Ventilation FAQ

 

 

 

Can COVID-19 be transmitted through HVAC ventilation system
The risk of spreading SARS-CoV-2, the virus that causes COVID-19, through ventilation systems is not clear at this time. Viral RNA has reportedly been found on return air grilles, in return air ducts, and on heating, ventilation, and air conditioning (HVAC) filters, but detecting viral RNA alone does not imply that the virus was capable of transmitting disease. One research group reported that the use of a new air-sampling method allowed them to find viable viral particles within a COVID-19 patient’s hospital room with good ventilation, filtration and ultraviolet (UV) disinfection (at distances as far as 16 feet from the patient). However, the concentration of viable virus detected was believed to be too low to cause disease transmission. There may be some implications for HVAC systems associated with these findings, but it is too early to conclude that with certainty. While airflows within a particular space may help spread disease among people in that space, there is no definitive evidence to date that viable virus has been transmitted through an HVAC system to result in disease transmission to people in other spaces served by the same system.

Healthcare facilities have ventilation requirements in place to help prevent and control infectious diseases that are associated with healthcare environments. For more information, see the CDC Guidelines for Environmental Infection Control in Health-Care Facilities.

Non-healthcare (e.g., businesses and schools) building owners and managers should, at a minimum, maintain building ventilation systems according to state and local building codes and applicable guidelines. Ensuring appropriate outdoor air and ventilation rates is a practical step to ensure good indoor air quality.

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Can ventilation filters effectively capture SARS- CoV-2 viral particles?
Yes, however, HEPA filters used in Portable HEPA Filtration are more efficient than filters for use in heating, ventilation, and air conditioning (HVAC) systems which are generally tested under procedures outlined in ANSI/ASHRAE Standard 52.2-2017-Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. This standard was developed by ASHRAE, a global society focused on building systems, indoor air quality, and sustainability in the built environment, Based on the filtration efficiency determined by the testing procedures, filters are assigned a Minimum Efficiency Reporting Value (MERV). The MERV provides a measure of the “filter efficiency” over the range of particle sizes prescribed in the test procedure. MERV values range from 1 to 16 and higher MERV values correspond to more efficient filters.

Research shows that the particle size of SARS-CoV-2 is around 0.1 micrometer (µm). However, the virus generally does not travel through the air by itself. These viral particles are human-generated, so the virus is trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are larger than an individual virus. Most of the respiratory droplets and particles exhaled during talking, singing, breathing, and coughing are less than 5 µm in size. CDC recommends using the highest efficiency ventilation filters possible, without having detrimental effects on overall HVAC system performance. ASHRAE has similar guidance; however, they recommend a minimum filtration efficiency target of MERV 13, provided there are not substantial negative impacts on the HVAC system performance and occupant comfort. A MERV 13 filter is at least 50% efficient at capturing particles in the 0.3 µm to 1.0 µm size range and 85% efficient at capturing particles in the 1 µm to 3 µm size range. Collectively these particles are capable of remaining airborne for hours and are most associated with deep lung penetration. A MERV 14 filter is at least 75% and 90% efficient, respectively, at capturing those same particles. Efficiencies for MERV 15 and MERV 16 filters are even higher. Thus, the recommended filters are significantly more efficient at capturing particles of concern than a typical MERV 8 filter, which is only around 20% efficient in the 1 µm to 3 µm size range and is not rated for capture efficiency of the smaller 0.3 µm to 1.0 µm particles.

Increasing filtration efficiency can increase the pressure drop across the filters. This can lead to increased fan energy, reduced airflow rates, and/or issues controlling indoor temperature and relative humidity levels. Scientific developments in filter design and manufacturing have reduced the amount of the increased pressure drop and its resulting impact on HVAC operations, but not all filters have adopted the newer technology. Prior to a filtration upgrade, the specific filters under consideration should be investigated for their pressure drop ratings at the flow rate(s) of intended use and the potential impacts of that pressure drop evaluated against the capabilities of the existing HVAC system.

High-efficiency particulate air (HEPA) filters are even more efficient at filtering human-generated infectious particles than MERV 16 filters. However, outside of a few unique applications, HEPA filters are rarely used in central HVAC systems. [See the question on Portable HEPA Filtration to learn more about them and their application in protective air cleaning].

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What is meant by directional airflow how and where should it be used?
Directional airflow is a protective ventilation concept where air movement flows in a clean-to-less-clean direction. This ventilation concept is applied to areas where the “clean” environment requires a higher level of protection and/or where the “less-clean” environment has a higher risk of containing airborne contaminants (activities or occupancy by individuals with a higher risk of being infectious). Examples of “clean” spaces might include healthcare facility triage stations or rooms/corridors adjacent to higher risk activities. Examples of “less-clean” spaces might include spaces that contain known/suspect infectious persons or spaces where a known activity has increased likelihood of generating infectious airborne particles.

The creation of directional airflow can be accomplished within a particular space or between two adjacent spaces. This can be done passively, through intentional placement of supply and exhaust heating, ventilation, and air conditioning (HVAC) grilles, or by the intentional creation of pressure differentials between adjacent spaces through specification of offset exhaust and supply air flow rates. Creation of the directional airflow can also be done actively, through the use of fans exhausting through open windows, strategic placement of ductwork attached to portable HEPA filtration units, or dedicated exhaust systems (installed or portable) that generate a desired airflow by exhausting air out of windows, doorways, or through temporary ducts. In specific settings, specialized local control ventilation interventions that establish the desired airflow directions can also be used.

Directional airflows must be evaluated carefully. Testing of the directional airflow effectiveness can be accomplished using visual tracer techniques that use “smoke tubes” or handheld “fog generators.” Other tools, such electronic monitors or visual aids to monitor pressure differences can be used when directional airflow is established between two adjacent spaces. To reduce the potential for directing airflow from infectious towards non-infectious space occupants, it is important that the “clean” and “less-clean” space determinations be established using infection control risk assessment considerations.

Research shows that the particle size of SARS-CoV-2 is around 0.1 micrometer (µm). However, the virus generally does not travel through the air by itself. These viral particles are human-generated, so the virus is trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are larger. Most of the respiratory droplets and particles exhaled during talking, singing, breathing, and coughing are less than 5 µm in size. By definition, a High Efficiency Particulate Air (HEPA) filter is at least 99.97% efficient at capturing particles 0.3 µm in size. This 0.3 µm particle approximates the most penetrating particle size (MPPS) through the filter. HEPA filters are even more efficient at capturing particles larger and smaller than the MPPS. Thus, HEPA filters are no less than 99.97% efficient at capturing human-generated viral particles associated with SARS-CoV-2.

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What is a HEPA filter and why use a portable HEPA air cleaner?
Portable HEPA filtration units that combine a HEPA filter with a powered fan system are a preferred option for auxiliary air cleaning, especially in higher risk settings such as health clinics, vaccination and medical testing locations, workout rooms, or public waiting areas. Other settings that could benefit from portable HEPA filtration can be identified using typical risk assessment parameters, such as community incidence rates, facemask compliance expectations, and room occupant density. While these systems do not bring in outdoor dilution air, they are effective at cleaning air within spaces to reduce the concentration of airborne particulates, including SARS-CoV-2 viral particles. Thus, they give effective air exchanges without the need for conditioning outdoor air.

In choosing a portable HEPA unit, select a system that is appropriately sized for the area in which it will be installed. This determination is made based on the air flow through the unit, which is typically reported in cubic feet per minute (cfm). Many portable HEPA filtration units are assigned a Clean Air Delivery Rate (CADR) which is noted on a label in the operators manual, on the shipping box, and/or on the filtration unit itself. The CADR is an established standard defined by the Association of Home Appliance Manufacturers (AHAM)

While smaller HEPA fan systems tend to be stand-alone units, many larger units allow flexible ductwork to be attached to the air inlet and/or outlet (note that larger ducted units don’t fall under the “room air cleaner” description and may not have a CADR rating). Using ductwork and placing the HEPA system strategically in the space can help provide desired clean-to-less-clean airflow patterns where needed. Ducted HEPA systems can also be used to establish direct source capture interventions for patient treatment and /or testing. Depending on the size of the HEPA fan/filter units and how the facility in which they are being used is configured, multiple small portable HEPA units deployed to high risk areas may be more useful than one large HEPA unit serving a combined space.

In testing portable HEPA filtration unit reduced the wait time before the room could be safely reoccupied, from 5 hours and 45 minutes to only 1 hour and 24 minutes, saving a total of 4 hours and 21 minutes. By adding the portable HEPA unit increased the effective ventilation rate and improved room air mixing. This resulted in over a 75% reduction in time for the room to be cleared of potentially-infectious airborne particles.

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Does ultraviolet germicide irradiation UVGI kill SARS- CoV-2?
Yes. Ultraviolet germicidal irradiation (UVGI), otherwise known as germicidal ultraviolet (GUV), is a disinfection tool used in many different settings, such as residential, commercial, educational, and healthcare settings. The technology uses ultraviolet (UV) energy to inactivate (kill) microorganisms, including viruses, when designed and installed correctly.

There is still a lot to learn about SARS-CoV-2 and the extent of airborne viral particles and spread. However, UVGI can inactivate viruses in the air and on surfaces.* The design and sizing of effective UVGI disinfection systems requires specific knowledge and experience.

Seek consultation with a reputable UVGI manufacturer or an experienced UVGI system designer prior to installing UVGI systems. These professionals can assist by doing necessary calculations, making fixture selections, properly installing the system, and testing for proper operation specific to the setting.

*Note: CDC’s recommendation for primary surface disinfection in occupied environments is to follow the CDC/EPA guidance for surface disinfection.

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What type of ultraviolet germicidal irradiation UVGI devices are available for cleaning and disinfecting in the workplace?

Upper-room UVGI
Upper-room (or upper-air) UVGI uses specially designed UVGI fixtures mounted on walls or ceilings to create a disinfection zone of ultraviolet (UV) energy that is focused up and away from people. These fixtures disinfect air as it circulates from mechanical ventilation, ceiling fans, or natural air movement. The advantage of upper-room UVGI is that it disinfects the air closer to and above people who are in the room. Since the 1980s, UVGI systems have been widely used for control of tuberculosis (TB). The CDC guidance Environmental Control for Tuberculosis: Basic Upper-Room Ultraviolet Germicidal Irradiation Guidelines for Healthcare Settings provides information on appropriate UVGI system design, related safe operation, and maintenance. Based on data from other human coronaviruses, a UVGI system designed to protect against the spread of TB should be effective at inactivating SARS-CoV-2 and therefore prevent spread. UVGI systems usually require a few UV fixtures to be effective. For example, a rectangular-shaped waiting room with 10–30 occupants will require 2–3 upper-air UVGI fixtures. As part of system installation, care must be taken to control the amount of UV energy directed or reflected into the lower occupied space below levels recognized as safe. Reputable UVGI manufacturers or experienced UVGI system designers will take the necessary measurements and make any required adjustments to prevent harmful UV exposures to people in the space.

Potential Application: Can be used in any indoor environment; most useful in spaces highly occupied with people who are or may be sick.

In-Duct UVGI
In-duct UVGI systems are installed within a heating, ventilation, and air-conditioning (HVAC) system. These systems are designed to serve one of two purposes:

1) Coil treatment UVGI keeps HVAC coils, drain pans, and wetted surfaces free of microbial growth. These devices produce relatively low levels of UV energy. This energy is continually delivered 24 hours a day, which is why they are effective. Coil treatment UVGI devices are not designed for disinfecting the air and should not be installed for the purpose of air disinfection.

Potential Application: Can be used to reduce HVAC maintenance and improve operational efficiency within large, commercial HVAC systems or residential HVAC systems; not recommended for inactivating airborne pathogens.

2) Air disinfection UVGI systems can be effective at applying intense UV energy to inactivate airborne pathogens as they flow within the HVAC duct. HVAC air disinfection UVGI systems generally require more powerful UV lamps or a greater number of lamps, or both, to provide the necessary UVGI required to inactivate pathogens in a short period of time. Air disinfection systems are often placed downstream of the HVAC coils. This location keeps the coil, drain pan, and wetted surfaces free of microbial growth and also disinfects the moving air.

Potential Application: Can be used inside any HVAC system to disinfect infectious airborne pathogens.

Far-UV (or Far-UVC)
Far-UV is one of many emerging technologies that have become popular during the COVID-19 pandemic. While standard UVGI fixtures emit UV energy at a wavelength around 254 nanometers (nm), far-UV devices use different lamps to emit UV energy at a wavelength around 222 nm. Aside from the wavelength, a major difference between the two technologies is that standard UVGI systems are specifically designed to avoid exposing people to the UV energy, while many far-UV devices are marketed as safe for exposing people and their direct environment to UV energy. A review of peer-reviewed literature indicates that far-UV wavelengths can effectively inactivate microorganisms, including human coronaviruses, when appropriate UV doses are applied. Questions remain about the mechanisms of killing microorganisms and overall safety. Far-UV might prove to be effective at disinfecting air and surfaces, without some of the safety precautions required for standard UVGI. Far-UV devices are best viewed as new and emerging technology.

Potential Application: Yet to be determined.

CDC does not provide recommendations for, or against, any manufacturer or product. There are numerous technologies being heavily marketed to provide air cleaning during the ongoing COVID-19 pandemic. Common among these are ionization, dry hydrogen peroxide, and chemical fogging disinfection. Some products on the market include combinations of these technologies. These products generate ions, reactive oxidative species (ROS, which are marketed using many names), or chemicals into the air as part of the air cleaning process. People in spaces treated by these products are also exposed to these ions, ROS, or chemicals.

While variations of these technologies have been around for decades, relative to other air cleaning or disinfection methods, they have a less-documented track record when it comes to cleaning/disinfecting large and fast volumes of moving air within heating, ventilation, and air conditioning (HVAC) systems or even inside individual rooms. This does not necessarily imply the technologies do not work as advertised. However, in the absence of an established body of peer-reviewed evidence showing proven efficacy and safety under as-used conditions, the technologies are still considered by many to be “emerging.”

As with all emerging technologies, consumers are encouraged to exercise caution and to do their homework. Registration alone, with national or local authorities, does not always imply product efficacy or safety. Consumers should research the technology, attempting to match any specific claims against the intended use of the product. Consumers should request testing data that quantitively demonstrates a clear protective benefit and occupant safety under conditions consistent with the intended use. When considering air cleaning technologies that potentially or intentionally expose building occupants, the safety data should be applicable to all occupants, including those with health conditions that could be aggravated by the air treatment. In transient spaces, where average exposures to the public may be temporary, it is important to also consider occupational exposures for workers that must spend prolonged periods in the space.

Preferably, the documented performance data under as-used conditions should be available from multiple sources, some of which should be independent, third-party sources. Unsubstantiated claims of performance or limited case studies with only one device in one room and no reference controls should be questioned. At a minimum, when considering the acquisition and use of products with technology that may generate ozone, verify that the equipment meets UL 867 standard certification (Standard for Electrostatic Air Cleaners) for production of acceptable levels of ozone, or preferably UL 2998 standard certification (Environmental Claim Validation Procedure (ECVP) for Zero Ozone Emissions from Air Cleaners) which is intended to validate that no ozone is produced.

Carbon dioxide (CO2) monitoring can provide information on ventilation in a given space, which can be used to enhance protection against COVID-19 transmission. Strategies incorporating CO2 monitors can range in cost and complexity. However, greater cost and complexity does not always mean greater protection.

Traditionally, CO2 monitoring systems are expensive, require extensive knowledge to accurately install and set up, and require sophisticated control programs to effectively interact with the building heating, ventilation and air-conditioning (HVAC) systems in real time. They were not designed to protect building occupants from disease transmission. Developers of whole-building CO2 monitoring equipment/software for HVAC system operations have been around for decades, with the technology most often applied in the pursuit of energy savings. As the current pandemic response has progressed, this technology has been marketed as a potential tool for providing an

indication of building ventilation efficacy, leading to questions about whether monitoring indoor CO2 concentrations can be used as a tool to help make ventilation decisions.

In some well-designed, well-characterized, well-maintained HVAC environments, the use of fixed CO2 monitors can be informative. When used, these monitors are often incorporated into demand-controlled ventilation (DCV) systems that are designed with a primary intent of maximizing energy efficiency through reductions in outdoor air delivery. However, guidance throughout the pandemic has been to exceed minimum ventilation whenever possible, in addition to masking, physical distancing, enhanced filtration, and other intervention-focused considerations. From the beginning of this pandemic’s response, both CDC and ASHRA icon have advised to deactivate DCV systems and operate the ventilation systems at maximum airflows, within the safety limitations of the equipment.

Fixed-position CO2 monitors measure CO2 concentration as an indicator of the number of people in the space. As the CO2 concentration increases, the HVAC DCV system increases the amount of outdoor air ventilation in the space to dilute CO2 (and vice versa). The number of CO2 sensors, the placement of those sensors, and their calibration and maintenance are collectively a large and complex issue that must not be overlooked. For example, the CO2 concentration measured by a fixed, wall-mounted monitor may not always represent the actual concentrations in the occupied space. If air currents from the room HVAC, or even make-up air from windows, flows directly over this monitor location, the corresponding concentration measurements will be artificially low. If the room has good air mixing, the measured concentration should approximate the true concentration, but rooms are rarely well mixed, particularly in older buildings with aging ventilation systems (or none at all). Also, if an elevated CO2 concentration results in an air flow increase to one room, that air may be “stolen” from other rooms on the same HVAC system. This may result in elevated CO2 concentrations in those other spaces which the HVAC system is unable to control.

Limited information exists regarding a direct link associating CO2 concentrations to a risk of COVID-19 transmission. Changes in CO2 concentrations can indicate a change in room occupancy and be used to adjust the amount of outdoor air delivered. However, CO2 concentrations cannot predict who has SARS-CoV-2 infection and might be spreading the virus, the amount of airborne viral particles produced by infected people, or whether the HVAC system is effective at diluting and removing viral concentrations near their point of generation. As a simple example, a small room with three occupants will have the same level of CO2 (and hence the same outdoor air ventilation rate controlled by the DCV system) whether no one has SARS-CoV-2 infection or whether one or more people are infected with the virus. Ventilation based on CO2 measurements cannot recognize the increased risk of transmission in the second scenario.

A more modest, cost-efficient, and accurate use of CO2 monitoring is the use of portable instruments combined with HVAC systems that do not have modulating setpoints based on CO2 concentrations. The CO2 meter can be purchased for under $300 and its measurements can be collected/logged near the breathing zones of occupied areas of each room. Under this approach, the HVAC outdoor air dampers could be set to introduce more outdoor air than code requires (as recommended by CDC and ASHRAE) and the resulting CO2 concentrations in rooms at the real-world occupancy levels documented using a calibrated, handheld portable CO2 meter. This documentation will be the CO2 concentration benchmarks for each room under the HVAC operating conditions and occupancy levels.

One potential target benchmark for good ventilation is CO2 readings below 800 parts per million (ppm). If the benchmark readings are above this level, reevaluate the ability to increase outdoor air delivery. If unable to get below 800 ppm, increased reliance on enhanced air filtration (including portable HEPA air cleaners) will be necessary. Once the benchmark concentrations are established, take periodic measurements and compare them to the benchmarks. As long as the ventilation airflow is unchanged (outdoor air or total air) and the occupancy capacity is not increased, future portable CO2 concentrations that are 110% of the benchmarks indicate a potential problem that should be investigated. Under the pandemic response, a pragmatic application of portable CO2 measurement tools is a cost-effective approach to monitoring building ventilation.

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Should temperature & humidity be used to help reduce the risk of COVID-19 transmission
For COVID-19, the first steps in reducing the indoor concentrations of the virus are wearing face masks, physical distancing, and reducing occupancy levels. Improved ventilation is an additional prevention strategy. For ventilation systems, increasing outdoor air above the code minimum requirements, increasing total ventilation, and increasing filtration efficiencies are more effective at controlling infectious disease transmission than controlling indoor temperature and humidity. However, the use of temperature and/or humidity to reduce the risk of disease transmission should be considered on a case-by-case basis, taking into account the building enclosure, heating, ventilation, and air-conditioning (HVAC) system capabilities, level of control and/or building automation, local COVID-19 transmission rates, any unique clinical features of the occupants, and local climate.

Both temperature and humidity can influence the transmission of infectious diseases, including COVID-19, but that influence has practical limitations. Research on the impact of temperature has shown that SARS-CoV-2, the virus that causes COVID-19, is sensitive to elevated temperatures, with over 99.99% inactivation in only a few minutes at 70°C (158°F). However, this temperature is far outside the limits of human comfort and could damage some building materials. While temperatures lower than 70°C (158°F) are also effective, the required exposure time for inactivation increases as the temperature decreases. So, elevated temperatures offer the potential for decontamination of SARS-CoV-2 virus in the air or on surfaces, but the use of increased temperature solely for decontamination is not generally recommended and is not realistic for occupied spaces. Another important consideration is that when the temperature in a space is elevated, the corresponding relative humidity level decreases.

Current evidence is not persuasive that humidity significantly reduces transmission of SARS-CoV-2 beyond the level resulting from good ventilation and filtration. Some research studies have shown that the survival of viruses, including human coronaviruses, may be reduced when the relative humidity is in the 40–60% range. However, the reductions are modest and there are outliers to these findings. Consequently, neither ASHRAE nor CDC recommends introducing humidification for the sole purpose of limiting transmission of COVID-19. While not affecting transmission, there are peer-reviewed studies that suggest preventing excessive dryness in the air could help maintain the effectiveness of the human body’s immune system.

Some HVAC systems can actively control both temperature and humidity. However, the majority of HVAC systems do not have dedicated humidification capabilities. Some dehumidification happens during warmer months as a byproduct of cooling humid warm air below its dew point and causing water to condense out of the air. Less common is the ability to limit low humidity by introducing water vapor into the dry supply air.

Most existing residential and commercial buildings located in cold climates are not constructed to resist the corrosion and excessive moisture accumulation that can result from long-term, whole-building humidification. If additional winter humidification is used to maintain comfort and prevent excessive dryness of nasal and ocular membranes, first analyze the building enclosure to verify that condensation and moisture accumulation will not become a problem. ASHRAE Standard 160 (Criteria for Moisture-Control Design Analysis in Buildings) provides guidance for hygrothermal analysis of building enclosures. For commercial buildings that are properly constructed to allow for long-term humidification, and which have humidification capabilities already installed, there is no reason not to humidify the air to comfortable levels during the winter months.

In residential settings, portable in-room humidifiers may be used for sensory comfort and to reduce excessively low relative humidity levels. In these instances, use a humidifier with a built-in humidistat and control the relative humidity level near 40%. Higher humidity levels are not necessarily better and may lead to localized mold growth, mildew, and other long-lasting indoor air quality issues. Maintenance and cleaning of portable humidification systems is very important. Change the water in the humidifier daily and maintain and clean the humidifier in accordance with manufacturer recommendations.

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Can fans be used to decrease the risk of COVID-19 transmission indoors?
Yes. While fans alone cannot make up for a lack of outdoor air, fans can be used to increase the effectiveness of open windows, as described in the CDC list of ventilation improvement considerations. Fans can also be used indoors to improve room air mixing. Improved room air mixing helps distribute supplied clean air and dilute viral particle concentrations throughout the room, which reduces the likelihood of stagnant air pockets where viral concentrations can accumulate. As with all fan use during the COVID-19 pandemic, take care to minimize the potential to create air patterns that flow directly across one person onto another:

  • Avoid the use of the high-speed settings

  • Use ceiling fans at low velocity and potentially in the reverse-flow direction (so that air is pulled up toward the ceiling)

  • Direct the fan discharge towards an unoccupied corner and wall spaces or up above the occupied zone.

Fans can also enable clean-to-less-clean directional airflow. Such applications should be evaluated closely to avoid unintended consequences and only adopted when supported by a safety risk assessment.

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Will using protective barriers interfere with improved ventilation?
Barriers can physically separate spaces that are next to each other. When used for infection control, the barrier is intended to prevent someone on one side of the barrier from exposing a person on the other side of the barrier to infectious fluids, droplets, and particles. Whether a barrier interferes with improved ventilation depends on how it is installed. Protective barriers can sometimes help improve ventilation, but they can sometimes hinder ventilation too. Sometimes they have no effect on ventilation.

Protective barriers can assist with improved ventilation when used to facilitate directional airflows or desired pressure differentials between clean and less-clean spaces. The barrier can be aligned with the intended airflow to help direct it towards a desired location, such as an HVAC return air grille or a portable air cleaner inlet. Example scenarios for this type of barrier deployment include those where there is a known source of potentially infectious aerosols, such as a dental operatory or COVID-19 testing station.

Alternatively, the barrier might be placed between two areas to better isolate one side of the barrier from the other. In this configuration, the barrier can also assist the HVAC design scheme in establishing a desired pressure differential between the adjacent spaces. If necessary, small pass-through openings or a retractable panel incorporated into the barrier can allow transfer of physical objects from one side to the other. Examples where this type of barrier application might be applied include a receptionist’s desk or a ticket booth.

When not carefully installed, barriers can sometimes hinder good ventilation. Barriers can unintentionally interrupt the airflow distribution within a space, thus allowing a concentration build-up of human-generated or other aerosols that may remain suspended in the air for minutes to hours. In this case, people could be exposed to higher concentrations of infectious aerosols than they would without the barriers in place. The larger the barrier, the greater the likelihood that this may occur. To reduce this likelihood, ensure that barriers are correctly positioned for the anticipated occupancy and that they are no larger than necessary to prevent direct transfer of respiratory droplets that could “spray” directly from one person onto another.

Any time barriers are deployed, airflow distribution testing with tracer “smoke” or handheld fog generators should be conducted. This testing can assist in evaluating airflow distribution within the occupied spaces. If stagnant air pockets are seen to occur, barrier redesign or reorientation can help to minimize the occurrence. Airflow distribution modifications such as adjusting the positioning of supply air louvers or the discharge of portable air cleaners can also assist in eliminating the development of stagnant air pockets.

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