Air Purifiers

This fact sheet provides information on air purifiers, including the different technologies used, selection, and use. Use of air purifiers can be an important strategy for helping to improve indoor air quality (IAQ).  For additional information on IAQ, please consult the EH&S guide on IAQ.

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Strategies for improving IAQ

The majority of indoor air quality issues result from poor ventilation, so simply increasing ventilation can often mitigate concerns. The three most effective ways to control indoor air pollution are (in order of most to least effective): 

• Source Control: Eliminate or control the sources of pollution, for example: Cleaning regularly to prevent dust accumulation
• Ventilation: Dilute and exhaust pollutants through outdoor air ventilation, for example: Increasing room air change rates by opening windows and maintaining HVAC systems
• Air Cleaning: Remove pollutants through proven air cleaning methods, for example:Using an air cleaner equipped with a HEPA filter

Using a portable air cleaner is the least effective way to clean the air, but it can be a good supplement to ventilation. If you decide to purchase a portable air cleaner, filtering air cleaners with High Efficiency Particulate Air (HEPA) filters are recommended because these do not emit ozone but do remove particulate matter from the air.  Electronic air cleaners (e.g. ionizers) and ozone-generators are NOT recommended due to the health and safety concerns of the ozone and other potential pollutants they can emit.

Ozone-Generating Air Cleaners

Ozone is a colorless gas that is found naturally in the Earth&#;s upper atmosphere and protects us from harmful ultraviolet solar rays, though it also can be formed at ground level. It is known to cause free-radical formation in biological systems, which damage tissues. One mechanism of damage results from ozone-olefin reactions, and another from ozone reacting with electron donors (such as glutathione). Inhalation of relatively small amounts of ozone can cause coughing, chest pain, throat irritation, and shortness of breath.

Ozone-generating air cleaners are devices that purposefully create ozone to clean the air through chemical interactions that alter the compounds of pollutants, yet this is also the mechanism by which ozone exerts harmful health effects. Furthermore, ozone is generally ineffective in controlling indoor air pollution at lower concentrations. In other words, if the concentration of ozone in the room is high enough to be effective in cleaning the air, then it&#;s also high enough to create an inhalation hazard for the people in the room.  Additionally, ozone may even react with existing chemicals in the air to create harmful by-products (e.g. formaldehyde).  For these reasons, EH&S does not recommend the use of ozone generating or electronic air purifiers under any circumstances.

Electronic Air Cleaners

Electronic air cleaners (including ionizers, electrostatic precipitators, hydroxyl generators, and UV light) use electric voltage to convert oxygen molecules, or other species, into their charged ionic components that inactivate airborne contaminants, in a process called bipolar ionization (BPI). The ionic components of oxygen are reactive radicals that are capable of removing hydrogen from other molecules. In the event of bipolar ionization, the positive and negative ions surround air particles, destroying present germs and pathogens, and the added mass helps the air particles to fall to the floor and get pulled into the building&#;s air filter. However, BPI can emit ozone and other free-radical species as a by-product, and may be less effective than other cleaning technologies since charged particles in the air sometimes stick to surfaces in the room (e.g. floors and walls) rather than getting filtered away.

Filtering Air Cleaners

Filtering air cleaners pass air through a filter, where polluting particles or gases become sequestered, and return the cleaned air into a room. There is some popular controversy surrounding the extent to which air cleaners can reduce the presence of larger particles (such as pollen, house dust allergens, mold spores, and animal dander), but most of these large particles settle on surfaces in the home or office, and cannot be removed by an air cleaner unless they are disturbed and re-suspended in the air. Therefore, regular cleaning is the best way to remove larger allergens.

Carbon Filters

Volatile Organic Compounds (VOCs) are compounds that have a high vapor pressure and low water solubility. VOCs are emitted as gases from certain solids or liquids, many of which can be found in household materials.  VOCs are generally responsible for odors.  Sources of VOCs include: paints, lacquers, paint strippers, cleaning supplies, building materials and furnishings, office equipment such as copiers and printers, correction fluids, craft materials like glues and adhesives, permanent markers, photographic solutions, cooking, pets and occupants. Carbon filters do not appreciably remove particles from the air.

HEPA Filters

HEPA (high efficiency particulate air [filter]) is a type of pleated mechanical air filter. This type of air filter can theoretically remove at least 99.97% of any airborne particles with a size of 0.3 microns (µm), which can include dust, pollen, mold, and bacteria.  HEPA filters are rated for containment ability of 0.3 μm particles because these are the most challenging size to capture, which means that efficiency of capture for other size particles is higher than 99.97%.  HEPA filters do not remove gases or VOCs from air.

Choosing the appropriate filter for a room

What is a Clean Air Delivery Rate?

Air cleaning units have air volume limitations which are identified by a &#;Clean Air Delivery Rate&#; (CADR). A CADR is the cubic feet per minute (CFM) of air that has had all the particles of a given size distribution removed. CADR indicates the volume of filtered air an air cleaner delivers per unit time, with three separate scores for smoke, pollen and dust, which represent different particle sizes. The higher the CADR number for each pollutant, the faster the unit filters the air for pollutants of a particular size range.

Room Size

The Association of Home Appliance Manufacturers (AHAM) recommends that the CADR of your air cleaner is equal to at least two-thirds of the room&#;s area. For example, a room with 120 square feet (10 ft by 12 ft) should have an air cleaner with a smoke CADR of at least 80. If your ceilings are higher than 8 feet, an air cleaner rated for a larger room will be necessary.

Particle Size

A carbon filter is the most appropriate unit for cleaning gaseous VOCs out of the air, such as when new furniture is off-gassing. However, if you notice an odor in a room, please contact EH&S first for an evaluation. The best course of action is to eliminate and control the odor&#;s source, and EH&S can perform an assessment to do that and develop a remediation plan.

HEPA filters are better for cleaning particles out of the air. The diameter specification of 0.3 microns responds to the worst case, or most penetrating particle size (MPPS). Particles that are larger or smaller are trapped with even higher efficiency. Using the worst case particle size results in the worst case efficiency rating (i.e. 99.97% or better for all particle sizes). Minimum Efficiency Reporting Values, or MERVs, report a filter&#;s ability to capture larger particles between 0.3 and 10 microns (µm). EH&S recommends HEPA filters for particles and aerosols.

Care and Maintenance

All filters need regular replacement as specified by the manufacturer in the product user manual. If a filter is dirty and overloaded, it will not work well. With proper care and upkeep, the portable air cleaner will continue to function properly and filter the air. Consider purchasing replacement filters with the air cleaner.  Carbon filters can passively absorbs VOCs from the air, so please ensure these remain sealed until installed in the unit to prolong their lifetime.

Do indoor plants clean the air?

According to the EPA, there is currently no evidence to suggest that a reasonable number of houseplants would be effective in removing significant amounts of pollutants from indoor air.  Indoor houseplants should not be over-watered because overly damp soil may promote the growth of microorganisms which can affect allergic individuals.

For more information:

EPA&#;s Guide to Air Cleaners in the Home https://www.epa.gov/sites/production/files/-07/documents/guide_to_ai...

EPA Indoor Air Quality website www.epa.gov/indoor-air-quality-iaq

Association of Home Appliance Manufacturers (AHAM) https://www.aham.org/

Pryor, W. A. (, November). Mechanisms of radical formation from reactions of ozone with target molecules in the lung. Free radical biology & medicine. Retrieved December 6, , from https://pubmed.ncbi.nlm.nih.gov//.

The FSG Blog. (, September 4). How bipolar ionization inactivates airborne pathogens and viruses in facilities. FSG Electric & Lighting. Retrieved December 6, , from https://fsg.com/bipolar-ionization-inactivate-pathogens-viruses-facilities/.

How to Choose a Room Air Cleaner

Clean air is not just important for the outdoor environment, it is important for your indoor living space. Follow the guidance below to help you choose the right room air cleaner.

Why Should I Buy a Room Air Cleaner?

Choosing the right portable room air cleaner (or air purifier) can help improve indoor air quality for you and your family. Indoor air contains pollutants that can affect human health and cause symptoms such as asthma and allergies. Some pollutants, such as wildfire smoke and pollen, can come from outdoors. Other pollutants are emitted from indoor sources and activities, such as cooking, cleaning, secondhand smoke, building materials, consumer products, and home furnishings. The best way to improve indoor air quality is to reduce sources of pollutants and to ventilate with clean outdoor air. However, using a portable air cleaner can help improve indoor air quality. When used properly and combined with other best practices recommended by CDC and other public health agencies, room air cleaners can also help reduce airborne pathogens in a home, such as the virus that causes COVID-19.[1]

For guidance about what to look for when choosing an air cleaner for your home, check out the steps below.

How Does a Room Air Cleaner Work?

Room air cleaners can use a variety of filtration methods to purify your indoor air. Many models use fans along with a combination of filters, such as High-Efficiency Particulate Air (HEPA) filters, that catch particles, and filters designed to remove gases, such as activated carbon filters. Others use electrostatic filters or precipitators that attract particles to purify the air.

Some air cleaners have an auto-setting, using sensors to adjust the speed of filtration in a space. Other models offer multiple speed settings that users manually select to choose the speed at which the air is filtered. Higher fan speeds and longer run times will increase the amount of air filtered, which will typically increase the amount of clean air.[2] With long run times comes significant energy use, making it even more important to find an energy-efficient model to save money while cleaning your indoor air.

In addition, some units are marketed as having quiet operation because they do not have a fan. However, units that do not have a fan are typically much less effective.[3]

What Should I Consider in Purchasing a Room Air Cleaner?

1. Find Your Room Size

Portable room air cleaners are designed to filter the air in a single room or area. Because there are many sizes of room air cleaners, determine the square footage of the room where you will use the room air purifier most often. Many devices specify a Clean Air Delivery Rate (CADR). Following the table in the next section, you can select a model with the CADR that aligns with the square footage of your room. Larger models use more energy, so unless you experience frequent, high air pollution events like wildfire smoke, choose a model that is sized appropriately to fit your room for the most efficient air purification. Oversizing your room air cleaner can result in paying a higher initial cost and increased operating costs (from increased energy consumption to  more expensive filter replacement).

2. Review the Clean Air Delivery Rate

The CADR measures how quickly a unit delivers filtered air and can be used to measure the performance of a room air cleaner. The higher the CADR, the larger the area the air cleaner can serve. It is important to select a room air cleaner specific for your room size. A general rule to consider is the &#;2/3 rule&#; &#; find an air cleaner with a CADR that is at least 2/3 the square footage of the space. Use the table below to help estimate which air cleaner is right for your home.

Portable Air Cleaner Sizing for Particle Removal

Room Area (ft2)

100

200

300

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400

500

600

700

800

900

Minimum CADR

65

130

195

260

325

390

450

515

580

Note: CADRs are calculated based on an 8-foot ceiling; if you have higher ceilings, you may want to select an air cleaner with a higher CADR. The CADR listed is based on the CADR required to remove 80% of smoke particles, assuming one room air exchange per hour, and the rating is typically measured at the air cleaner&#;s highest speed.

CADRs for air cleaners are measured across three common particulate types &#; smoke, dust, and pollen. Think through the following considerations in understanding which CADR(s) &#; smoke, dust, pollen or all three &#; may be most relevant for you:

Allergies: If you suffer from allergy symptoms, check the CADR for the particulates most relevant to your allergies&#;dust, pollen, or smoke. For example, if you have spring allergies and live in an area with high pollen count, look for air cleaners with a high pollen CADR.

Please note that room air cleaners, by themselves, may not address all allergy issues within a home, and additional remediation steps may be required. For example, some large allergens may settle out of the air quickly and accumulate on surfaces like furniture or floors before they reach an air cleaner, which may be addressed through regular vacuuming, mopping, and dusting. Another example is mold, which requires removing the moisture source to clean up the mold and to prevent further mold growth.

COVID-19: By itself, air cleaning or filtration is not enough to protect people from COVID-19. But when used properly along with other best practices recommended by CDC, air purifiers can be part of your plan to reduce the potential of airborne transmission indoors1. Be sure it meets at least one of the following criteria on the air cleaner packaging, label, or website description:

1. The filter is designated as High-Efficiency Particulate Air (HEPA),
2. It has high CADR rating for smoke (vs. pollen or dust) because smoke has a smaller particulate size than pollen or dust,
3. The manufacturer states that the device will remove most particles in the size range below 1 µm.

Wildfires: You may decide to purchase an air cleaner as part of your preparation for a wildfire emergency to help improve your indoor air quality during a wildfire smoke event. If you experience frequent, very high smoke concentrations, you may want to purchase an air cleaner with a higher CADR than suggested in the table above.

3. Ensure the Air Cleaner Does Not Emit Ozone

Some air cleaning technologies may emit ozone, which is a known lung irritant. In fact, there are products called ozone generators, that intentionally generate ozone, that are marketed as air cleaning devices. Even small amounts of ozone can cause chest pain, coughing, shortness of breath, and throat irritation.[1] All ENERGY STAR certified products are third-party verified to not exceed safe levels of ozone generation.

4. What is Air Changes per Hour (ACH)?

Air changes per hour (ACH) is another metric sometimes reported by the manufacturer. ACH primarily measures ventilation &#; or how fast air is added and removed, or exchanged &#; in your home or room in an hour. Higher amounts of ventilation help to remove particulates and other pollutants; however, air cleaners don&#;t actually ventilate. Modern homes are designed and built to reduce energy loss so your home maintains its temperature as efficiently as possible, and as such, we add ventilation into homes through vent fans in kitchens and bathrooms, energy-recovery ventilators, or opening a window. Adding a room air cleaner into your space simulates ventilation by removing most particulates from the air that passes through the device. The rate at which air is treated by an air cleaner can also be measured as ACH, for a specified room size.

EPA recommends looking for air purifier models that provide at least 4.8 ACH for the recommended room size to help improve your indoor air quality. A device labeled to provide 4.8 ACH in a 10&#;x12&#;x8&#; room can treat the air in a room of that size 4.8 times each hour. In general, a device with much higher ACH for a room is not recommended, as it provides minimal air quality improvements for higher energy costs.

It is important to note that claims regarding ACH and CADR have not been substantiated unless the product has been tested to the ANSI/AHAM AC-1 test method through a program that third-party verifies products such as the ENERGY STAR program.

5. Look for the ENERGY STAR!

Air cleaners that are ENERGY STAR certified can help you save money and the planet! Room air cleaners that have earned the ENERGY STAR are 27% more energy-efficient than standard models. If all room air cleaners sold in the United States were ENERGY STAR certified, the energy cost savings would grow to more than $400 million each year, and more than 9 billion pounds of greenhouse gas emissions would be prevented, equivalent to the emissions from nearly 900,000 vehicles.

Models that earn the ENERGY STAR are independently certified based on testing in an EPA-recognized lab to ensure they save energy and money without sacrificing performance. ENERGY STAR certified room air cleaners measure energy efficiency by using a CADR-to-watt ratio, which means they deliver the same amount of filtered air as a standard model while using less energy. 

Please note that EPA does not endorse manufacturer claims regarding the degree to which a specific product will produce healthier indoor air.

Where Do I Find an ENERGY STAR Certified Room Air Cleaner?

Ready to start looking for the best air cleaner option for your home? Use the ENERGY STAR Product Finder to find and compare certified models from a wide variety of manufacturers. You can also find available rebates in your area.

Other Tips!

• Check the filter &#; If a filter is dirty, it won&#;t work as well. Review the product manual for recommendations on how often to replace the filter.
• Placement &#; Read the product manual or instructions on where to place your room air purifier to achieve optimum performance. Make sure the airflow is not obstructed. Keep air cleaners away from curtains and items that might block airflow.

[1] EPA, Ozone Generators that are Sold as Air Cleaners, https://www.epa.gov/indoor-air-quality-iaq/ozone-generators-are-sold-air-cleaners.

[1] EPA, Air Cleaners, HVAC Filters, and Coronavirus (COVID-19), https://www.epa.gov/coronavirus/air-cleaners-hvac-filters-and-coronavirus-covid-19.

[2] EPA, Guide to Air Cleaners in the Home 2nd Edition, https://www.epa.gov/sites/default/files/-07/documents/guide_to_air_cleaners_in_the_home_2nd_edition.pdf, July .

[3] EPA, Residential Air Cleaners: A Technical Summary 3rd Edition, https://www.epa.gov/sites/default/files/-07/documents/residential_air_cleaners_-_a_technical_summary_3rd_edition.pdf, July .

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