Ventilation FAQs

Updated July 9, 2021.

These FAQs are intended to provide basic information about building ventilation as it relates to reducing the possible spread of COVID-19. They may be some of the questions businesses get from their customers. 

This document was created originally by Portsmouth’s Citizen Response Task Force/Health Subcommittee with member and CRTF co-chair James Petersen, as lead author, to serve the interests of local businesses, organizations, residents, and visitors during the COVID-19 pandemic. James Petersen, a licensed professional engineer, is founder/owner of Petersen Engineering in Portsmouth, and has 34 years of experience design heating, ventilation and air-conditioning (HVAC) systems for buildings.


How does the coronavirus spread?

  • The virus is thought to spread mainly from person-to-person through close contact.
  • People without symptoms are able to spread the virus.  
  • These facts support basic rules of behavior to reduce the spread of the virus:
    • Wear a mask
    • Keep a safe distance (six feet or more)
    • Avoid crowds
    • Wash your hands
    • Opt for the outdoors
  • This Journal of Infectious Diseases study reported that five identified studies found a low proportion of reported COVID-19 infections occurred outdoors (<10%) and the odds of indoor transmission were very high compared to outdoors (18.7 times). 

Here in New England, being outdoors is not always possible or comfortable. What do we know about how the virus spreads indoors?

  • Reports from several outbreak investigations have shown that the virus can spread particularly effectively in crowded, confined indoor spaces.
  • The virus has been shown to spread via small droplets in the air, and via even smaller particles in the air called aerosols.
  • Actions such as coughing, sneezing, shouting, or singing may release virus into the air with more force than simply breathing.  At times, these particles can travel up to 27 feet.
  • Several reports suggest that the virus can be spread by the flow of air from air conditioning systems.
  • Poor ventilation in confined indoor spaces is associated with increased spread of respiratory infections including COVID-19


What is HVAC?

“HVAC” stands for Heating, Ventilation and Air-Conditioning. The three tend to work together through shared ducting in a building, so are called ‘the HVAC system.’

What is a building enclosure? (Also referred to as the building envelope.)

The building enclosure is what separates the indoors from the outdoors including exterior walls, windows, doors, the lowest level floor and the roof. During the winter, the enclosure keeps the heat in and the cold out.

What is ventilation?

Ventilation is the process of maintaining clean and uncontaminated indoor air by adding or removing the fresh or recirculated air in a building. Indoor contaminants include particulates such as smoke, bacteria and viruses, gases, vapors, and mists.

What is infiltration?

Infiltration is air that unintentionally leaks into or out of a building enclosure. On windy days there is more infiltration due to differences in wind pressure. On cold days there is more infiltration because of the large temperature difference between indoors and outdoors.

What is natural ventilation?

Natural ventilation is the air that moves in or out of a building through intentional openings such as operable window and doors.

What is mechanical ventilation?

Mechanical ventilation uses fans or blowers to force air into or out of a building.

What is exhaust air?

Air that is forced by a fan or blower out of a building.

What is supply air?

Air that is forced by a fan or blower into a building.

What is local ventilation?

Local ventilation exhausts air from a point close to unwanted odor, moisture, heat or other contaminants including particulates, gases, vapors and/or mists. Common examples of local ventilation include bathroom exhaust fans and kitchen range hoods. Local ventilation is the preferred method for removing contaminants; however in the case of viruses, people – who move around -- are the source of the contaminant so local ventilation is not an option.

What is dilution ventilation?

Dilution ventilation uses supply and exhaust air to flush out the building to reduce the contaminant to a safer level.

What is recirculated air?

Air recirculation mixes existing air within the building. Recirculated air can serve to dilute a contaminant from a local area of high concentration to a lower concentration throughout the building. Systems that recirculate air can also be used to clean the air using filters and other technologies.

What is fresh air?

Fresh air is outdoor air that is brought into a building.

What is ACH?

Air Changes per Hour (ACH) is a term used to measure airflow: how often the air volume of the building is replaced with new air, or the how much air is recirculated by the HVAC system within a given air volume in a building. One ACH of fresh air replaces the building’s entire volume of air with new outdoor air in one hour. 12 ACH replaces indoor air with outdoor air 12 times in one hour. One ACH of recirculated air is a building’s worth of air moved by an HVAC system in one hour.

CDC guidelines on air exchangeThis table is intended for healthcare settings but it does show time needed for the removal of airborne contaminants, depending on the rate of air changes per hour (ACH) in the room

What is CFM?

Cubic Feet per Minute (CFM) is another rate of air exchange used to express airflow.

Why is outdoors safer than indoors?

The amount of fresh air exchanged indoors is small compared to the outdoors. Contaminants in the air outdoors are quickly diluted, thereby helping to protect humans from harm.

Example: Take a home with 9-foot ceilings and 1000 sq. ft. of living space. If there were no walls and a light breeze of 5 MPH flowed through the home, the result would be ventilation at a rate of 820 ACH (air changes per hour). This is a lot of air movement. By comparison, the ventilation code requires a minimum of just 0.35 ACH for homes. So, when you are outside, that 5 mph breeze you feel is providing more than 2,300 times more ventilation than a code-compliant home. Even if you increased the home ventilation from 0.35 to what the code requires in a hospital operating room – 4 ACH – the breeze would provide over 200 times more ventilation.

What is the Ventilation Code?

The Ventilation Code is part of the standardized Building Code that defines the requirements for ventilation systems, including rates of change for fresh air and recirculated air. Ventilation rates in healthcare facilities are expressed as ACH in the code, and rates in all other building types are expressed as CFM in the code.

What is filtration?

Filtration -- by air filters, ultraviolet germicidal irradiation (UVGI) and electronic air filters – uses various methods, other than fresh air, to remove contaminants from buildings.

What is HEPA?

High-Efficiency Particulate Air (HEPA) filtering is a type of mechanical air filtration.

What is ASHRAE?

The American Society of Heating Refrigeration and Air-conditioning Engineers (ASHRAE) is a resource for research and “best practices” for improving ventilation system performance.

What is MERV?

Minimum Efficiency Reporting Value (MERV) is a rating system for filters ranging from MERV 1 to MERV 20. The higher the MERV rating the better the filter is at capturing small particles of various sizes. ASHRAE guidelines suggest using MERV 13 filters in existing systems if possible. Higher-rated MERV filters are difficult to retrofit in existing HVAC system because of physical size constraints, because they take additional fan energy to push the air through the filter which can be impractical in older systems.

As a contaminant how does coronavirus compare to other contaminants?

Coronavirus is characterized as follows:

  • Potentially severe in its effects
  • Highly contagious
  • Associated with a significant percentage of infectious, although asymptomatic, individuals
  • Transmitted person-to-person
  • Initiates respiratory infection through inhalation and contact with the eyes, nose, and mouth
  • Has an unknown infectious dose range

What is the particle size of the coronavirus that causes COVID-19?

SARS-CoV-2, the virus that causes COVID-19 is very small at about 0.12 microns. For comparison, a human hair has a diameter of 50 microns, 400 times larger than the virus. At 0.12 microns the coronavirus is sufficiently small to remain suspended in the air and is difficult to remove.

What is ionization?

Ionization technology adds positively or negatively charged ions to the air. The ions cause particles to attract each other, creating larger particles that drop out of the air by gravity or are now large enough to be captured by an air filter. Although ionization technology has been around for many years, there is little data on its effectiveness so If used, ionization-treated air should be used to supplement not replace outside air and should be used with other measures such as filters, masks and physical-distancing to reduce the risk of COVID-19.

What about electronic “air cleaning devices” using ionization and other technologies?

Ionization technology adds positively or negatively charged ions to the air. The ions cause particles to attract each other, creating larger particles that drop out of the air by gravity or are now large enough to be captured by an air filter. Although ionization technology has been marketed for many years, there is little proof these devices can remove COVID-19 virus from the air and ASHRAE considers it to be “an emerging technology.”

In April 2021, ASHRAE circulated a letter and report, from a panel of 14 of the most knowledgeable researchers and engineers in the indoor air quality community, that appeals to those in authority for investment decisions to avoid wasting money and resources on electronic air cleaners. ASHRAE commented that the proven, effective and less expensive alternatives of filtration and ventilation are and remain available for use in COVID-19 risk reduction.

They said:

“…We appeal to school district facility managers and administration leadership, as well as the relevant national and international bodies and Architecture, Engineering and Construction (AEC) industry consultants and professional organizations, to recognize the unproven nature of many electronic air cleaning devices. Such devices are typically electrically powered air-cleaners intended to remove particles from airstreams or to inactivate pathogens. As they are unproven, it is critical to avoid wasting valuable emergency COVID relief aid dollars installing them within school district facilities.

Studies indicate a much lower degree of effectiveness in real-world conditions than typically claimed by manufacturers. Studies also indicate that chemical compounds at harmful concentrations can be produced in real-world settings, directly as a part of the process or as byproducts created from the chemical reactions occurring within the space. In the absence of regulation and with presently very little peer-reviewed research, significant questions remain regarding effectiveness and the potential impacts on human health…”

This advice focuses on public institutions, especially schools but would apply to homes as well. The experts stress that the devices’ effectiveness is much less than typically claimed by their manufacturers and that they might even produce chemical compounds in concentrations that could be harmful. The proven, effective, and less expensive alternatives for improving air quality improvement including open windows, ultraviolet light and mechanical filtration like HEPA, along with such practices as physical distancing, masking and hand-washing are better approaches to COVID-19 risk reduction.

How does the ventilation system on a commercial jet work?

Commercial jets use a sophisticated air circulation system that brings fresh air into the passenger cabin and mixes it with filtered cabin air to recirculate it. A constant replenishing of all the air in the cabin with sub-zero outside air would be prohibitive to heat, therefore the mix of fresh air and filtered/recirculated air is used. The air flow in the passenger cabin is regulated by the way the air is introduced at the top of the cabin and ventilated at the floor. This contains particulates such as virus to a more limited space than the full cabin, but masks help protect against that exposure, which is why the Federal government still requires the wearing of masks on all US commercial flights. For an animated illustration of air circulation in jet cabins, created by the New York Times,click here.


Industrial Ventilation, A Manual of Recommended Practice, 24th Edition, ACGIH (American Conference of Governmental Industrial Hygienists)

ASHRAE (American Society of Heating, Refrigeration, Air-conditioning Engineers), 2020 ASHRAE Handbook, HVAC Systems and Equipment

International Mechanical Code, International Code Council, Inc.

ANSI/ASHRAE/ASHE Standard 170-2017, Ventilation of Health Care Facilities

This summary information was updated January 27, 2021.

For the latest (January 21, 2021) bulletin on ventilation from ASHRAE, click here.

What experts know and believe about coronavirus and the COVID-19 pandemic may change over time. Please refer to the websites below for further detail and the most up-to-date information.

State of NH Department of Health and Human Services

Centers for Disease Control


What is the best way to reduce the risk of COVID-19 in my home?

The best way is to prevent COVID-19 from entering your home is to minimize the number of people entering your home and to restrict the network of people you and members of your household come into contact with.

What can I do to improve ventilation in my home?

Opening windows allows the largest amount of fresh air into your home and dilutes any contaminants, but when you allow hot or cold outdoors air in, you do compromise your comfort and HVAC efficiency. During these times, we must strike a balance between the benefits of open windows, occupant comfort, and energy use. See above, regarding "electronic air-cleaning" devices.

Do I need to worry about ventilation in my home if no one at home has COVID-19?

Because individuals can be contagious and have no symptoms the coronavirus is a difficult contaminant to manage. Improving ventilation in your home will reduce risk.

What are other effective ventilation measures, besides opening windows?

  • If your home has a ducted all-air HVAC system, call your service contractor to see if you can upgrade your filters to higher efficiency filters (MERV-13, or electronic filters)
  • Consult with your service contractor to increase the amount of fresh air that is supplied to the home by the HVAC system.
  • Add portable air cleaners in rooms where you spend the most time. (Warning: Avoid brands that generate ozone -- O3 – which is a common pollutant and makes matters worse for indoor safety.)

What can I do about ventilation when someone in my home has COVID-19?

There are many steps recommended for reducing the risk of infection among other members of the household, but in terms of ventilation the following measures can be taken:

  • Isolate the sick individual in one room or area
  • Allow supply air from the HVAC system into the room but seal off the return air opening. (This will prevent the spread of the virus throughout the home via duct work.)
  • Open a window to allow room air out
  • If the room is not heated through duct work, open two windows and provide heat or air-conditioning
  • Install a portable air cleaning device (see caution above).
  • There’s greater air circulation if it’s colder outdoors than in. So you don’t need to open the window as wide to get good circulation. If the indoor and outdoor temperatures are the same a box fan can help increase air circulation. 

What are long-term changes I can make at home to improve ventilation?

Adding an Energy Recovery Ventilator (ERV) provides fresh air while incorporating a heat exchanger to recover energy from the exhaust air-stream. ERVs increase the amount of outside air brought into the house without compromising comfort or increasing energy use.


I need to travel in a car with someone who is not a member of my household. Are there any measures we can take to reduce the risk of transmission while in the vehicle together?

Traveling with another person in a vehicle increases the risk of transmitting infection due to its enclosed space. Wearing a mask and putting as much distance as possible between driver and passenger -- passenger in right rear (RR) seat diagonally distant from the driver in the front left (FL) --  helps.

New research on air flow in the cabin of the vehicle has provided important new information:  

  • All 4 windows fully open provides the maximum air exchange (ACH), but may not be practical, especially in winter.
  • Having the driver and the passenger each roll down their “own” windows (FL and RR) provides more air flow than keeping all the windows closed, but offers only a limited reduction in the possible transmission of aerosol droplets (e.g. the virus).
  • The maximum benefit in air exchange that reduces transmission between driver and passenger is to open the front RIGHT and rear LEFT windows.

The New York Times reported the results of this study along with further information from one of the researchers. In additional work, researcher Varghese Mathai found that opening the windows halfway seemed to provide about the same benefit as opening them fully, while opening them just one-quarter of the way was less effective. He said the general findings would hold for most four-door, five-passenger vehicles. The flow in trucks and the effects of opening sunroofs/moonroofs are not yet reported.


How can I use ventilation to reduce the risk of COVID-19 in my commercial building?

The key is to dilute the concentration of any coronavirus particles by exchanging of “contaminated” indoor air with fresh air. Open windows and doors on opposite sides of rooms for the most effective air exchange.

What if there are only windows on one wall?

Use a box fan, pulling outdoors air in, to increase the air exchange rate.

What if we can’t open windows or doors? What else can be done? instead of or in addition to opening windows and doors?

Many buildings use a ducted HVAC system to heat, air-condition and provide ventilation. Generally, these systems introduce a percentage of fresh air while recirculating most of the air. The following steps, in order of priority, can improve the efficiency of these systems:

1.       Confirm that system is well-serviced and is introducing fresh air.

2.       Because HVAC systems vary in terms of how much fresh air they can supply to a building, ask your mechanical service contractor to evaluate the limits of your system and increase the percentage of outside air, if possible.

3.       Operating the system 24 hours per day is better for reducing risk than operating the system only when occupied. (This may be impractical when outdoor temperatures are extremely cold.)

4.       Increase the efficiency of filters at air-handling units. MERV-13 is a good target efficiency since many existing systems can physically accommodate MERV-13 filters and there is little or no downside impact on the HVAC system’s ability to heat, cool and ventilate effectively.

5.       Add air cleaning technology (UV or Ionization) at the air-handling unit. A mechanical service contractor can determine what technology can be accommodated for individual circumstances.

What other ventilation measures reduce the risk of COVID-19 in commercial buildings?

Stand-alone portable air cleaning equipment is available from multiple manufacturers. This equipment does not introduce fresh air into the building. Instead it uses various methods such as high-efficiency filters, UV light, or ionization to reduce indoor air contaminants. The equipment draws in room air, cleans it to some extent, and delivers it back to the room. As individual cleaning units would need to be placed close to contagious individuals to be effective, this approach is probably the least effective ventilation method to reduce risk in commercial buildings.

As an employer or business owner, what can I do with the ventilation systems currently in place to improve air circulation?

Two ways to improve ventilation and reduce the chance of spreading Covid-19 in your place of business are: 

1) Turn on restroom ventilation fans and leave them on to circulate air continuously during business hours. Restrooms generally are closed spaces with limited outdoor air flow and air exchange. By leaving the ventilation fan on, you increase the air circulation in those spaces.

2) Consider turning on your building's ventilation system(s) 2 hours prior to opening and 2 hours following the close of business to optimize air replacement before and after operating hours.

What about when it gets cold out?

Cold outdoor temperatures make open windows unattractive from a comfort standpoint. Each building manager will need to strike a balance between open windows and occupant comfort, though the more people accommodate by wearing warmer clothing, the more fresh air can be brought in to reduce the risk of virus contamination. Of course introducing more cold air means increasing heating costs.

I lease space for my business. What measures can I ask my landlord to take to improve the ventilation in the building?

  • There are many different configurations of HVAC systems in buildings depending on many variables such as the age of the building, the size of the building, the number of stories, etc.
  • Renters should work with their landlords to determine what type of modifications can be made to the HVAC system to reduce risk.
  • If the HVAC is provided by a unit on the roof, a contractor can service the unit, improve filter efficiency, and add ionization or UV measures to the HVAC unit.
  • If the rented space is in an older building with perhaps no ductwork, the landlord can upgrade exhaust fans to move more air and can recondition windows and doors to make them easier to operate.

Is there any research reported in the ventilation in outdoor structures such as 'igloos'?

Outdoor seating isn’t always safer than indoor dining. Some structures may be relatively safe, others could be worse, trapping aerosols inside. For a report on recent research, click here.


CDC on how COVID-19 is spread(accessed on August 27, 2020)

AMA/Anthony Fauci MD on blunting the resurgence of COVID-19 (accessed on August 27, 2020)

ECDC COVID-19 transmission (accessed August 27, 2020)

CDC report on Ventilation and COVID-19 (December 21, 2020).