Important Basics about Air purifiers

Posted September 10, 2019 by DonnPeffley

Air purifiers alone cannot adequately remove all indoor pollutants from homes. This is especially true when the sources emit a large amount of pollution, or when the pollutants settle rapidly on surfaces.

Many types of indoor pollution problems are found in homes. The most effective approach to solving these problems is usually to remove or reduce the sources of indoor pollution.

Ventilation also helps remove indoor air pollutants, but not as effectively as source reduction.Air purifiers can also be helpful when used along with source reduction and ventilation. The best solution to the air pollution within your home will depend, of course, on the type and scope of the particular problem, your home’s characteristics, and your family’s specific health concerns and budget. The information below will help you decide if an air cleaning device would be useful as a part of your solution to indoor pollution, and if so, what type might best meet your needs. Many of the principles discussed here also apply to offices, schools, and vehicles.

Important Basics about Air purifiers

Air purifiers alone cannot adequately remove all indoor pollutants from homes. This is especially true when the sources emit a large amount of pollution, or when the pollutants settle rapidly on surfaces. The most common air purifiers will remove some of the particles from the indoor air, but will not effectively remove certain types of pollutants, such as very fine particles, bacteria and viruses, carbon monoxide, radon, odors, and some allergens such as those from roaches and pets. Specific types of filtration devices and air cleaners are needed for such pollutants: high efficiency filters and air cleaners, discussed further below, can remove very small particles, and devices with sufficient quantities of charcoal (carbon), Purafil, or similar sorbent materials can remove some odors and gaseous pollutants.

Air purifiers are available as portable, stand-alone appliances or as filters or air cleaners in a central air system. Portable units are usually best for single room use, rather than multiple room or whole-house use, because of their limited capacity to circulate large volumes of filtered air. Air cleaners and specialized filters designed for use in central heating and air conditioning systems, if properly configured and maintained, may have the greatest potential to improve your entire home’s air quality because most can circulate very large volumes of filtered air throughout the home.

The health benefits of air purifiers are not clear, based on the limited scientific evidence that is currently available, but high efficiency filtration may be an effective strategy, and it is currently being studied by the California Air Resources Board (ARB) and others. However, it is clear that you should never use an air cleaner that deliberately produces ozone, sometimes called “ozone generators”. Ozone generators cause indoor pollution and do not clean the air.

The ARB has taken regulatory action to limit ozone emissions from portable air cleaners, as discussed later in this fact sheet.

How Is a Filter’s Particle Removal Efficiency Rated?

Several standardized methods have been developed by trade groups or organizations to allow comparison of filters. These standards are designed to rate a filter’s particle removal efficiency.

No recognized standard has been developed to rate a filter’s gaseous or chemical vapor removal rate. Currently, most filters are rated using the Minimum Efficiency Reporting Value (MERV) rating system that has been developed by the American Society of Heating,

Refrigerating, and Air-Conditioning Engineers (ASHRAE). The MERV rating system ranges from a low of 1 to a high of 20, with each rating indicating the percentage of particles removed within a specific particle size range (measured in microns, where a micron is one millionth of a meter).

For illustration purposes, the diameter of a human hair is about 40-50 microns wide. Particles smaller than 10 microns, especially those smaller than one micron, are of greatest concern to health because they penetrate farthest into the lungs. Generally, as the MERV rating increases, so does the filter’s removal efficiency for smaller-sized particles.

How Do I Improve My Existing Air Filter in My Central Heating and Air System?

As a first step, you should check your central air system filter and upgrade to a higher MERV filter if possible. This may be all that is needed to provide clean air in your home. Central forced air systems in homes usually accommodate a rectangular, one-inch thick fiberglass filter that slides underneath the furnace fan, or into a wall or ceiling register where the air returns to the furnace.

These filters typically remove less than 20% of the particles that reach the filter and are usually rated at only MERV 1-4. They are disposable and typically cost $2 to $4.

Often, merely upgrading this filter to a medium- or high efficiency filter will improve the air quality in your home.

Medium-efficiency filters are typically pleated, woven material. Those that use synthetic media may help reduce bacterial growth on the filter itself, and recent research shows that synthetic filters may not emit formaldehyde as some fiberglass filters do. They are rated at 20% to greater than 70% efficiency for removing particles of 3 to 10 microns and carry a MERV 5 to 8 rating. Mediumefficiency, disposable one-inch filters cost about $4 to $9. Washable medium efficiency filters for residential use tend to carry a MERV 8 or lower rating, and an average size filter costs about $20. Some medium-efficiency filters use static electricity created by air flow, but the effectiveness of such filters may decline as the static charge decreases over time.

Higher efficiency filters begin to remove much smaller particles and are rated at less than 50% to greater than 80% efficiency for removing particles 1 to 3 microns in size, and they carry a MERV 9 to 12 rating. 

They cost about $10 to $20 for disposable one-inch models. Before using a filter rated MERV

9 or higher, you may want to have an HVAC technician check your central system to be sure it can handle any added airflow resistance that higher efficiency filters may cause. Some higher efficiency filters are designed to avoid excessive airflow reduction (e.g. the filter is highly pleated, or has deeper pleats), so you may want to use one of those.

High efficiency filters remove very small particles and are rated as removing less than 75% to

greater than 95% of particles 0.3 to 1 microns in size, and carry a MERV 13 to 16 rating.

Premium one-inch, MERV 13 disposable filters are becoming more common and cost up to about $25. 

Most high efficiency filters are typically two to five-inches deep (deeper filters are meant to prevent reduced airflow and typically have a longer service life), and may only be installed in central air systems

designed or modified to accommodate them. High efficiency filters are sometimes mistakenly called HEPA or True HEPA ( filters. HEPA filters are 99.97% efficient

for removing particles less than 0.3 microns, require correctly sized fans and central duct work, and are now becoming more readily available for use in home central forced air systems.

Note that many filters sold in retail stores and online do not show a MERV rating; you may have

to check the manufacturer’s website or contact the manufacturer to determine the MERV rating

of the filter you wish to purchase. Sometimes other information is provided on the filter packaging that provides information similar to the information on the MERV table above; this can help you choose a suitable filter even if the MERV value is not specified.

Before purchasing any portable air cleaner, be sure to check ARB’s list of certified air cleaners, available at

ARB-certified air cleaners typically emit little or no ozone, and may not emit more than 0.050 parts per million (ppm) of ozone. (One ppm is equal to about one drop of water in 15 gallons of water). They also meet the applicable electrical safety standards of Underwriters Laboratories, Inc. Confirm the presence of an ARB-certified label on the device’s packaging. In addition, ARB has published a list of potentially hazardous ozone generators sold as air cleaners that you should avoid; that list is available at

Where Can I Obtain More Information?

The references listed below provide extensive, useful information on residential air cleaning

devices. If you have questions or would like to receive a printed copy of ARB’s Indoor Air

Quality Guidelines and Supplements or other documents listed below, please contact us at:

Indoor Air Quality and Personal Exposure Assessment Program

Research Division

California Air Resources Board

P.O. Box 2815

Sacramento, CA 95812

Phone: 916-322-8282, Fax: 916-322-4357

This document is also available on the Internet at the following URL:

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Last Updated September 10, 2019