Celebrating 125 Years

Frequently Asked Questions (FAQ)

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    ASHRAE members and other subject matter experts researched and provided responses to these frequently asked questions (FAQ), which were submitted by media and other members of the public. Have a question for the FAQ? Email us at COVID-19@ashrae.org

  • Healthcare FAQ

    Q1: What kind of personal protective equipment (PPE) should building engineering staff wear when changing filters or accessing Air Handling Units (AHUs) or spaces where potentially contaminated air may be relieved?

    A: Recommend wearing the same PPE as a healthcare worker (N95 mask, vinyl gloves, disposable coverall and shoe covers). After maintenance is complete, double bag and dispose of PPE, change clothes, shower and wash hands thoroughly.

    See “Filtration and Air-Cleaning Systems to Protect Building Environments”, NIOSH 2003 for additional guidance.

    Q2: What kind of PPE should building engineering staff wear when accessing clinical areas for maintenance?

    A: Follow clinical guidance. Refer to American Society of Health Care Engineers (ASHE) and clinical staff for requirements. Measures may be taken to protect patients from potential infection by staff in non-COVID-19 wards. In wards with patients infected with COVID-19 and where the virus may be present (e.g. soiled utility corridors and rooms), staff should be protected from infection by using PPE.

    Q3: What temperature, RH to run space or building at?

    A: Normal operating temperature set points should be maintained based on the existing licensing requirements for the space use and occupancy. Consider maintaining relative humidity between 40%-60% RH. ASHRAE Research Project CO-RP-03 reports that scientific literature generally reflects the most unfavorable survival for micro-organisms when the RH is between 40%-60%.

    Q4: What are some general parameters for air changes per hour (ACH), temperature, filtration, and relative humidity for non-COVID-19 emergency surge spaces outside of a licensed hospital?

    A: Minimum 2ACH Outdoor air and 2 ACH Total air, though higher total air is desired (basis is patient room from FGI 1997).

    For large volume spaces with high ceilings, such as conference centers, air changes may be calculated based on a ceiling height of 10 feet; however, for supply air temperatures above room temperature, minimum OA and total ACH may be calculated as 2ACH Outdoor divided by 0.8 ACH, or 2.5. Total air, (ASHRAE 62.1 ventilation effectiveness) and supply air temperatures kept no more than 15 degrees F above room temperature to minimize stratification and short circuiting of air within the space.

    No less than MERV 13 and MERV 14 preferred for systems that are not serving specialized environments that may require even higher efficiency filtration.

    Temperature 70 – 75 degrees

    Humidity – Recommend 40-60% RH

    Refer also to Minnesota Department of Health, Methods for Temporary Negative Pressure Isolation

    Q5: Where might it be necessary to increase filtration level in healthcare setting?

    A: In areas with confirmed or potential COVID-19 patients, the healthcare facility may deem it desirable to increase the filtration rate for the HVAC systems serving those areas. The system filters should be rated for the maximum filter efficiency available while not adding more restriction in the HVAC systems to cause reductions in system airflow to the point that the system can no longer maintain indoor temperature and humidity set points or desired pressure relationships. HVAC Filters rated at MERV 14 or above are better and should be considered if the HVAC system can accommodate them. Isolation wards, temporary vestibules, and COVID-19 positive patient rooms specifically may benefit from HEPA filtration.

    Q6: How to or what to consider with vaporized hydrogen peroxide generators?

    A: Seal the HVAC system to the room being treated. Following decontamination, Vaporized Hydrogen Peroxide (VHP) system should be set up to clear all VHP from the space. Arrange for confirmation of complete space purge before reoccupying. VHP may be better utilized for equipment rather than room sterilization. There are additional considerations aside from those related to the HVAC system. Consult the system manufacturer for additional considerations.

    Q7: Guidance for mask disinfection or other manufacturing facilities

    A: Technology for the Decontamination and Re-use of N95 respirators has been established by the Food and Drug Administration. Specific details can be found at this link.

    A variety of approaches have been experimented with and while there is debate about the complete effectiveness, a shortage of PPE creates a need to re-use masks. This guidance from the Anesthesia Patient Safety Foundation provides information about the CDC’s official guidance on the short term and long-term reuse of N95 masks.

    Q8: What is the proper amount of outside air to spaces where infections exist or are known to have occurred?

    A: Ventilation rates should be increased and maintained 24/7 especially in buildings where known infections exist or are known to have occurred. The minimum ventilation rates during occupied periods should be at least meet the recommendations of ASHRAE 62.1. The maximum amount of outside air can be as high as 100% depending on outdoor air conditions (temperature, humidity, pollution content), indoor conditions trying to be maintained (temperature and humidity), capacity of the HVAC system to condition (filter, cool and dehumidify) the outside air or outside air/return air mixture to conditions that will maintain the desired indoor conditions. Applying a combination of increased filter efficiency and increased ventilation rates may be the best way to achieve better indoor air quality and help dilute COVID-19 or other infectious aerosol concentrations indoors. Facility operators and building owners should consult a local HVAC engineer or service professional to help them make these kinds of operational changes to their HVAC systems. As with anything, when making changes to existing systems, make small changes at a time and only one or two changes at a time and monitor the system and indoor conditions to confirm that no unexpected consequences or detrimental conditions, such as pressurizing a contaminated space relative to an adjacent uncontaminated one, appear so that the systems can be returned to pre-modification conditions if needed before any damage is done.

    Q9: What is the proper level of negative pressure for isolation and how can this be measured un-scientifically?

    A: Healthcare facilities should follow the recommendations in ASHRAE Standard 170-2017. Specific guidance is provided for the various spaces in a hospital including for Airborne Infection Isolation Rooms (AIIRs) can be found in Table 19.1 of Standard 170-2017. Standard 170 provides good guidance for any building that is housing, isolating, and caring for people infected with COVID-19. The principals specified in Standard 170 consider several criteria: whether the space should be kept at a positive or negative pressure with respect to the other space around it in the building, the number of air changes per hour for ventilation or outside air and the total number of air changes an hour the HVAC system capable of providing to the space. Spaces where COVID-19 infected people are being cared for or where they are self-quarantined should be kept at a negative pressure with respect to the other spaces around it in the building. Unscientific ways to determine whether the space is negative to its surrounding spaces is include, but not limited to:

    Use of a lightweight streamer tied to the end of a pencil such as a piece of string or ribbon. Crack the door that enters the room from the corridor or other space while standing in the room where the COVID-19 person is staying. Hold the streamer up to the crack in the door and if the streamer moves in the direction of the room or toward you, the room is negative with respect to the other space. If the streamer moves toward the crack in the door or toward the space outside where you are standing, the room is positive and airflow in the room should be adjusted so that is negative by increasing the return or exhaust air in the room and possibly blanking off a portion of the supply air to the room. Make small changes at a time and monitor the space to make sure temperature and humidity conditions do not become uncomfortable of other unexpected or undesired conditions do not occur. Consult a local HVAC engineer or HVAC service professional when needed.

    Q10: Air doors or air curtains?

    A: Temporary vestibules or ante rooms with HEPA filtration may be useful for containment at the entry to COVID-19 patient wards, at the entry to COVID-19 patient rooms, and for utilizing an operating room for a CVOID-19 patient. See specific information under the Healthcare Guidance section of the ASHRAE COVID-19 website.

  • HVAC System Operation During Building Shutdown FAQ

    Q1: Does ASHRAE have any recommendations for building owners for how to "mothball" their buildings for an undetermined amount of time and how to operate the HVAC systems during the shutdown?


  • How to Return the HVAC System to Normal Operation FAQ

    Q1: Does ASHRAE plan to provide guidance on how to re-occupy a building, especially what measures should be taken to return the HVAC system to normal operation?

    A: The intent of this question is for when the work-remote orders are retracted, and the threat of exposure is greatly reduced. Those are listed below for many systems in the building. If you are restarting a building still at a high-level threat of exposure, please review the Occupancy Guides at www.ashrae.org/Covid19.

    General recommendations:

    1. Prior to starting the building, operators may want to create a strategic plan that includes the following:
      1. Create measures to make occupants feel safer
      2. Ensure supply chain for critical items, such as filters, as confirmed for delivery
      3. Review contractual agreements with tenants with regards to building support
      4. Establish a communication protocol with tenants and include key contacts
      5. Prepare and provide training for tenants on safety measures

    It is important to note, that if you are opening when PPE requirements are still in place, the Occupancy Guides should be referenced as they deal with functioning buildings during the epidemic.

    1. Notify relevant people - include exact dates and times that the building will be reopened.
    2. Follow all local, state and federal executive orders, statutes, regulations, guidelines, restrictions and limitations on use, occupancy and separation until they have been officially relaxed or lifted.
    3. Follow CDC advice regarding PPE
    4. Follow OSHA Guidelines
    5. Ensure that custodial scope includes proper cleaning procedures built from EPA and CDC guidance on approved products and methods:
      1. Disinfect high-touch areas of HVAC and other building service systems (e.g. on/off switches, thermostats)
      2. Disinfect interior of refrigerated devices, e.g. refrigerators, where the virus can potentially survive for long periods of time.
    1. In buildings with operable windows, if the outside air temperature and humidity are moderate, open all windows for two hours minimum before the reoccupation.
    2. Review programming to provide flushing two hours before and post occupancies. This includes operating the exhaust fans as well as opening the outside air dampers.
    3. Run the system on minimum outside air when unoccupied.
    4. Garage exhaust, if any, should run two hours before occupancy.
    5. Install signage to encourage tenants to use a revolving door, if any, rather than opening swing doors in lobby area.
    6. Review all procedures to consider the addition of “touchless” interactions where applicable. As an example, auto-flush valves are considered “touchless”.
    7. Consider future renovations, to be included in the capital budget, to incorporate some of the strategies to mitigate transmission of viruses as indicated in the ASHRAE Position Document “Infectious Aerosols” as well as the Occupancy Guides.

    Heating, Ventilating and Air-Conditioning:

    1. ASHRAE recommends that all building owners and service professionals follow the requirements of ASHRAE Standard 180-2018 which has tables to show the typical maintenance on equipment that has been in operation.
    2. Consider PPE when maintaining ventilation materials, including filters and condensate. Consult additional guidance before duct cleaning.
    3. Check if all the setbacks and setup modes are reversed back to normal.
    4. Open outside air intake dampers to their maximum, 100% preferred, four hours minimum, before the reoccupation. The maximum position the outside air dampers may be opened will depend on the time of year, local climate, the temperature and humidity of the outside air, and the capability of the HVAC equipment to condition the outside air so that the system is able to maintain acceptable indoor temperature and humidity. When operating in this “flush out” mode, monitor the system continuously to make sure that unexpected or unacceptable conditions inside do not develop. Upon completion of the flush, the damper positions should be corrected to provide design levels.
    5. Check to see that space temperature and relative humidity levels are being controlled to the acceptable setpoints.
    6. Check the status of any heat recovery wheels in the systems for leakage and cross-contamination. Consider deactivating these wheels until a service technician checks the operation and condition.

    Airside systems:

    1. Check to see that the fans have turned on, and that air is moving in and out of the building.
    2. Check to make sure the dampers (outside and return) are working properly as this helps control the fresh air to the building. If the building increased its outside air (OA) during the epidemic, rebalancing the dampers may be required to achieve design air flows.
    3. Check overall building pressure to make sure it is positive. Do the same for any critical interior spaces.
    4. Check that the filters are still in acceptable condition. Facility staff should wear PPE, assuming the system may have been contaminated prior to shut down or upon restarting.
    5. Operator should consider increasing the level of filtration in the Air Handling Units (AHUs) for one or two replacement cycles upon opening the building. Make sure the air handling systems and fans can overcome the additional pressure drop of the new filters and still maintain air flow at acceptable levels. Refer to the Filtration Guidance www.ashrae.org/covid19.

    If higher filtration is not available, portable units in the high-traffic areas may be used for a few months.

    Cooling systems:

    1. Check the refrigerant pressures to make sure the system is adequately charged.
    2. Check the water quality in the systems and add chemicals as needed.
    3. Check coil leaving air temperatures to make sure the systems are providing dehumidification.
    4. Check the water levels and make-up water source for cooling towers to ensure they are available.
    5. Check pump operation and that water is flowing.

    Heating System:

    1. Check the fuel source to make sure it is on and available. Old fuel oil may need to be replaced.
    2. Confirm that the flues and make-up air paths are open prior to engaging boilers.
    3. Check that the coil actuators are controlling to temperature, or that heating elements are turned on at the disconnect.
    4. If the boiler system(s) were shut down, follow state boiler codes and the manufacturer's written instructions for starting up, and bring hot water and steam heating systems and plants back online.

    Building Automation System:

    1. Check that the devices and sensors are within an acceptable calibration for controlling space comfort and ventilation.
    2. Check that the alarms are set up and their communication path is correct (it is notifying the right person).
    3. Consider an update to the programming that would incorporate HVAC strategies to reduce virus transmission prior to future events. Automate the control sequences applied as “Epidemic Mode” operation that can be manually selected by the operator with one stroke.
      1. Refer to Occupancy Guides for suggested HVAC strategies to employ when operating the building in an epidemic.

    Plumbing Systems:

    1. Many facilities have a water risk management plan such as an ASHRAE Standard 188-2018, Legionellosis: Risk Management for Building Water Systems, to provide guidance and protocols to minimize the risk of waterborne pathogens, such as legionella pneumophila in their utility water systems.
    2. Turn on the water and run the drinking fountains, lavatories, urinals, water closets, and pantries to ensure water quality before usage.
    3. Make sure all P and U-traps on plumbing drains are wet.
    4. Distributed domestic hot water systems - if possible, keep these systems circulating. Keep water above 140°F to avoid microbial incursion. Do not let it drop below 120°F. If circulation was stopped, try to circulate once every two weeks for two hours at temperature. If the hot water recirculating system goes down for extended duration, do a high temperature flush and pull the strainers before going back online.
    5. Maintenance should wear epidemic-level PPE when maintaining any of the sewage ejectors and lift stations until those systems are sterilized.

    Electrical Systems:

    1. 1. Plug in all appliances that were unplugged to avoid phantom electrical loads, including but not limited to:
      1. Computers
      2. Routers
      3. Modems
      4. Televisions
      5. Printers
      6. Chargers
      7. Microwaves
      8. Things that turn on with a remote control

    Special Systems:

    1. Check on fire alarms and other equipment with battery backup power supplies. Consider having an electrical technician come and check that everything is working properly.
    2. Have fire protection sprinkler systems, fire alarm systems, emergency lighting systems and other life-safety systems inspected by local authorities having jurisdiction (AHJs), if required by state and local statutes and ordinances, and by contract service professionals who routinely maintain these systems.
    3. Check on the battery backup power supplies for Information Technology (IT) and Internet of Things (IOT) devices, especially the ones that are mission critical. That would include servers, building automation systems (BAS), communication systems, lighting control systems and security systems.
    4. If the building is equipped with an emergency or backup generator, arrange to have it tested as required by codes, local jurisdictions and the manufacturer’s recommendations.
  • Residences FAQ

    Q1: What can I do in my home to reduce the risk of COVID-19 spread?

    A: ASHRAE recommends following guidance from the CDC which includes minimizing contact, having a household plan which includes basic information on how to protect yourself and how to keep your home sanitary.

    Additionally, follow the guidance in the Task Force’s Technical Guidance for Residences presentation which is intended to supplement CDC recommendations with options related to controlling virus transmission through the air using the home’s heating, ventilating and air-conditioning (HVAC) equipment and related factors.

    This Technical Guidance for Residences includes guidance for all homes, homes with forced-air systems, multi-family homes and guidance for homes with COVID-19 positive or high-risk individuals.

    Q2: Should I operate my heating/cooling system in my home?

    A: Yes, guidance on how to run your residential Heating Ventilating and Air-conditioning (HVAC) system can be found in the Task Force’s Technical Guidance for Residences. This document includes guidance for all homes, homes with forced-air systems, multi-family homes and guidance for homes with COVID-19 positive or high-risk individuals.

    Q3: Can my home heating/cooling system spread COVID-19?

    A: In general, a well-designed, maintained and operated heating, ventilating and air-conditioning (HVAC) system that meets applicable standards for ventilation and filtration will reduce the risk of transmission in your home. If, however, there is an infected individual, care should be taken to isolate them from the rest of the household which may include isolating parts of the HVAC system. See guidance for creating an isolation space.

    Note also that it is ASHRAE’s position that:
    Ventilation and filtration provided by heating, ventilating and air-conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air. Unconditioned spaces can cause thermal stress to people that may be directly life threatening and that may also lower resistance to infection. In general, disabling of heating, ventilating and air-conditioning systems is not a recommended measure to reduce the transmission of the virus.

    Guidance on how to run your residential heating, ventilating and air conditioning system can be found in the Task Force’s Technical Guidance for Residences

    Q4: What can I do if someone in my home has COVID-19?

    A: When a household member is known to be infected, additional precautions are required in order to reduce the risk to other household members. The CDC has provided general guidance for this situation. A key aspect is to use a separate room essentially as an isolation space. The following additional actions should be considered when creating such an isolation space:

    • Select isolation space
    • Separate HVAC systems
    • Install air barriers
    • Operate exhaust ventilation

    Please see the Technical Guidance for Residences for more details.

    Q5: What can I do if someone in my home is a high-risk individual?

    A: The CDC recognizes various categories of high-risk individuals and has general guidance for high-risk individuals. If there is not a known infected individual but there is a high-risk household member, additional protection can be afforded by creating a protected space for the high-risk individual. The following actions should be considered:

    • Select protected space
    • Separate HVAC systems
    • Install air barriers
    • Operate supply ventilation
    • Operate stand-alone air cleaner

    Please see the Technical Guidance for Residences for more details.

    Q6: I want to upgrade the filter in my central heating and cooling system to a higher effectiveness one. Is this safe and do I need to adjust my fan setting?

    A: Generally, this is safe and some minor adjustments are in order.

    The typical 1-inch (thick) filter in most systems can be replaced with a filter up to MERV 13. Above this MERV rating the additional airflow resistance of high-performance filters may impact the performance of the HVAC system. If you have a 2-inch or greater filter then you can upgrade it safely with filters up to MERV 16. A 4-inch or greater filter will need to be changed far less frequently.  If installing a new filter grille or holder, consider one that can take a 4-inch filter.

    You will need to adjust the fan setting to make the fan run even if there is no heating or cooling. Many thermostats have a “fan only” setting that can be used or you may have a smart thermostat or fan controller that can be used to provide a minimum operating time of at least 15 minutes per hour.  In hot humid climates, be aware that continuous fan operation may reduce the ability of your system to control humidity. Please refer to the user’s manual of your system or consult an HVAC contractor for more information, and consider asking your contractor to upgrade your filters. Care must be taken when replacing filters because of exposure from surface contaminants.

    Q7: I am considering using a germicidal ultraviolet system in my home, but I want to know if these systems produce ozone. Is there equipment I can buy that does not produce ozone?

    A: Yes, but it is important to avoid ozone production because it is a contaminant of concern. Look for equipment meeting UL Standard 2998 Environmental Claim Validation Procedure (ECVP) for Zero Ozone Emissions from Air Cleaners.

    This kind of germicidal equipment uses mercury vapor lamps that produce ultraviolet light at 254nm, which is effective at inactivating many pathogens including SARS-CoV-2. Light at this wavelength does not produce ozone, but the mercury vapor lamps also produce ultraviolet light at shorter wavelengths that do produce ozone. Manufacturers have overcome this problem by using titanium-doped quartz in the lamps, which block the ozone-producing wavelengths. These products require careful design, installation and maintenance to be most effective.

    Q8: Will letting daylight into my home reduce COVID-19 risks?

    A: Perhaps. Direct sunlight has disinfectant properties in addition to having other benefits for people. Research has not yet determined exactly how strong that effect is for the SARS-CoV-2 virus. Window glass absorbs a lot of the ultraviolet frequencies where the disinfectant benefit comes from so, it is best to let sunlight in through open windows. However, this practice should only be used in addition to and not as a substitute for following all the recommendations from the CDC and the ventilation and air cleaning guidance found here. One must also take care to balance open windows with other concerns including thermal stress and pressure balancing.

    Q9: Is it safe to continue use my ERV or HRV to provide ventilation in my single-family home?

    A: Generally, yes. While there can be a potential to recirculate a small amount of contaminated air if someone is infected, the ERV/HRV will still be an overall benefit in single-family homes.

    Q10: Should I upgrade the outdoor air filter of my ventilation system to prevent the virus from entering my house?

    A: In general, it is not necessary to upgrade the filter treating the supply of outdoor air. According to current knowledge, there is a very low probability that the virus will enter from outdoors into the house through the mechanical ventilation system. In addition, a higher effectiveness filter may be more prone to clogging and will reduce air flow. One factor that might change this recommendation is if the outdoor air intake is near another building’s exhaust vent or near a location where people congregate.

    Q11: My spouse and I are teleworking from home full time. Are the ventilation rates in ASHRAE Standard 62.2 enough for our dwelling?

    A: Generally, yes. If your work involves typical use of a computer and normal accessories and other routine office work, the ventilation requirements in ASHRAE Standard 62.2 should be enough. However, if your office work is more intense (e.g., heavy duty use of laser printers, use of a 3d printer, soldering or other activities not typically performed at home), you may wish to look to ASHRAE Standard 62.1 for guidance on adequate ventilation.

  • Transportation FAQ

    Q1: What can I do as a rider of mass transit to help reduce the risk of COVID-19 spread?
    A: The easiest way to avoid becoming infected while traveling is to not travel in the first place. If travel is not essential, it is advisable to reconsider your travel plans. However, many workers in essential services still require the use of public transportation systems, and riders of mass transit should factor this into their plans. Please see Technical Guidance for Transportation – Guidance for Mass Transit Riders for more information.
    Q2: What guidance is available for air passengers to help reduce the risk of COVID-19 spread?
    A: Air passengers can help reduce the spread of COVID-19 by wearing facemasks, practicing social distancing, and travelling only if the travel is necessary. Please see Technical Guidance for Transportation – Guidance for Air Passengers, for detailed suggestions.    

    Q3: Is there guidance available for Transportation Facility Operation?  Is guidance for similar commercial buildings appropriate?
    A: Many transportation facilities are very much like other buildings that superficially share usage types. But there are very important differences between transportation facilities and other similar commercial buildings (office buildings, malls, etc.):

    1. Occupants of the facility are often on their way to or have come from other locations (other neighborhoods, cities, states, and countries).
    2. The facility may be subject to a great deal more regulation.
    3. The facility may have tenants (operators of the transport services using the facility) that are subject to even more regulation by a different set of agencies or bodies.

    ASHRAE recommends following the guidance in the Task Force’s Technical Guidance for Transportation – Guidance for Transportation Facility Operation.

    Q4: What is the best/safest place to sit on an airplane?
    A: Experimental data show there are no particularly good or bad seat locations in typical airliners with respect to risk for airborne disease transmission. The primary factor in this risk is how well ventilation air is supplied to a given location as it is this ventilation air that flushes airborne pathogens out of the cabin. The more quickly and effectively this supply air flushes airborne pathogens from the air, the lower the risk, all other factors being equal. 

    Airliners are designed to supply ventilation air uniformly along the length of the cabin. However, that feature alone does not ensure that every seat gets the same ventilation. Experiments were conducted in full-sized mockups of both wide-body and narrow body aircraft using actual aircraft air delivery systems and design flow rates. 1,2 Effective local ventilation rates were measured at each seat, 30 seats in the narrow-body and 77 seats in the wide-body. Effective local ventilation is a measure of the amount of air supplied to the cabin adjusted by the effectiveness with which it reaches a given location.  For the narrow body aircraft, the effective local ventilation rates varied from 29 to 32 air changes per hour (Note: Since HEPA filtration of recirculated air is standard practice in nearly all airliners, effective ventilation rate is based on total airflow, not outside airflow, for airborne pathogen applications.).  For the wide body, it varied from 24 to 28 air changes per hour.  These differences within an aircraft are not of practical importance and there was no discernable pattern as to which seats had the higher values (e.g. window versus aisle).

    Another study showed that the biggest factor in exposure risk, not surprisingly, is distance from the infected person. 3 The further you are from the infected person in any direction, the better.  While it is not possible to know in advance the location of infected people, the fewer people in the general vicinity of a seat and especially the fewer people within 2-3 seats in any direction, front-back, right-left, the lower the risk.  This same study indicated the worst place to be for airborne transmission may be directly in front of an infected person. 

    Since seats are spaced further apart in both directions in business and first-class sections, the risk in these sections is likely somewhat less in a fully loaded aircraft.  However, a fully loaded first class section may have a higher occupant density than a lightly loaded main cabin, thus, there is no guarantee that a 1st class seat will reduce exposure.


    1) J. Patel, B. Jones, and M. Hosni, Experimental Analysis of Ventilation Effectiveness and Tracer Gas Dispersion in a Boeing-737 Mockup Cabin, Proceedings of the Second International Conference on Energy and Indoor Environment for Hot Climates, ASHRAE, February 2017, Doha, Qatar.

    2) J. Patel, B. Jones, M. Hosni, A. Keshavarz, Experimental Investigation of Ventilation Effectiveness in an Airliner Cabin Mockup, Proceedings of the ASME 2016 International Mechanical Engineering Congress and Exposition, IMECE2016,  Phoenix AZ, November 12, 2016

    3) J.S. Bennett PhD , B.W. Jones, M.H. Hosni, Y. Zhang, J.L. Topmiller, and W.L. Dietrich, Airborne Exposure Patterns from a Passenger Source in Aircraft Cabins,  HVAC&R Research, 22 Nov 2013.

  • Glossary

    CIDRAP = U of MN Center for Infectious Disease Research and Policy

    DMHC = ASHRAE Design Manual for Hospitals and Clinics (First Edition)

    WHO = World Health Organization


    ASHRAE PD = ASHRAE Position Document on Infectious Aerosols

    FGI = Facility Guidelines Institute


    Term Definition Source
    Aerosol generating procedure (AGP) Procedures that are likely to induce coughing. Procedures that are believed to generate aerosols and droplets as a source of respiratory pathogens include positive pressure ventilation (bi-level positive airway pressure [BiPAP] and continuous positive airway pressure [CPAP]), endotracheal intubation, airway suction, high-frequency oscillatory ventilation, tracheostomy, chest physiotherapy, nebulizer treatment, sputum induction, and bronchoscopy. AGPs should ideally take place in an airborne infection isolation room (AIIR). CDC
    Aerosol, infectious An infectious aerosol is a system of liquid or solid particles uniformly distributed in a finely divided state through a gas, usually air. (They are small and buoyant enough to behave much like a gas yet they can be filtered out of the gas.) ASHRAE PD
    Aerosol, Short-range transmission Transmitting disease by inhalation of aerosols near the source. The distance for this transmission has not been studied beyond two meters. CIDRAP
    Age of Air The time that has elapsed after the air enters a space (at any given point.) DMHC
    Air change rate Airflow in volume units per hour divided by the building space volume in identical volume units (normally expressed in air changes per hour [ACH or ACPH]) DMHC
    Air irritant A particle or volatile chemical in air that causes physiological response when in contact with mucosa in the eye, nose, or throat. DMHC
    Air volume migration The volume of air that is exchanged during room entry/exit (through a door-way between a room and the area beyond its door) DMHC
    Air, exhaust Air removed from a space and discharged outside the building by mechanical or natural ventilation systems. DMHC
    Air, makeup Any combination of outdoor and transfer air intended to replace exhaust air and exfiltration. DMHC
    Air, outdoor (1) Air outside a building or taken from the outdoors and not previously circulated through the system;
    (2) Ambient air that enters a building through a ventilation system, through intentional openings for natural ventilation, or by infiltration.
    Air, recirculated Air removed from a space and reused as supply air. DMHC
    Air, supply Air delivered by mechanical or natural ventilation to a space that is composed of any combination of outdoor air, recirculated air, or transfer air. DMHC
    Air, transfer Air moved from one indoor space to another. DMHC
    Airborne droplet nuclei Small-particle residue (5 µm or smaller) of evaporated droplets containing microorganisms that remain suspended in air and can be dispersed widely by air currents with a room or over a long distance. DMHC
    Airborne infection isolation room (AIIR) A room designed with negative pressurization to protect patients and people outside the room from the spread of microorganisms (transmitted airborne droplet nuclei) that infect the patient inside the room. DMHC
    Airborne infectious agent An airborne particle that can cause an infection. DMHC
    Airborne pathogen An airborne particle that can cause disease. DMHC
    Airborne transmission Airborne transmission is defined as "dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain infective over time and distance." An important requirement of airborne transmission is that it can occur only at a long distance from the source, according to the CDC. CIDRAP
    Air-cleaning system A device or combination of devices used to reduce the concentration of airborne contaminants, such as microorganisms, dust, fumes, respirable particles, other particulate matter, gases and/or vapors in air. Related term: HEPA Filter. DMHC
    Anteroom A room separating an isolation room from a corridor. DMHC
    Bay (patient) A space for human occupancy with one hard wall at the headwall and three soft walls. FGI
    Bioaerosol Particles or droplets suspended in air that consist of or contain biological matter such as bacteria, pollens, fungi, skin flakes, and viruses. DMHC
    Building air infiltration Uncontrolled inward leakage of air (that may contain entrained water vapor) through cracks and interstices in any building element and around windows and doors of a building, caused by the pressure effects of wind or the effect of differences in the indoor and outdoor air density. DMHC
    CADR Clean Air Delivery Rate which is the combined effect of actually how much air is moved through the filter and the filter efficiency. ASHRAE 52.2-2017
    Community acquired infection An infection present or incubating in a patient upon admission to a hospital (or who subsequently shelters in place outside the hospital). DMHC
    Contaminant or Pollutant Any impurity, any material of an extraneous nature, associated with a chemical, a pharmaceutical preparation, a phuysiologic principle, or an infectious agent. DMHC
    Contaminant, airborne An unwanted airborne constituent that may reduce the acceptability of air. DMHC
    Contamination The act of contaminating, especially the introduction of disease germs or infectious material into or on normally sterile objects. DMHC
    COVID-19 COVID-19 is the short name for “coronavirus disease 2019" WHO
    Cubicle A space intended for human occupancy that has at least one opening and no door and is enclosed on three sides with full height or partial height partitions. FGI
    Droplet transmission Droplet transmission is defined as "respiratory droplets carrying infectious pathogens that transmit infection when they travel directly from the respiratory tract of the infectious individual to susceptible mucosal surfaces of the recipient, generally over short distances, necessitating facial protection." Close contact involves hand transfer of surface contamination to mouth, nose or eyes, hand washing and gloves being common controls. CIDRAP
    Epidemiology Study of the distribution and determinants of disease. DMHC
    HEPA filter (or absolute filter)  High efficiency particle air filter with an efficiency of 99.97% removal of particulates larger than 0.30 microns. DMHC
    Hospital Acquired Infection (HAI) See Nosocomial infection.
    Intensive care rooms (ICU)(also critical care rooms CCU) Rooms in which the level of patient care and electronic monitoring of patients are greatly increased over conventional patient rooms. FGI
    MERV Minimum Efficiency Reporting Value: The fraction of particles removed from air passing through a filter is termed “filter efficiency” ASHRAE 52.2-2017
    Nosocomial infection (or Hospital Acquired Infection [HAI]) An infection that is acquired in a hospital and that was not present or incubating upon admission. DMHC
    Occupationally acquired infection An infection acquired while working in a medical care setting. DMHC
    Opportunistic organism An ordinarily non-infectious agent that becomes infectious in an immunocompromised host. (any novel organism, especially aerosolized respiratory viruses for which there is no vaccine or herd immunity becomes an opportunistic organism.) DMHC
    Pneumonia Inflammation of lung tissue. DMHC
    PPE Personal Preotective Equipment is equipment worn to minimize exposure to hazards that cause serious workplace injuries and illnesses. Occupational Safety and Health Administration, PPE
    Pressurization A difference in pressure between a space and a reference pressure. DMHC
    Room A space enclosed by hard walls and having a door. FGI
    SARS-CoV-2 Severe Acute Respiratory Syndrome CoronaVirus 2 International Committee on Taxonomy of Viruses (ICTV)
    UV Ultraviolet irradiation. DMHC
    UVGI Ultraviolet germicidal irradiation. DMHC
    Ventilation A process of supplying air to or removing air from a space for the purpose of controlling air contaminant levels, humidity, or temperature within the space. Such air may or may not have been conditioned. DMHC
    Ventilation effectiveness The ability of a system to remove contaminants generated by a source in a room. DMHC

Information on these pages is provided as a service to the public. While every effort is made to provide accurate and reliable information, this is advisory, is provided for informational purposes only, and may represent only one person’s view. These are not intended and should not be relied upon as official statements of ASHRAE.