This Q and A contains responses to questions that were raised at a webinar SEDAC delivered on August 18, 2020: Considerations, Best Practices, and Energy Implications for Reopening Critical Community Facilities in the Pandemic. Answers have been developed in consultation with webinar presenters Dr. Ty Newell, Dr. Helen Nguyen, Robert Nemeth and other SEDAC building engineers.

Have a question about building modifications during COVID-19? Contact SEDAC at or 800.214.7954.

Interested in receiving a COVID-19 Readiness Building Assessment? SEDAC's COVID-19 Readiness Assessments give you the independent, expert advice you need to identify medium to high risk spaces and make building modifications to reduce the risk of virus transmission.

Q: How much is virus transmission connected to indoor environments?
Indoor transmission is much higher than outdoor transmission, indicating a concentration of virus indoors compared to outdoors. In one study from China of 300 outbreaks (3+ people infected), all of the outbreaks occurred indoors (80% at home, 20% elsewhere). Studies reviewed by WHO found that transmission is much more likely in enclosed spaces with up to one hour of contact time. Outdoor transmission is rare, but possible with extended close contact.

Q: Can COVID-19 be transmitted through HVAC systems? Are there any documented cases of COVID-19 transmission through HVAC systems? 
A: While there are no documented cases of COVID-19 transmission through HVAC systems, we know that it can happen. Here's why.

  • The virus has a "half-life" in the air of ~60-80 minutes, more than enough time to transport from one region of a building to another through an HVAC system [1]. Viral laden droplets travel in "cloud" patterns much further than 6 feet and large droplets take several minutes to "fall." [2]
  • We know the virus can spread via intra-space airflows [3].
  • We know that viral matter can move efficiently through an indoor space and be transported through an HVAC system. For instance, in a study from University of Nebraska, more than twice as much viral matter was present on air exhaust duct grills than on any other surface, including cell phones and bathroom surfaces. Viral matter under beds in isolation rooms was as concentrated as that on cell phones and bathroom surfaces.

Q: Is 6 ft enough distance between people?
A: The 6 ft distancing rule was based on the assumption that the dominant transmission path for the virus is large droplets expired during sneezes and coughs, as is the case with other viruses such as MERS, SARS and influenza. However, recent studies have shown transmission can occur from finer particles that spread farther from the source. Virus laden droplets seem to travel farther than bacteria laden droplets. According to one study, bacteria laden droplets fell within 6 ft of the respiration/cough emission source, while virus laden droplets fell much farther away than 6 ft from the source.

Though COVID-19 can spread farther than 6 ft, the viral load in the air is highest near an infected person. Santarpia and colleagues found a 50% reduction in viral load at a 6 ft distance from an infected person.  Best practices: Maintain at least 6 ft distance, wear face masks, and use proper hygiene.

Q: How does contact time impact virus transmission?
Infection also relies on viral load which is impacted by contact time with droplets or aerosols containing the virus, ventilation/dilution rates, humidity, and even temperature. Momentary contact outdoors or indoors is unlikely to result in infection. Usually transmission is associated with a close-contact exposure with an infected individual of 5 minutes or more.

Q: What's the difference between airborne and droplet transmission?
A: Airborne and droplet transmission are the same thing (respiratory emitted droplets). Respiratory droplets range from microns (so-called "big" droplets of 10 microns that take 10 minutes to "fall" to the ground), to sub-micron (airborne). The good news is that viral-laden droplets across the spectrum can be filtered as well as killed with UVGI (ultraviolet germicidal radiation).

Q: What is the COVID-19 decay limit? 
A: The "Infection Parameter" is a ratio of persons infected by an infectious person over a 2-week period. It has a mathematical boundary value of 2.72 (the mathematical constant "e"). IP values greater than 2.72 is the infection accelerating region and values less than this is a region of decay. Dr. Ty Newell has posted two papers on MedRxiv that provide more background on IP, and its dependence on human behavior through social distancing and disease transmission efficiency (paper 1paper 2).

Q: Does the wide variety of the virus affect the accuracy of test results for current or past infections? 
A: The tests recommended by the CDC are tracking the conserve regions of the genome. For this reason, the infection will still be detected.

Q: What are your top recommendations to reduce virus spread through HVAC modifications?
Top 3 recommendations for businesses, public gathering places, and homes:

  • Increase air flow of outside air. The target is 40 cfm per person fresh air, resulting in 800 ppm of carbon dioxide concentration with occupants at sedentary metabolism. This is easiest to achieve through "smart" or demand control ventilation that adjusts to building occupancy level.
  • Improve filtration. At least MERV 11 filters, but preferably MERV 13 filters to manage particulates and to filter out 90% of infectious particles.
  • Add ultraviolet germicidal irradiation (UVGI) to recirculation air stream. 0.02 Wuv per cfm of building airflow for 85% single pass kill efficiency.  Conversion of electric energy to UV irradiation is approximately 25%, so UV sanitation of 200cfm of air requires ~4Wuv, or 16W electricity to sanitize.

Of course, these measures are no substitute for face masks, physical distancing, and hygiene, which are essential.

Q: Are there any special guidelines for hospitals and other medical facilities? 
A: Hospital-acquired infections are a growing concern with today's drug-resistant bacteria and immune suppression treatments that increase patients' susceptibility to bacteria, virus and fungal infections. Medical facilities should have systems reviewed to be sure components and system operations meet guidelines for ventilation (ASHRAE 62.1). Medical facilities should be operating with high ACH (air changes per hour) with enhanced filtering that meets or exceeds our guidelines. Airflow patterns should be examined in high-risk spaces.

Q: Any special recommendations for schools? 
A: It's hard to provide specific guidance because each school is so unique. Contact SEDAC for specific recommendations. See also the CDC's guidance on reopening k-12 schools.

Q: Do these measures apply to homes as well?
Yes. Increased air flow, improved filtration (above MERV 11) and UVGI will reduce the probability of someone in your household catching a cold, the flu or COVID-19, and are well worth the cost, even if they downgrade your home's energy certification rating.

Q: Do you recommend keeping these measures in place after the pandemic is over? What are the benefits in non-virus periods? 
A: We recommend that the increased ventilation, improved filtration, and UVGI be continued after the pandemic ends. The recommendations for increased ventilation and improved filtration will help dilute any future pathogens, along with the current COVID-19 pathogen, as well as improve employee productivity and reduce sick days (see below). UVGI in addition to killing viruses helps keep the coil and condensate pan clean of biofilms that tend to build up over time, reducing maintenance costs for cleaning the coil system [4].

Q: How can I make the case that measures to improve air quality and reduce virus spread are beneficial and cost effective, beyond the pandemic?
Pandemic concerns aside, improving air quality in buildings should be a much bigger priority for organizations that care about the well being of their building occupants. A study by the Harvard TH Chan School of Public Health found that doubling fresh air flow (decreasing building CO2 from ~1100 ppm to 800 ppm) resulted in employee productivity gain valued at $6500/employee per year, far exceeding the added energy cost of the measure. Potential operational cost increases for improved comfort, fresh air, and air filtration/sanitation generally amount to ~$0.01/employee hour of operational cost and a similar level of capital cost with significantly higher return (~$2.5/employee-hour) due to improved productivity (~10%) and reduced sick days. (~40%).  We recommend that facility managers and HR personnel work together to make the case for these improvements.

Q: Are there roles for private sector service providers to help with these efforts? 
A: Absolutely! Improving and maintaining a building's indoor air quality (IAQ) and energy systems at a higher quality level will require the assistance of a variety of service providers,  from mechanical engineers, those who install or sell "smart" online monitoring systems, to boots-on-the-ground HVAC contractors and maintenance workers.

Q: Spraying disinfectants between every customer makes me dizzy. Any recommendations on disinfectants? 
A: We use 70% ethanol wipes to avoid this problem.

Q: Is 100% outside air needed? What are the energy impacts of increasing air flow?
100% outdoor air generally is not necessary. The 800 ppm CO2 recommendation for indoor air quality provides a good balance between improving air quality and energy efficiency. This recommendation results in significantly improved air quality while minimizing utility bill impacts. And though there will be a relatively small increase in energy costs, these costs will be more than offset by improved employee productivity and fewer sick days.

Q: What percentage of outside air is recommended for crowded spaces?
 Crowded hallways, entrance spaces, and other confined spaces need sufficient air flow to keep carbon dioxide 800 ppm (40 cfm per person). During the pandemic, distancing must be practiced to keep direct-hit infections reduced. A 12' by 60' hallway should keep occupant density to no more than 24 people (~30 sqft per person), with a fresh air flow of 960 cfm. If the ventilation system operates at 2000 cfm, outside air dampers should be set to 50% fresh air. Ideally, CO2 sensors that can modulate fresh air should be used for active fresh air control.

Q: At what level of concentration does CO2 affect cognitive thinking or cause minor discomfort affecting performance?
A: According to a study from the Harvard TH Chan School of Public Health, cognitive performance in multiple areas decreases when CO2 levels exceed 1000 ppm. See this follow up study to explore value of cognition performance versus cost of ventilation. Our recommendation of 800 ppm CO2 is well within recommended guidelines for CO2 levels.

Q: Should I open windows for increased ventilation? 
A: When the temperature and humidity is appropriate, natural ventilation is more cost effective than mechanically supplied air, but it's very important to consider how the air moves through the space. Air entering through windows is uncontrolled and variable. Wind on the face of a building will blow into the building while an open window on the opposite side of the building may be drawing air out of the building.  This can be potentially problematic if an asymptomatic person is sitting next to a window with air blowing into the room. The air from the window may carry contaminants from that person into the room.  The opposite could happen on the other side of the building.  Open windows will affect airflow as the pressures outside an open window fluctuate. We recommend using supplemental fans when opening windows to ensure circulation.

Q: What should I do about increasing fresh air if my building doesn't have fresh air intake? 
A: Simply providing recirculated air will not dilute contaminants and in fact can help spread them. A high efficiency filter may capture some of the contaminants, but only so much. Overall, concentration of pathogens in the air will increase over time without fresh air dilution. Depending on the layout of your system, you may be able to hire a contractor to add fresh air ducts and damper controls to your system at a reasonable cost. The lowest cost option is to open windows (see above). You can keep a local exhaust fan on to draw out air from the space, and let air in through an open window.

Q: How much of a benefit is additional outside air vs. focusing on improving comfort through retro-commissioning?  
A: Both air quality and comfort  improvements should be considered together; don't assume improvement of one requires exclusion of the other. Clearly, occupant comfort is an important concern, as both ventilation and comfort impact productivity. In general, increasing fresh air ventilation and improving filtration will make a space more comfortable, in addition to promoting health.

Q: How effective are filters at blocking the virus?
A: Filters with a MERV rating of 11 or higher are very effective at blocking the virus. In a study by the Illinois Institute of Technology, researchers investigated airborne virus and bacteria movement in a residential HVAC system. MERV 8 filtration was no different than no filter in removing viral matter. MERV 11 and MERV 16 (similar to HEPA) were found to remove 85% and 95% of viral matter, respectively [5].

Q: How often should air ducts and vents be cleaned?
A: In most cases, duct cleaning is unnecessary if filters are changed regularly and the air handling units are cleaned and functioning properly. The key is prevention.  Increasing filters from today's MERV 8 to MERV 13 will improve duct cleanliness. Duct dust can harbor contaminants, but multiple studies have found that duct cleaning rarely improves indoor air quality and sometimes can make it worse [67]. Duct cleaning is appropriate if you have:

  • Persistent duct water damage
  • Slime or microbial growth observed in ducts
  • Debris build-up that restricts airflow
  • Dust discharging from supply diffusers
  • Offensive odors originating in ductwork or HVAC components.

Q: What are some things I should consider when restarting air handling units after a period of disuse? 
A: Contact HVAC professionals for basic system startup and maintenance (cleaning out cobwebs, checking filters, etc). Maintenance will be similar to restarting AC in the spring and heating in the winter. Increased ventilation air flow may loosen accumulated dust/dirt on duct walls of older systems. Cleaning ducts prior to increased fan ventilation can prevent the release of duct wall debris (and lower particulates in indoor air).

Q: What are some considerations when deciding whether to use a portable in-room air cleaner (with HEPA filters and UVGI) or mount one on the wall or ceiling? 
A: In-room air cleaners can be portable or fixed. According to a review of literature by the Ontario Health Technology Advisory Committee, fixed are preferable because they have a greater degree of reliability. The effectiveness of these cleaners depends on their correct placement. Some things to consider when deciding between wall mounted or ceiling mounted include airflow patterns, number of occupants, time of occupancy, and type of air cleaning technology used.  Consult with ventilation engineers, infection control experts, and industrial hygienists to install the air cleaner.

Q: Is there UVGI  that can kill the virus? What is the best way to apply UVGI in a recirculating airstream?
A: UVGI bulb fixtures are available for easy retrofit to duct systems and have been shown to be effective at killing the virus. Duct mounted UVGI in a large scale, blind study  conducted over 1 year with alternating periods of operation showed significant improvement in building occupant health (especially among people with respiratory sensitivities). Installing UVGI fixtures after the return filter and before heating/cooling coils will also provide coil drain pan sanitation that has been shown to provide relief for respiratory-sensitive building occupants (e.g., asthma, allergenic rhinitis, etc.). Installing UVGI just downstream of AC coils is also good, especially if the light can "see" upstream to the cooling coils and drain pan. See this white paper from UV Resources for a description of UVGI in ducts and duct placement instructions.

Q: Can you provide examples of facilities that have used UVGI airstream treatment? 
A: UVGI is very common in hospital HVAC units and laboratory AHUs that test microbes.

Q: Should we be considering UVGI surface sanitization in addition to disinfectants to clean surfaces? 
A: There are UVGI lamp fixtures that can be mounted on ceilings and walls to sanitize surfaces in a room. In one study exploring UVGI surface sanitation, microbial contamination on surfaces was reduced from 30% to 3%. Some UVGI lamp fixtures are even portable. These systems are often used in medical facilities, but could also be considered in places that frequently serve large groups of people.

Q: Should we be considering needle point bipolar ionization products (no ozone), such as GPS, PlasmaAir?
A: While ASHRAE has investigated UVGI (ultraviolet germicidal irradiation) for air sanitation, limited research is available on ionization effectiveness and ionization design principles, to our knowledge.

SEDAC recommends conducting a risk assessment to identify the areas of a building where COVID-19 transmission risk is highest. SEDAC reached out to  3Flow, a company that recently developed a tool to help facility managers complete a risk assessment.  3Flow's building risk assessment tool can be accessed here.  We are sharing it with their permission. This tool helps you analyze airflows, building layouts, and occupancy patterns to identify the areas of your building that are at highest risk for COVID-19 transmission.

Q: Can I use the risk analysis tool to analyze different building types? What if I don't know how to rank some of the inputs in the spreadsheet?
A: The risk analysis tool can be used for most building types. The inputs and all weighting factors for each input can be altered, but it is rarely necessary to do so. In most instances, it should be used as-is. Use your best estimate when entering data into the spreadsheet, and err on the side of caution. You can also contact SEDAC for assistance.

Q: Are there other methods to study airflow in a space? 
A: Tracer gasses are used to study how particulates migrate in a space, or space to space. Tracer gas studies require sophisticated equipment being installed in a space, locating sensors around the room, releasing a tracer gas, and recording concentrations at various locations.

Q: What is computational fluid design (CFD) analysis, what is is used for, and how expensive is it?
A: CFD provides the ability to model how air--and contaminants--move and mix throughout a room. Building spaces depend on air distribution from mechanical ventilation on air-conditioning systems to provide thermal comfort and good air quality. Without adequate air distribution, excessive air movement may occur in some zones, and stagnant air may be present in others. Poor air distribution can affect the indoor climate, degrade air quality, and contribute to virus transmission. One CFD expert estimated that evaluating and modeling existing conditions would be approximately $7,500. Simple alterations to that model to examine different conditions would be $2,500 to $3,000, and more complex alterations would be more.

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