVAC retrofits, enVerid Systems white paper models show that when advanced filtration is combined with ASHRAEs 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.

5. What is the difference between a MERV 13, 14 and HEPA filter?

MERV (Minimum Efficiency Reporting Value) is an ASHRAE-created standard that evaluates filter efficiency for three particle size ranges: 0.3-1 micron, 1-3 microns, and 3-10 microns.

Standards testing performed by ASHRAE show that MERV 7 filters (along with MERV 8, commonly used in commercial buildings) achieve a 42.2% average efficiency in filtering bioaerosols. MERV 13 filters achieve an 85.9% efficiency for particles ranging from 0.3-10 microns. MERV 17-20 filters are often referred to as HEPA filters. HEPA stands for High Efficiency Particulate Air filters.

 6. What about ionization products?

In a 6/8/20 Ask the Expert KCBS interview, Max Sherman, a member of the ASHRAE Epidemic Task Force and retired Senior Scientist at the Lawrence Berkeley National Laboratory had this to say:

“Bipolar ionization is one of those things that we really don’t know enough [about]. ASHRAE recommends making sure that the systems dont emit ozone, and some types of ionizing systems can emit ozone… The issue with ionizers where the ions leave the equipment itself and go into the space is one that hasnt been thoroughly investigated as to whether they will actually reduce COVID-19 exposure or not.”

Learn more: read our white paper “HVAC Design in Commercial Buildings to Mitigate COVID-19: Improve Filtration, Don’t Increase Ventilation”

Israel Biran

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