Air Cleaners, HVAC Filters, and Coronavirus (COVID-19)

Portable air cleaners (also known as air purifiers) may be particularly helpful when additional ventilation with outdoor air is not possible without compromising indoor comfort (temperature or humidity), or when outdoor air pollution is high.

Caution: The use of air cleaners alone cannot ensure adequate indoor air quality, particularly where significant pollutant sources are present and ventilation is insufficient. Read EPA’s “Guide to air cleaners in the home" (PDF).

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

How to select a portable air cleaner for a residence that can effectively remove viruses

Choose a portable air cleaner that is intended for the room size in which it will be used and be sure it meets at least one of the following criteria:

1. it is designated as High-Efficiency Particulate Air (HEPA),
2. it is CADR rated for smoke, or
3. the manufacturer states that the device will remove most particles in the size range below 1 um.

Most manufacturers provide this information on the air cleaner packaging, label or website description.

Do not use air cleaners that intentionally generate ozone in occupied spaces or that do not meet state regulations or industry standards for ozone generation.

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

Do not use ozone generators in occupied spaces.

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 or other events when air quality is poor and other indoor air filtration options are unavailable.

Evidence from multiple studies indicates that well-built DIY air cleaners can be of comparable effectiveness to commercial air cleaners in reducing airborne particles (including viral particles). However, their performance does vary based on the design selected and the quality of materials and assembly. Each time a DIY air cleaner is re-assembled after changing a filter, its performance may be different. Commercial devices, on the other hand, are tested for performance, and this performance information can be used to match them to the size of a room.

Therefore, 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). However, this recommendation should not be interpreted to discourage the use of DIY air cleaners in circumstances when commercially available portable air cleaners or other products of known performance are not available. Using a DIY air cleaner that was inadequately designed or assembled does not worsen indoor air quality and may still offer some benefits.

To address concerns that box fans in DIY air cleaners might be associated with increased risk of fire, 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 are many possible DIY designs and variations of those designs, and limited tests have been done to see how well some of them work.

• Initial costs for single filter designs can be lower than designs that use multiple filters, but operation costs for single filter designs may be higher, for the same performance.
• Multi-filter designs can be harder to put together, and it can be harder to replace their filters. They are also bulkier, and more difficult to move around than single filter designs. However, multi-filter designs generally have superior performance, and they can be more cost effective.
• Using multiple single-filter units in the same room is also worth considering, when balancing performance, costs, space, and ease of assembly for your specific needs.

Some example designs are: How to build a low-cost air filter (pdf) (from the University of Washington, School of Public Health) Spanish version (pdf) 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 recommends not using DIY air cleaners built with older model box fans (built before 2012), because their fire hazard is unknown. If older fans are used, they should not be used unattended or while sleeping.
• Use filters of approximately the same shape and size as the box fan. Filters that only partially overlap the fan will result in reduced performance. Filters that are bigger than the fan may be unnecessarily more expensive.
• 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.

Features That Can Improve DIY Air Cleaner Performance

• Increase the number of filters in the design. Some designs can have 2, 3, 4 or 5 filters. This feature generally improves performance the most.
• 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. Thicker filters generally provide a large improvement in 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. This approach generally provides a large improvement in performance. You can use cardboard, duct tape, or wood to make the cover – some DIY fan designers call these “shrouds”. This cover can also be made from the cardboard box in which the fan was packaged, at no additional cost.
• 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. This is less important for performance than the other features listed above.

Tips For Operating a DIY Air Cleaner

• Run the device whenever the room is occupied.
• Make sure the device is free from obstructions and air can flow through it.
• Run the device at the highest speed setting acceptable to you.
• Check the filter(s) regularly and replace when dirty.

Air Cleaner Operation

• Place DIY air cleaners in the rooms where people are spending the most time, in general. To protect especially vulnerable people, place the air cleaner where they spend most of their time. If someone is isolating because they could be transmitting an infectious disease (such as COVID-19 or flu), place the air cleaner nearest them.
• Make sure air can flow to the device and away from it, keeping it clear from obstructions. A central place in a room works best, but it is not essential as long as air flow is free. Do not operate an air cleaner inside a closet, as this would limit its effectiveness.
• Consider running DIY air cleaners the entire time a space is occupied. The longer they run, the more particles they will likely remove.
• Consider running the fan at higher speed settings. Air cleaning performance improves at higher fan speeds, although noise and air movement in the room also increase.
• 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.

Additional Information

• 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: Improving Ventilation in Your Home CDC Interactive Ventilation Tool (for Homes)

Return to Indoor Air and Coronavirus (COVID-19).