Air Cleaners, HVAC Filters, and Coronavirus (COVID-19)
When used properly, air cleaners and HVAC filters can help reduce airborne contaminants including viruses in a building or small space. By itself, air cleaning or filtration is not enough to protect people from COVID-19. When used along with other best practices recommended by CDC and other public health agencies, including social distancing and mask wearing, filtration can be part of a plan to reduce the potential for airborne transmission of COVID-19 indoors.
Air cleaners and HVAC filters are designed to filter pollutants or contaminants out of the air that passes thru them. Air cleaning and filtration can help reduce airborne contaminants, including particles containing viruses.
In order for an air cleaner to be effective in removing viruses from the air, it must be able to remove small airborne particles (in the size range of 0.1-1 um). Manufacturers report this capability in several ways. In some cases, they may indicate particle removal efficiency for specific particle sizes (e.g. “removes 99.9% of particles as small as 0.3 um”). Many manufacturers use the Clean Air Delivery Rate (CADR) rating system to rate air cleaner performance. Others indicate they use High Efficiency Particulate Air (HEPA) filters. In order to select an air cleaner that effectively filters viruses from the air, choose: 1) a unit that is the right size for the space you will be using it in (this is typically indicated by the manufacturer in square feet), 2) a unit that has a high CADR for smoke (vs. pollen or dust), is designated a HEPA unit, or specifically indicates that it filters particles in the 0.1-1 um size range.
Air cleaners and HVAC filters in Homes
Where to place a portable air cleaner in your home
Choosing where in your home to place a portable air cleaner to help protect from airborne infections depends on the situation. Put the air cleaner in the room where most people spend most of their time (e.g., a living room or bedroom) unless:
- Someone in a household is especially vulnerable to the risks from infection, then, place the air cleaner where they spend most of their time or
- If someone is isolating because of an active infection, then, place the air cleaner where they are isolating. See CDC guidance for creating isolation spaces - COVID-19 Quarantine and Isolation.
- Read EPA’s “Guide to air cleaners in the home” for more information on HVAC filters and placing and operating a portable air cleaner.
- Learn how to decrease levels of virus particles during and after a guest visits a home. (Centers for Disease Control and Prevention's Interactive Ventilation Tool)
Air cleaners and HVAC filters in Offices, Schools, and Commercial Buildings
The HVAC systems of large buildings typically filter air before it is distributed throughout a building, so consider upgrading HVAC filters as appropriate for your specific building and HVAC system (consult an HVAC professional). The variety and complexity of HVAC systems in large buildings requires professional interpretation of technical guidelines, such as those provided by ASHRAE and CDC. EPA, ASHRAE and CDC recommend upgrading air filters to the highest efficiency possible that is compatible with the system and checking the filter fit to minimize filter air bypass.
Consider using portable air cleaners to supplement increased HVAC system ventilation and filtration, especially in areas where adequate ventilation is difficult to achieve. Directing the airflow so that it does not blow directly from one person to another reduces the potential spread of droplets that may contain infectious viruses.
Air cleaning may be useful when used along with source control and ventilation, but it is not a substitute for either method. Source control involves removing or decreasing pollutants such as smoke, formaldehyde, or particles with viruses. The use of air cleaners alone cannot ensure adequate air quality, particularly where significant pollutant sources are present and ventilation is insufficient. See ASHRAE and CDC for more information on air cleaning and filtration and other important engineering controls.
- See CDC's Interactive School Ventilation Tool to learn how to improve ventilation.
Air Cleaning Devices that use Bipolar Ionization, including Portable Air Cleaners and In-duct Air Cleaners used in HVAC Systems
Some products sold as air cleaners intentionally generate ozone. These products are not safe to use when people are present because ozone can irritate the airways. Do not use ozone generators in occupied spaces. When used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biological pollutants.
Bipolar ionization (also called needlepoint bipolar ionization) is a technology that can be used in HVAC systems or portable air cleaners to generate positively and negatively charged particles. Provided manufacturers have data to demonstrate efficacy, manufacturers of these types of devices may market this technology to help remove viruses, including SARS-2-CoV, the virus that causes COVID-19, from the air, or to facilitate surface disinfection of surfaces within a treated area. This is an emerging technology, and little research is available that evaluates it outside of lab conditions. As typical of newer technologies, the evidence for safety and effectiveness is less documented than for more established ones, such as filtration. Bipolar ionization has the potential to generate ozone and other potentially harmful by-products indoors, unless specific precautions are taken in the product design and maintenance. If you decide to use a device that incorporates bipolar ionization technology, EPA recommends using a device that meets UL 2998 standard certification (Environmental Claim Validation Procedure (ECVP) for Zero Ozone Emissions from Air Cleaners).
Please note that there are many air cleaning devices that do not use bipolar ionization – the device packaging or marketing materials will typically indicate if bipolar ionization technology is being used.
DIY Air Cleaners
Do-it-yourself (DIY) air cleaners are indoor air cleaners that can be assembled from box fans and square HVAC (or furnace) filters. They are sometimes used during wildfire events when air quality is poor and other filtration options are unavailable. There have been questions about whether DIY air filters can be effective in reducing virus particles in indoor environments. DIY air cleaners may provide some benefits for reducing concentrations of viruses and other indoor air pollutants, but research is limited and there are several important considerations explained below.
EPA does not recommend the routine use of DIY air cleaners as a permanent alternative to products of known performance (such as commercially available portable air cleaners). The performance of different DIY air cleaners will vary and cannot be reliably assessed without specialized instruments. Commercial devices have been tested for performance and can be chosen to match the size of a room.
EPA and Underwriter Laboratories evaluated the use of DIY air cleaners and the risk of fire. Fans that were built since 2012 and met UL standard 507 did not pose a fire hazard under the conditions tested in the study. (See Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors for more information.)
Tips - If You Choose to Use a DIY Air Cleaner
- Use government, state, tribal, university or other expert instructions for building the device. There is limited evidence on the effectiveness of DIY air cleaners. There are many possible DIY designs and variations of those designs, and few tests have been done to see how well they work.
- Cost and Design Considerations:
- Initial costs for single filter designs can be lower than designs that use multiple filters.
- Designs that use more than one filter can be harder to put together, bulkier, and more difficult to move than single filter designs.
- Designs with more than one filter may also be harder to disassemble in order to replace the filters.
- Some example designs are:
- How to build a low-cost air filter (pdf) (from the University of Washington, School of Public Health)
- One filter flat against the fan (from the Washington Dept of Ecology)
- Two filters taped with cardboard to form a triangle against the fan (from the Confederated Tribes of the Colville Reservation)
- Four filters used to create an air filtration box, also known as the Corsi-Rosenthal box (pdf) (from the University of California, San Diego)
- Use a newer box fan (made since 2012) with a UL (Underwriters Laboratory) or ETL (Intertek) logo because they have verified safety features to reduce the risk of the fan overheating. EPA does not recommend using DIY air cleaners built with older model box fans (built before 2012), but if they are used, they should not be used unattended or while sleeping.
- Consider running DIY air cleaners the entire time a space is occupied. The longer they run, the more particles they will likely remove.
- When assembling a DIY air cleaner, choose a high-efficiency filter, rated MERV 13 or higher, for better filtration. Align the arrows on the filter to be in the same direction of the air flow through the fan. Create a good seal between the fan and the filter.
- Change the filters periodically. Longer run times, higher fans speeds, and higher levels of air pollution will mean that the filter will be removing more particles from the air, but the filter will also get dirty more quickly. Change the filter when it appears dirty.
- When changing the filter(s), wear gloves, an N-95 respirator or similar, and goggles (without holes) for personal protection. Remove the filters gently - outdoors if possible. Avoid shaking or banging the filters to minimize the release of accumulated dust. Dispose of the filters in garbage bags.
Features that can improve DIY air cleaner performance:
- Cover the outside corners of the front of the box fan, so that air flows only through the center part of the fan where the blades are visible. You can use cardboard, duct tape, or wood to make the cover – some DIY fan designers call these “shrouds”.
- Use a thick HVAC filter that is 2” or 4” thick instead of a 1” filter. Generally, thicker filters are more expensive than thinner filters, but need to be changed less often.
- Increase the number of filters in the design. Some designs can have 2, 3, 4 or 5 filters.
- Improve the seal where the filters are attached to the fan or each other. Seal the edges using duct tape, for example, instead of ties or clamps.
- See EPA Air Cleaners and Air Filters in the Home for more information.
- Read ASHRAE guidance.
- Schools and universities (pdf) (1.93 MB)
- Commercial buildings (pdf) (1.32 MB)
- Multifamily owners/managers (pdf) (1.19 MB)
- Core Recommendations for Reducing Airborne Infectious Aerosol Exposure (pdf) (152.72 KB)
- CDC websites for more information: