Ventilation in buildings
CDC recommends a multi-pronged approach to reducing exposures to SARS-CoV-2, the virus that causes COVID-19. This approach includes the use of multiple mitigation strategies, including improved building ventilation, to reduce the spread of the disease and decrease the risk of exposure. In addition to improved ventilation, the layered approach includes physical distancing, mask wearing, hand hygiene, and vaccination.
SARS-CoV-2 virus particles spread more easily between people indoors than outdoors. Indoors, theconcentration of virus particles is often higher than outdoors, where even a light wind can quickly reduce concentrations. Indoors, ventilation mitigation strategies can help reduce the concentration of viral particles. The lower the concentration, the less likely viral particles are to be inhaled into the lungs (which can reduce the inhaled dose), come into contact with the eyes, nose, and mouth, or fall into the air to accumulate on surfaces. Protective ventilation practices and interventions can reduce airborne concentrations and reduce the overall viral dose to occupants.
Reoccupation of a building during the COVID-19 pandemic should not, in most cases, require new building ventilation systems. However, upgrading or improving the ventilation system can increase the supply of clean air and dilute potential contaminants. Consult experienced heating, ventilation and air conditioning (HVAC) professionals when considering changes to HVAC systems and equipment. Buildings that provided healthy, code-compliant indoor air quality prior to the pandemic can be improved for pandemic occupancy using less expensive interventions. Below is a list of ventilation interventions that can help reduce the concentration of viral particles in the air. They represent a list of "tools in the mitigation toolbox," each of which can contribute to a reduction in risk. Implementing multiple tools at the same time is consistent with CDC's layered approach and will increase the overall effectiveness of ventilation interventions. These ventilation interventions can reduce the risk of exposure to the virus and reduce the spread of disease, but they will not completely eliminate the risk.
Although the tool list can be applied universally in indoor environments, their application to different types of buildings, occupancies, and activities under environmental and seasonal changes can be challenging. The specific combination of tools chosen for use at any given time may change. It will be up to the building owner or operator (with expert consultation if necessary) to identify the appropriate tools for each building throughout the year. In addition to buildings, vehicles - including public transportation such as buses, subways, trains, school buses, carpools and vanpools - are also areas where ventilation improvements can be applied to reduce the spread of the virus and decrease the risk of exposure.
Tools for improving ventilation
Some of the following interventions are based on the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Guidance for Building Operations During the COVID-19 Pandemicpdf iconexternal icon. Not all interventions will work in all scenarios. Use caution in highly polluted areas when increasing outdoor air ventilation.
Consider using some or all of the following tools to improve ventilation:
Increase the introduction of outside air:
Open outside air dampers beyond the minimum settings to reduce or eliminate HVAC air recirculation. In mild weather, this will not affect thermal comfort or humidity. However, this can be difficult to do in cold, hot or humid weather, and may require consultation with an experienced HVAC professional.
Open windows and doors, weather permitting, to increase outside air circulation. Do not open windows and doors if doing so poses a safety or health risk (e.g., fall hazard, triggering asthma symptoms) to building occupants. Even a slightly open window can introduce beneficial outside air.
Use fans to increase the effectiveness of open windows:
To do this safely, the placement of fans is important and will vary depending on the room configuration. Avoid placing fans so that contaminated air can flow directly from one person to another (see FAQ below on using fans indoors). A useful strategy is to use a window fan, safely and securely placed in a window, to exhaust air from the room to the outside. This will draw outside air into the room through other open windows and doors without generating strong drafts in the room. Similar results can be achieved in larger installations by using other ventilation systems, such as gable fans and roof ventilators.
Ensure that ventilation systems are operating properly and providing acceptable indoor air quality for the current occupancy level of each space.
Rebalance or adjust HVAC systems to increase total airflow in occupied spaces where possible.
Disable any demand-side ventilation (DSV) controls that reduce air supply based on occupancy or temperature during occupied hours. In homes and buildings where HVAC fan operation can be controlled by the thermostat, set the fan to the "on" position instead of the "auto" position, which runs the fan continuously, even when heating or cooling is not needed.
Improve central air filtration:
Increase air filtration - external icon - to the highest level possible without significantly reducing design airflow. Increased filtration efficiency is especially helpful when outside air supply options are limited.
Ensure that air filters are properly sized and within their recommended life expectancy.
Inspect the filter housing and brackets to make sure they fit properly and that air is flowing around the filter instead of through it.
Ensure that toilet 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 whenever these spaces are occupied. Consider running them even when the specific space is not occupied to increase overall ventilation in the occupied building.
Consider using Portable high efficiency particulate air (HEPA) ventilation/filtration systems. to improve air cleaning (especially in high-risk areas such as a nurse's office or areas frequently inhabited by people with a higher probability of having COVID-19 and/or an increased risk of contracting COVID-19). See the FAQ below on HEPA filters and portable HEPA air cleaners. (Note: Portable air cleaners that use less efficient filters than HEPA filters also exist and can help clean the air in the room. However, they must be clearly labeled as non-HEPA devices).
Generate clean to less clean air movement by evaluating and repositioning, if necessary, supply louvers, exhaust grilles and/or damper settings. See the FAQ below on directional airflow. This recommendation is easiest to accomplish when the supply and exhaust points are located in a ceiling grille system.
Consider using ultraviolet germicidal irradiation (UVGI ) as an additional treatment to inactivate SARS-CoV-2, especially if options for increasing room ventilation and filtration are limited. Upper room UVGI systems can be used to clean air in occupied spaces, and duct UVGI systems can help improve air cleaning in central ventilation systems.
In non-residential environments, consider running the HVAC system at maximum outdoor airflow for two hours before and after building occupancy.
The ventilation interventions listed above come with a range of initial and operating costs that, along with risk assessment factors-such as community incidence rates, facemask compliance expectations, and room occupancy density-can affect tool selection. Examples of cost estimates for ventilation interventions include:
No cost: opening windows; dedicated exhaust ventilation inspection and maintenance; deactivating DCV controls; repositioning outside air dampers.
Less than 100 $: use of fans to increase efficiency of open windows; repositioning of supply/exhaust diffusers to create directional airflow.
500 $ (approx.): adding portable HEPA fan/filter systems.
1500 to 2500 $ (approx): add UVGI system in upper rooms.