Second Edition July 22, 2020
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Ventilation and Filtration

Buildings can be the front line of defense against COVID-19, working within a layered system of strategies to reduce transmission. Improving the ventilation of buildings can reduce airborne transmission of SARS-CoV-2, the virus that causes COVID-19. The concentration of virus in indoor air can be reduced by:

  • bringing more fresh outdoor air into the building (i.e. improving ventilation)
  • filtering re-circulated air to remove airborne viral particles in the ductwork
  • using portable air cleaners, or through more advanced air cleaning techniques such as ultraviolet germicidal irradiation (UVGI) to inactivate viral particles.

Improving ventilation and filtration in theater spaces does not necessarily require significant capital investment. Theaters should consult with their building managers and local HVAC experts to learn more about their individual ventilation systems and understand the capacity of their existing system for enhanced ventilation and filtration.

The following decision tree may be helpful in developing a ventilation and filtration strategy.

Key elements to discuss when considering ventilation and filtration are:

  • Increasing the amount of outdoor air moving through the space. Higher amounts of outdoor air brought into a space can dilute the indoor concentration of airborne virus, as well as help reduce the concentration of volatile organic compounds (VOCs) in the air from enhanced cleaning and disinfection practices.
  • Improving the filtration of recirculated air. Theaters should consider upgrading filters in HVAC systems to a minimum of MERV-13 filters, which capture a higher percentage of airborne particles than the standard MERV-8 filters. In systems that deploy a pre-filter/final filter system, a MERV-13 can be used as the final filter in combination with a MERV-8. Consult with your HVAC company before changing your filter.
  • Disabling demand control ventilation (DCV) systems and extending HVAC operating hours to ensure proper dilution of airborne virus and other pollutants. Demand control ventilation systems are designed to deliver only enough outdoor air for the number of occupants actually in a building, rather than the number of occupants the building was designed for. These systems measure carbon dioxide (CO2), of which humans are the main source indoors, as a proxy for occupancy and then adjust the amount of outdoor air ventilation based on current occupancy. This adjustment of ventilation can result in periods when minimal fresh air is being delivered to spaces, leading to less dilution or displacement of any airborne virus present and higher risk of airborne viral transmission. Disabling demand control ventilation systems would reduce this risk of airborne viral transmission. Likewise, energy-saving practices, such as restricting the hours of HVAC operations, may result in reductions in outdoor air supply that could pose a risk for occupants of the space during non-operational hours. Consequently, it is important to make sure that HVAC system operating hours extend to cover any hours when people may be in a building, as well as some buffer hours before and after the building is occupied each day.
  • Ensuring proper ventilation in all occupied areas of the building. Indoor spaces such as lobbies, scene shops, dressing rooms, classrooms, and all other areas of the building should be provided with sufficient ventilation relative to the maximum potential occupancy at any given point in time.
  • Verifying bathroom exhaust. Viral particles can be shed in stool. Therefore, ensuring that exhaust ventilation is functioning properly in bathrooms is an important control strategy. Theaters can check whether exhaust systems are operational and meet standards, and whether airflow is moving into the bathroom rather than from the bathroom into adjacent spaces (i.e. the bathroom is negatively pressurized relative to common areas).
  • Considering the use of supplemental control technologies, where warranted. Some building systems may not physically be able to accommodate higher ventilation rates and higher efficiency filters. In those instances, supplemental air-cleaning strategies may be warranted, including the use of upper room ultraviolet germicidal irradiation (UVGI) or other air-cleaning technologies. Particular care must be taken when using these technologies, and expert advice should be sought before implementing them. For smaller spaces, such as rehearsal rooms, shops, and administrative offices, the use of portable air purifiers with true HEPA filtration should also be considered. Additional details about portable air purifiers and UVGI are provided below.

Questions to discuss when considering changes to ventilation and filtration systems include:

  • How much outdoor air can my system bring in, across different seasons?
  • Can my system handle higher-efficiency filters?
  • Are the filters installed correctly, with no bypass?
  • How big is the facility?
  • Are there other tenants (or neighbors) to consider besides the theater?
  • Does the ventilation system serve one space or multiple spaces?
  • What are the local climate conditions?
  • How will occupant health and comfort be balanced with energy use and building health?
  • How much will changes cost, including initial outlay and ongoing operational expense?
  • How long will the changes take to implement?

A.R.T. is currently undertaking a comprehensive audit of ventilation and filtration systems in use across the theater’s spaces. It should be noted that the theater’s prop shop is isolated from the ventilation and filtration systems that serve the rest of the building and will require a separate update. As a means of avoiding the recirculation of construction-related particles through other spaces, this isolation of shops’ ventilation systems may be common in other theater facilities. In future editions, this Roadmap will share interventions and adjustments made by A.R.T. to facilitate re-occupancy of the theater’s facilities.

Supplemental Air-Cleaning Technologies

Portable Air Purifiers

Portable air purifiers with High Efficiency Particulate Air (HEPA) filters may help remove airborne virus from air in smaller rooms that cannot be ventilated sufficiently. When choosing a portable air purifier, it is important to evaluate whether the noise it generates is tolerable and whether the unit is correctly sized for the room where it will be deployed. To size a unit, it’s important to take into account both the clean air delivery rate of an air purifier, and the air exchange rate desired for a particular space.

  • The clean air delivery rate (CADR) value assigned to an air purifier by its manufacturer can be used to determine how many units are needed to treat the air in a room. A common rule of thumb is that for every 250 square feet of space in a room where a portable air purifier is to be placed, a CADR of approximately 100 cubic feet per minute (cfm) is recommended. A more specific way to use the CADR value (usually in cfm) is to multiply it by sixty to calculate an hourly delivery rate in cubic feet per hour. Then this calculated hourly delivery rate can be divided by the measured volume of the room where the portable unit will be placed (in feet3) to estimate the equivalent air exchange rate (AER) provided by the air purifier (in hours-1 or “per hour”).
  • In general, an air exchange rate (AER) indicates how much air in a room is replaced over a given period of time, with higher air exchange rates being associated with having more fresh outdoor air; for example, an air exchange rate of 0.5 hour-1 or 0.5 “per hour” means that half the air in the room is replaced each hour. ASHRAE 62.1 is a guidance document that lists recommended ventilation rates for different kinds of spaces; these ventilation rates can be converted to air exchange rates following the processed described above. If the ASHRAE recommendations cannot be met through ventilation, the calculated equivalent air exchange rate associated with an air purifier that is added to a room can be compared with the ASHRAE recommendation for the room to determine if the combination of ventilation and portable air purifiers can help the room reach the recommended amount of air exchange.

Facilities teams looking for further guidance on the above topics might consult HVAC engineers or civil/environmental engineers who work with HVAC systems.

Upper Room Ultraviolet Germicidal Irradiation (UVGI)

Upper room ultraviolet germicidal irradiation (UVGI) uses low-wavelength ultraviolet light (UVC light) to destroy microorganisms including viruses. This technology could be used to reduce airborne SARS-CoV-2 transmission. In buildings, UVC-generating lamps can be installed near the ceiling so that UVC light directed across the unoccupied upper part of the room can irradiate airborne virus without exposing building occupants to the UVC light, which can cause eye or skin irritation. This technology has been used in schools, prisons, and homeless shelters, but should not be used without consulting a UVGI expert, as the effectiveness of UVGI depends on how it is installed and on how well-mixed the room air is. Theaters should also consider the impact of UVC light on materials used in performance spaces and production elements.

Building Commissioning

Commissioning is the term used to describe the process of verifying system performance. Over the years, buildings and their systems suffer changes that may negatively impact their performance. Enlisting trained professionals to perform commissioning regularly can ensure that all building systems are operating as designed, that building HVAC systems are providing the right amount of air to all occupied spaces, that air filters are installed correctly, and that pressure differentials are kept as designed.

Building commissioning often results in buildings becoming healthier and more comfortable for building occupants. A 2011 publication from Lawrence Berkeley National Laboratory (LBNL) demonstrated that around 45% of existing buildings studied and just over 60% of newly constructed buildings studied reported improved indoor air quality after commissioning and around 80% of existing buildings studied and around 70% of newly constructed buildings studied reported improved thermal comfort after commissioning. In addition to health and comfort benefits, building commissioning can also result in energy savings. Another LBNL study in buildings across multiple sectors showed that commissioning led to median annual savings on energy costs of 6% with a median payback of 2.2 years. Similarly, they found that monitoring-based commissioning resulted in a median net annual savings of $0.14 per square foot after accounting for annual software and labor costs.

Measuring Ventilation Performance with CO2 Monitoring

In between commissioning events, which verify system performance, outdoor air ventilation can be tracked approximately using carbon dioxide (CO2) measurements to estimate ventilation rates. Occupants in a space exhale CO2, and the concentrations can be used to approximate the rate of outdoor air supply in a space. For most spaces, keeping CO2 levels below 1000 ppm suggests that the minimum ventilation level is being met. For more details, follow the ASTM Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation. Note that CO2 measurements can only be used as an indicator of outdoor air ventilation; they will not indicate whether supplemental air cleaning by UVGI or by portable air purifiers is effective because these interventions do not remove CO2 when they remove or inactivate airborne virus. Testing should be performed by trained individuals.

No-Contact Infrastructure

No-contact infrastructure can be adopted in as many places as possible to reduce the likelihood of fomite transmission. In bathrooms, for example, automatic touchless faucets, soap dispensers, towel dispensers, and toilet flushers could be installed. Throughout buildings, conventional doors could be replaced with automatic doors or doors with foot handles. In places where payments occur, such as concession stands, no-contact payment platforms could be used. Theaters may plan to transition away from paper tickets and move as many audience communication materials as possible (programs, flyers) to a digital format. The more shared objects or surfaces that are eliminated, the more risk is minimized.

Physical Barriers

Physical barriers are a useful tool to reduce droplet and airborne transmission. For example, clear plexiglass walls like those now used at many grocery store checkout stations could be installed at a box office window, reception desk, or concession stand to hinder the transfer of virus through the air between staff and patrons; they could also be deployed to separate workspaces within shared offices and/or shops. It is important to regularly clean and disinfect these barriers to ensure that they don’t contribute to fomite transmission if they are used to protect multiple people throughout the day.

Physical barriers are not a failsafe intervention on their own, and are best used in combination with social distancing, de-densifying spaces, and proper use of personal protective equipment such as masks.


Restrooms represent a particular challenge for all public facilities in the time of COVID-19. Many people need to use the same facilities and share the same doors, sinks, and toilets. The virus has been found to exist in stool; there is also evidence that the coronavirus may be transmitted through bioaerosols, making restroom air a potential source of additional risk. In theaters, these problems are potentially exacerbated when the entire audience often needs to use the restroom in the same window of time. While each venue will need to plan for its own specific architecture and facilities, here are a few strategies to consider in addition to the engineering controls (e.g., exhaust ventilation and no-touch faucets and paper towel dispensers):

  • Require masks. Masks may help prevent the wearer, who may not know they are infectious, from spreading the virus to others. Masks also provide some level of protection to the wearer from others who might be infectious. Universal mask wearing, including in bathrooms, is encouraged.

  • Communicate with audience members. Prior to arrival, audience members could receive notification that there will be efforts to limit overcrowding of restrooms, and that they should prepare accordingly.

  • Manage bathroom flows and overcrowding. Coordinating bathroom use and managing flows of people is another useful measure. Limiting the number of people who can use the bathroom at one time is important, but needs to be balanced against the potential for overcrowding in common areas. Strategies to consider include: increasing the length of intermissions, requiring people to remain seated or standing at their seat during intermission unless moving directly to or from the bathroom, installing plexiglass dividers between urinals, and major capital improvements to increase capacity of restrooms.

  • Consider adjustments to traditional intermission practices. If possible, theaters might consider beginning their programming with shows that do not require an intermission. For longer performances, consider multiple intermissions and message that the intermission will be prolonged to accommodate all patrons and reduce anxiety.

  • Add additional capacity. Additional facilities, such as restroom trailers or other moveable toilets placed outside the building, may be possible for some theaters as a means to reduce pressure on existing restroom facilities and help prevent long lines. They can also be used by patrons immediately prior to entering, and immediately after exiting, which may reduce pressure for some patrons to use the bathrooms during intermission.

  • Increase the frequency of cleaning. Theaters should consider cleaning their restrooms multiple times per day.

  • Provide wipes in the restroom. Consider providing sanitizing wipes for individuals to wipe down door knobs, toilet seats, and sinks prior to their own use, both for their comfort and as a way to increase frequency of cleaning.

  • Use portable air purifiers. Portable air purifiers may be more effective in restrooms than in other parts of the theater, because the volume of the space is relatively small and noise may not be a concern. Increasing the filtration of the air may help reduce the risk of transmission by bioaerosols.

  • Add hand washing and hand sanitizer stations. Theaters should consider making available many options for patrons to clean their hands without entering the restroom. Post signage about proper hand washing technique in restrooms and at all hand washing stations.

  • As stated above in Ventilation and Filtration:

  • Verify bathroom exhaust. Viral particles can be shed in stool. Therefore, ensuring that exhaust ventilation is functioning properly in bathrooms is an important control strategy. Theaters can check whether exhaust systems are operational and meet standards, and whether airflow is moving into the bathroom rather than from the bathroom into adjacent spaces (i.e. the bathroom is negatively pressurized relative to common areas).

Cleaning and Disinfecting Procedures

Cleaning and disinfection can help reduce the chance of fomite transmission of SARS-CoV-2. Cleaning, typically done with soap and water, removes microorganisms, while disinfection kills them. Coronaviruses, including SARS-CoV-2, are easy to disinfect on non-porous surfaces through the proper use of EPA-registered and approved disinfectants. Appropriate cleaning and disinfection techniques vary depending on the type of surface being treated; different techniques may be appropriate for soft and hard surfaces. Appropriate cleaning and disinfection techniques also depend on the frequency of an object’s use. If an object has not been used for a few days, it is unlikely that live virus is still present, and aggressive disinfection may not be necessary. When possible, for surfaces that have some downtime between uses, it is useful to wait as long as possible after a use to clean and disinfect the surfaces to reduce potential viral exposures to cleaning staff.

The Centers for Disease Control and Prevention (CDC) have published two sets of guidance that explain how to clean different types of surfaces, what products to use for disinfection, and cleaning frequencies to protect against COVID-19.


Other Healthy Building Strategies

When bringing a building back online after a long absence, facilities managers should re-commission their HVAC systems (see Building Commissioning) and maximize delivery of fresh air forty-eight hours before repopulating the building. Water systems should be purged by opening faucets and by flushing valves and other water lines and equipment to eliminate stagnant water and to ensure all plumbing traps remain wet. For more details, follow guidance in ASHRAE Standard 188-2018 “Legionellosis: Risk Management for Building Water Systems”. CDC also offers a free resource for legionella and mold.

Maintaining indoor relative humidity between 40% and 60% may help reduce the viability and transmission of SARS-CoV-2. Moreover, maintaining relative humidity in this range may provide benefits to the respiratory system’s natural defenses against viral infection. Manipulating indoor relative humidity can be achieved by placing portable humidifiers in small rooms. At a whole-building level, it may be possible to manipulate indoor humidity by air sealing the building (i.e. sealing off cracks where cold, dry air infiltrates) or by installing a whole-building humidification system that is built into the HVAC system. Care must be taken around this intervention to avoid creating conditions for condensation and mold growth.

Healthy Buildings

This roadmap is provided for informational and educational purposes only. It is not intended as a set of directions. Please see About the Use of This Resource for further explanation.