Ventilation in Buildings

Summary of Recent Changes

• Added Key Strategies, a simplified summary of the most important recommendations on this page.
• Added discussion on “How much ventilation is enough?” with a recommendation to get at least 5 air changes per hour of clean air in occupied spaces.
• Updated the minimum filter recommendation to Minimum Efficiency Reporting Value (MERV) 13.
• Updated the guidance on post-occupancy flushing of building air.
• Included more information on up-front, maintenance, and energy cost considerations for ventilation strategies.
• Added a Frequently Asked Question on “Do-It-Yourself (DIY) Air Cleaners.”
• Updated all Frequently Asked Questions to include a concise answer, followed by more detail.
• Updated the discussion on Whole-Room Ultraviolet Germicidal Irradiation (also called Far UV) in Frequently Asked Question #7.

View Previous Changes

On This Page

DEFINITION

Ventilation

Ventilation is a term with different meanings to different people. For the purpose of this webpage, “ventilation” includes:

• Indoor air movement and dilution of viral particles through mechanical or nonmechanical (also called natural) means.
• Filtration through central heating, ventilation and air conditioning (HVAC) systems and/or in-room air cleaners (portable or permanently mounted).*
• Air treatment with Ultraviolet Germicidal Irradiation (UVGI) systems (also called Germicidal Ultraviolet or GUV).*
• These air cleaning techniques are sometimes referred to as “equivalent ventilation.” They are not a substitute for meeting minimum outdoor air delivery requirements that may be specified in national, state, and local building codes.

What You Need to Know

• An important principle to remember when contemplating mitigation strategies is to “First do no harm.” Building owners and operators should strongly consider seeking expert consultation when considering mitigation strategies if they lack knowledge and experience in implementing those strategies.
• Airborne viral particles spread between people more readily indoors than outdoors. Indoors, the concentration of viral particles is often higher than outdoors. Protective indoor ventilation practices can reduce the airborne viral concentrations and the overall viral exposure to occupants.
• Ventilation system upgrades or improvements can increase the delivery of clean air and dilute potential contaminants. Buildings that provided healthy, code-compliant indoor air quality prior to the pandemic can be improved for pandemic and post-pandemic occupancy using less costly interventions.
• While the mitigation strategies can be universally applied across many indoor environments, applying them to different building types, occupancies, and activities under environmental and seasonal changes can be challenging. The building owner or operator should identify which strategies are appropriate for each building throughout the year.
• Implementing multiple building-level mitigation strategies at the same time is consistent with CDC’s layered approach and will increase overall effectiveness of ventilation interventions.
• Building owners and operators can participate in the Clean Air in Building Challenge to improve indoor air quality and protect public health.
• Get answers to Frequently Asked Questions below about ventilation and safe building practices during and after the COVID-19 pandemic.

Ventilation Mitigation Strategies

When indoors, ventilation mitigation strategies can help reduce viral particle concentration. The lower the concentration, the less likely viral particles can be inhaled into the lungs (potentially lowering the inhaled dose); contact eyes, nose, and mouth; or fall out of the air to accumulate on surfaces. Although it isn’t known exactly how much the concentration of viral particles in air needs to be reduced to start reducing risk of viral infection, ventilation mitigation strategies still provide a reasonable approach to reducing risk. Not all interventions will work in all scenarios and their selection must be carefully evaluated prior to adoption.

These ventilation interventions can reduce the spread of disease, but they will not eliminate risk completely. These interventions are intended to lower transmission risk by lowering the concentration of infectious aerosols in a room. However, the overall transmission reduction is less likely to apply to people who are very close (e.g. face-to-face) to the infectious source. Some of the following interventions are based on COVID-19 Technical Resources published by ASHRAE (a professional organization formerly known as the American Society of Heating, Refrigerating, and Air Conditioning Engineers).

The Clean Air in Buildings Challenge is a national call to action that highlights a range of recommendations and resources available to assist with reducing risks from airborne viruses and other indoor contaminants. Create your clean indoor air action plan today.

In addition to buildings, vehicles – including public transportation such as buses, subways, trains, school buses, carpools, and rideshares – are also areas where ventilation improvements can be applied to reduce the spread of airborne viruses and lower the risk of exposure.

The recommendations presented here are not intended to replace guidance that may already exist in national, state, and local standards and guidelines. For example, some healthcare spaces have specified ventilation requirements intended to prevent and control infectious diseases. For spaces where existing standards and guidelines specify lower ventilation rates than the recommendations presented here, building owners and managers are encouraged to consider adoption of the more protective guidance.

Important: For interventions listed below that are marked with “**,” consulting with professionals experienced in the proper selection, implementation, and commissioning of HVAC improvements is strongly encouraged. Their experience should preferably include the building, system, and occupancy types under evaluation. Other interventions may require similar consultation, depending upon the knowledge and experience of the individuals responsible for implementing changes to the building.

Improving Air Circulation

1

If you do nothing else, ensure existing HVAC systems are providing at least the minimum outdoor air ventilation requirement in accordance with ventilation design codes.

• Applicable codes are based on the year of building construction or latest renovation and intended building occupancy.
• Preferably, upgrade HVAC system performance to meet current ventilation code requirements at current occupancy levels.**
• This will develop a strong and lasting baseline upon which further interventions can be implemented.

2

Increase the introduction of outdoor air beyond code-minimum requirements.

This measure will potentially increase energy costs. Use of an energy recovery ventilator (ERV) can lessen the potential energy and system implications of increased outdoor air.

• Open the outdoor air dampers on your HVAC equipment beyond minimum settings to reduce or eliminate HVAC air recirculation. In mild weather, this will not affect thermal comfort or indoor humidity. However, this may be difficult to do in cold, hot, or humid weather.**
• Open windows and doors, when weather conditions allow, to increase outdoor air flow. Even a slightly open window can introduce beneficial outdoor air.
• Do not open windows and doors if doing so poses a safety or health risk (e.g., risk of falling, triggering asthma symptoms) to building occupants. Use caution in highly polluted areas when increasing outdoor air ventilation.

3

Use fans to increase the effectiveness of open windows.

• To safely achieve this, fan placement is important and will vary based on room configuration.
• Avoid placing fans in a way that could potentially cause contaminated air to flow directly from one person to another (see FAQ #11 below on indoor use of fans).
• One helpful strategy is to use a window fan, placed safely and securely in a window, to exhaust room air to the outdoors. This will help draw outdoor air into the room via other open windows and doors without generating strong room air currents. Similar results can be established in larger facilities using other fan systems, such as gable fans and roof ventilators.

4

Rebalance or adjust HVAC systems to increase total airflow to occupied spaces when possible.

• Increase total airflow to increase room air mixing and reduce viral particle concentration and subsequent exposure potential.**
• Turn off any demand-controlled ventilation (DCV) controls that reduce air supply based on occupancy or temperature during occupied hours.**
• In homes and buildings where the HVAC fan operation can be controlled at the thermostat, set the fan to the “on” position instead of “auto,” which will operate the fan continuously, even when heating or air conditioning is not required.
• Ensure restroom exhaust fans are functional and operating at full capacity when the building is occupied.
• Inspect and maintain exhaust ventilation systems in areas such as kitchens, cooking areas, etc. Operate these systems any time these spaces are occupied. Operating them even when the specific space is not occupied will increase overall ventilation within the occupied building.
• In non-residential settings where an infectious source was not known to have been present, run the HVAC system at maximum outside airflow for 2 hours, or until the building has achieved at least 3 air changes, after the building is no longer occupied. If an infectious source was present, see FAQ #2.
• Generate clean-to-less-clean air movement by evaluating and repositioning as necessary, the supply louvers, exhaust air grilles, and/or damper settings.** See FAQ #4 below on Directional Airflow. This recommendation is easier to accomplish when the supply and exhaust points are included as part of a “drop ceiling.”

Improving Air Cleanliness

Upgrade central HVAC filter efficiency to a Minimum Efficiency Reporting Value (MERV)-13 or better. When compatible with your HVAC system, increased filtration efficiency is especially helpful when enhanced outdoor air delivery options are limited.

2

Inspect HVAC systems.

• Ensure ventilation systems operate properly and are up to date on maintenance.
• Make sure air filters are properly sized and within their recommended service life.
• Inspect filter housing and racks to ensure appropriate filter fit and minimize air that flows around, instead of through, the filter.

3

Use portable or built-in high-efficiency particulate air (HEPA) fan/filtration systems (also called air cleaners or air purifiers).

• Use HEPA systems to enhance air cleaning (especially in higher risk areas such as a medical office or areas frequently inhabited by people with a higher likelihood of having COVID-19 and/or an increased risk of getting COVID-19). See FAQ #5 below on HEPA filters and in-room HEPA air cleaners.
• In-room air cleaners that use filters less efficient than HEPA filters also exist and can contribute to room air cleaning. However, they should be clearly labeled as non-HEPA units.
• Some air cleaners/air purifiers use technologies other than filtration. See FAQ #8 for a detailed discussion of factors to consider before using these other technologies.

4

Use UVGI (also called GUV) as a supplemental treatment to inactivate airborne viruses, such as SARS-CoV-2. UVGI can be effective in many spaces, but it is especially useful as an additional layer of protection to reduce infectious particles in indoor spaces that host large gatherings or where the risk of disease transmission is high. It is also helpful when options for increasing room ventilation and filtration beyond code requirements are limited.**

• Upper-room UVGI systems can be used to provide air treatment within occupied spaces.
• In-duct UVGI systems can help enhance air cleaning inside central ventilation systems.
• See detailed discussion in FAQs #6 and #7.

How Much Ventilation Is Enough?

Aim for 5 Air Changes per Hour (ACH)

When possible, aim for 5 or more air changes per hour (ACH) of clean air to help reduce the number of germs in the air.

This can be achieved through any combination of central ventilation system, natural ventilation, or additional devices that provide equivalent ACH (eACH†) to your existing ventilation. Supplying or exhausting an amount of air (use the larger of the two values but do not add them together) that is equal to all the air in a space is called an air change. Multiplying that amount by 5 and delivering it over one hour results in 5 ACH.

To calculate the ACH (or eACH):

1. Determine (or measure) the airflow through the system in cubic feet per minute (cfm).
2. Determine the area of the room = length (ft) x width (ft)
3. Determine the height of the room (ft).
4. Calculate ACH:

ACH=ACH=cfm×60Area×Height

1. When multiple strategies are used, repeat the ACH calculation for each system then add together for a total ACH value (which could be compared to the minimum 5 ACH recommendation).

Note: See FAQ #2 and FAQ #5 for examples on how the ACH calculation may be applied.

While there is insufficient science to identify an optimum ventilation strategy for all spaces, 5 ACH is what portable air cleaners provide (as eACH) when properly sized following the Environmental Protection Agency’s guidance [2.9 MB, 7 pages] on the selection of portable air cleaners. Five ACH will not guarantee totally safe air in any space, but it reduces the risk of exposure to germs and other harmful air contaminants.

Rather than a hard-and-fast rule, the 5 ACH target provides a rough guide to air change levels likely to be helpful in reducing infectious particles. For example, increasing ventilation from 2 to 5 ACH substantially reduces the time to remove airborne contaminants.

• Large volume spaces with very few occupants (e.g., a warehouse) may not require 5 ACH and spaces with high occupancy or higher-risk occupants may need higher than 5 ACH.
• While ACH levels higher than 5 (e.g., those used in airborne isolation rooms in hospitals) may reduce infectious aerosols further, the potential benefits of increased ventilation should be balanced with the additional upfront, periodic maintenance, and energy costs that may be incurred.
• Some limited studies have demonstrated this protective benefit of increased ACH, although an optimum number remains uncertain.
• A Lancet Commission Report [249 KB, 33 pages] that draws on available scientific evidence proposes ACH levels of 4 as “Good,” 6 as “Better,” and >6 as “Best,” underscoring that ACH (to include eACH) represents a continuum.
• It is unknown exactly how much this will reduce the risk of getting a viral infection in an indoor space.
• However, the improvements are reasonable for indoor environments when additional protection is desired. More research is needed to evaluate the influence of central ventilation, portable air cleaning, and UV air treatment on respiratory infectious disease transmission.

† Some air cleaning and air treatment devices do not bring in outdoor air. Instead, they clean or treat the indoor air to reduce the concentration of infectious particles. Thus, they give eACH without the need or expense of conditioning outdoor air. Note that eACH depends on the contaminant. An air treatment device that provides eACH for particles may not be effective against other contaminants such as gases and vapors.

Cost Considerations

The ventilation interventions listed above come with a range of initial costs and operating costs, which, along with risk assessment factors – such as community incidence rates and the adoption of other interventions – may affect the selection of ventilation mitigation strategies. The following are examples of cost estimates for different strategies:

Intervention Strategy

Up-front Cost

Ongoing Daily Interaction

Ongoing Maintenance Requirements and Incremental Energy Usage

Opening windows

No

• No ongoing maintenance requirements
• Incremental energy usage varies, depending on ambient outdoor conditions

Expanded operation of dedicated exhaust ventilation

No

No

• Periodic preventive maintenance required
• Incremental energy usage varies, depending on exhaust system capacity and ambient outdoor conditions

Repositioning HVAC outdoor air dampers

No

No

• Periodic preventive maintenance required
• Incremental energy usage varies, depending on HVAC system capacity and ambient outdoor conditions

Switching thermostats from “Auto” to “On” or adjusting building HVAC control systems to disable Demand Controlled Ventilation (DCV)

No

No

• Periodic preventive maintenance required
• Incremental energy usage varies, depending upon fan energy consumption

Using fans to increase effectiveness of open windows

< $100

Yes

• No ongoing maintenance requirements
• Incremental energy usage varies, depending on HVAC system capacity and ambient outdoor conditions

Repositioning supply/exhaust diffusers to create directional airflow

< $100

No

• No ongoing maintenance requirements
• No incremental energy usage

Adding in-room HEPA fan/filter systems

$500 (approximately)

• Must inspect/replace HEPA filter per manufacturer instructions
• Low incremental energy usage

Adding upper room UVGI

[Typical classroom requires 2-3 fixtures]

< $1500 (approx. per fixture)

No (unless manual activation)

• Must clean/inspect/replace UVGI lamps per manufacturer instructions
• Low incremental energy usage

Adding in-duct UVGI to treat moving air

Varied, more cost-effective (<$0.25/cfm) with larger systems

No

• Must clean/inspect/replace UVGI lamps per manufacturer instructions
• Low incremental energy usage

Ventilation Frequently Asked Questions