Do Air Purifiers Work?

One of the common responses many folks have when they are feeling sick is to fire up an air purifier. This is particularly true when the sick person is a young child with a form of respiratory disease such as childhood asthma. Clients ask, “When do air purifiers work and when not. Is it worth the cost of the purchase and operation?”

The Environmental Protection Agency (EPA) is the primary authority on air quality in the US. EPA does not certify or recommend any particular brands of air cleaning devices but does offer guidance.  They point out that indoor air contaminants generally fall into two categories:  particulate matter and gaseous pollutants.  Particulate matter includes dust particles, asbestos fibers and mold spores.  Gaseous pollutants include volatile organic compounds (VOCs), formaldehyde and environmental tobacco smoke.

There are several types of air cleaning devices and filters on the market. Air cleaning devices can be installed directly into the heating, ventilation, and air conditioning (HVAC) system or they can be portable devices.  For particulate matter contaminants, there are two primary types of filters that are available. There are mechanical filters that capture particles on filter materials such as high efficiency particulate air (HEPA) filters, and electronic air cleaners that use a process called electrostatic attraction to trap charged particles.

Gaseous pollutants may either be removed or chemically changed. Gas-phase air filters remove pollutants through the use of a sorbent, such as activated carbon or potassium permanganate. Ultraviolet germicidal irradiation (UVGI) cleaners use ultraviolet radiation from UV lamps to destroy biological organisms on coils and other HVAC surfaces.  Photocatalytic oxidation cleaners (PCO) use a UV lamp in conjunction with a catalytic surface to chemically transform gaseous pollutants into inert byproducts such as water and carbon dioxide.   Ozone generators use UV light or an electrical charge to create ozone, which reacts with gaseous pollutants to chemically transform them to harmless byproducts.

With so many different products, how does one decide what type of filter or cleaner to use? It is important to understand the advantages and limitations of each filter before making a decision.  The following information will provide the basics.

1. Is there a contaminant source in the home or office?

One of the cardinal rules of industrial hygiene is the need to identify and control a contaminant before it reaches a receptor population, meaning the people in the building. It is only in rare cases that we allow the contaminant source to remain and try to protect folks with protective equipment. This is because of the many limitations of the equipment and because it requires the people who are affected to properly use the equipment. So it is always more effective to eliminate the contaminant, or control it at its source, rather than protect people as they breathe.

Another factor related to this is that, if you are running an air purifier where the people breathe, the contaminant reaches the purifier and the people simultaneously. So the exposure still occurs, even if you have selected the right filter and the contaminant is reduced. If there is a contaminant source, it is essential that the source be identified and eliminated.

2. What are we trying to filter?

 One fundamental question is, “Are we filtering the right thing?” This is particularly true for biological sources such as mold and bacteria. There is scientific consensus that a variety of contaminant types are produced by microorganisms: particulates such as spores and hyphae, semi-volatiles such as mycotoxins and endotoxins, and volatiles such as alcohols, ketones and aldehydes. The science is unclear as to what folks actually respond to. So how can you filter when you don’t know what the contaminant is?

3. Particulate Filters: Strengths and Weaknesses

Particulate filters are some form of either (HEPA) filter or electronic air cleaners. Mechanical filters capture particles on filter materials. These filters usually come in flat or pleated form, pleated being more efficient due to increased surface area.  These filters, particularly HEPA (high efficiency particulate air) are good at trapping larger particles such as dusts, mites, dander, pollen, some allergens, and mold spores. As these filters are used, they generally become more efficient as they become loaded.  Routine replacement of these filters is necessary.  The disadvantage is many larger particles will settle out in the indoor environment before making their way to the filter.  Mechanical filters also do not remove gaseous or biological pollutants.

Electronic air cleaners use electrostatic attraction to trap charged particles on filter materials. There are two types: electrostatic precipitators and ion generators.  Electrostatic precipitators pull air through an ionizing section, which charges the particles passing through this section.  The charged particles then stick to collector plates that have an opposite charge as the particles.  Ion generators release charged ions into the air by means of corona discharge or UV light, which in turn charge the particles.  These charged particles then stick to nearby surfaces such as furniture and walls.  These electronic cleaners require minimal maintenance.  Like mechanical filters, electronic air cleaners do not remove gaseous or biological pollutants.  There is also the risk of ozone production with the use of these cleaners.  Ozone may be hazardous to human health, and ozone may react with other chemicals in the home forming ultrafine particles or aldehydes.

4. Chemical Filters: Gas Phase

Gas-phase air filters use a sorbent such as activated carbon to remove gaseous pollutants from the air. The activated carbon adsorbs chemicals through weak intermolecular forces. Adsorption can be reversible given different variables such as high temperature and humidity.  Gas-phase air filters may also be impregnated with reactive agents such as potassium permanganate.  These reactive agents react with gases to form a stable chemical bond through a process known as chemisorption to remove the pollutants from the air.  These filters are generally ineffective for highly volatile gases such as formaldehyde. Gas-phase air filters usually have a high initial efficiency, but vapor penetration increases as the filter becomes loaded.  Depending on the level of pollutants in the environment, these filters may require frequent replacement.  Gas-phase air filters are not designed to remove particulates or biological pollutants.

5. Ultraviolet Filters

UVGI cleaners are designed to kill biological contaminants with UV radiation, which is known to have germicidal effects. There are two types of UVGI cleaner designs:  airstream disinfection and surface disinfection.  UVGI cleaners are typically installed in the HVAC system downstream of the filter and upstream of the cooling coils.  Through airstream disinfection the UVGI cleaner is supposed to kill the biological contaminants as the air passes by.  Surface disinfection takes place in a moist area in the system such as the cooling coils.  The UVGI cleaners are somewhat effective in killing viruses, but not very effective in killing bacteria or mold spores.  The bulb strength in these units is not high enough to kill bacteria, and mold spores are resistant to UV radiation.  Longer exposure time may increase the efficiency on surfaces, though airstream disinfection typically does not allow for long exposure times.

PCO cleaners are intended to remove Volatile Organic Chemicals (VOCs). They work by combining oxygen, water, and VOCs on a catalyst surface (typically titanium dioxide) in conjunction with a UV light to form water and carbon dioxide.  There may be incompletely oxidized VOCs leading to higher levels of formaldehyde and acetaldehyde, though there may be a reduction of VOCs overall.  There is also the risk that the degradation of certain VOCs by this method may form other indoor pollutants such as phosgene and chlorides.  PCO combined with a secondary filter of manganese oxide may lower formaldehyde and acetaldehyde levels further than a PCO alone.  Further research is needed to evaluate the effectiveness of these cleaners.

6. Ozone Generators

Ozone generators use corona discharge or UV lights to create ozone, which is dispersed by a fan to the surrounding area. Manufacturers claim that the ozone reacts with particulates, biological contaminants, and gaseous contaminants to disinfect or chemically change contaminants.  Ozone is a regulated pollutant in its own right and is known to be irritating to humans when inhaled.  Scientific studies have shown that ozone levels below allowable public health limits are not effective in controlling indoor air pollutants as manufacturers claim.  Furthermore a buildup of ozone in an enclosed space may induce asthma attacks, coughing, chest discomfort, and other respiratory symptoms.  These ozone generators are generally viewed as ineffective as advertised and potentially harmful to human health as backed by scientific studies.  To learn more about ozone generators, visit the EPA’s web page “Ozone Generators that are Sold as Air Cleaners”.


In summary, EPA recommends the following hierarchy of controls:

  1. Source control – eliminate or control the sources of pollution.
  2. Ventilation – dilute and exhaust pollutants through outdoor air ventilation.
  3. Air cleaning – remove pollutants through proven air cleaning methods.


Air cleaning is a last resort and generally not sufficient on its own. For more in-depth information on air cleaners, visit the EPA’s web pages “Residential Air Cleaners (Second Edition): A Summary of Available Information” and “Guide to Air Cleaners in the Home”.