Functional schematic of an air purifier

An air purifier or air cleaner is a device which removes contaminants from the air in a room to improve indoor air quality. These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and at reducing or eliminating second-hand tobacco smoke.

The commercially graded air purifiers are manufactured as either small stand-alone units or larger units that can be affixed to an air handler unit (AHU) or to an HVAC unit found in the medical, industrial, and commercial industries. Air purifiers may also be used in industry to remove impurities from air before processing. Pressure swing adsorbers or other adsorption techniques are typically used for this.


In 1830, a patent was awarded to Charles Anthony Deane for a device comprising a copper helmet with an attached flexible collar and garment. A long leather hose attached to the rear of the helmet was to be used to supply air, the original concept being that it would be pumped using a double bellows. A short pipe allowed breathed air to escape. The garment was to be constructed from leather or airtight cloth, secured by straps.[1] In the 1860s, John Stenhouse filed two patents applying the absorbent properties of wood charcoal to air purification (patents 19 July 1860 and 21 May 1867), thereby creating the first practical respirator.[2]

In 1871, the physicist John Tyndall wrote about his invention, a fireman's respirator, as a result of a combination of protective features of the Stenhouse's respirator and other breathing devices.[3] This invention was later described in 1875.[4]

In the 1950s, HEPA filters were commercialized as highly efficient air filters, after being put to use in the 1940s in the United States' Manhattan Project to control airborne radioactive contaminants.[5][6]

The first residential HEPA filter was reportedly sold in 1963 by brothers Manfred and Klaus Hammes in Germany,[7] who created the Incen Air Corporation which was the precursor to the IQAir corporation.[citation needed]

Use and benefits

Dust, pollen, pet dander, mold spores,[8] and dust mite feces can act as allergens, triggering allergies in sensitive people. Smoke particles and volatile organic compounds (VOCs) can pose a risk to health. Exposure to various components such as VOCs increases the likelihood of experiencing symptoms of sick building syndrome.[9]


See also: Impact of the COVID-19 pandemic on public transport § Research and development

A Sharp FU-888SV Plasmacluster air purifier.
The same air purifier, cover removed.

Joseph Allen, director of the Healthy Buildings program at Harvard's School of Public Health, recommends that school classrooms use an air purifier with a HEPA filter as a way to reduce transmission of COVID-19 virus, saying "Portables with a high-efficiency HEPA filter and sized for the appropriate room can capture 99.97 percent of airborne particles."[10]

One fluid dynamic modelling study from January 2021 suggests that operating air purifiers or air ventilation systems in confined spaces, such as an elevator, during their occupancy by multiple people leads to air circulation effects that could, theoretically, enhance viral transmission.[11] However, real-life testing of portable HEPA/UV air filters in COVID-19 wards in hospital demonstrated complete elimination of air-borne SARS-CoV-2.[12] Interestingly this report also showed a significant reduction in other bacteria, fungal and viral bioaerosol, suggesting that portable filters such as this may be able to prevent not only nosocomial spread of COVID-19 but also other hospital-acquired infections.

Purifying techniques

An air purifier placed under a table
Air-purifiers with air flow generated by bladeless fan. Some models can act as heaters or humidifiers and may feature oscillation and adjustment of air flow angle.

There are two types of air purifying technologies, active and passive. Active air purifiers release negatively charged ions into the air, causing pollutants to stick to surfaces, while passive air purification units use air filters to remove pollutants. Passive purifiers are more efficient since all the dust and particulate matter is permanently removed from the air and collected in the filters.[13] Several different processes of varying effectiveness can be used to purify air. As of 2005, the most common methods were high-efficiency particulate air (HEPA) filters and ultraviolet germicidal irradiation (UVGI).[14]


Air filter purification traps airborne particles by size exclusion. Air is forced through a filter and particles are physically captured by the filter. Various filters exist notably including:

Other methods

An air purifier which can use HEPA, ionization, PCO, UVGI, and ozone generation

Consumer concerns

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Other aspects of some air cleaners are hazardous gaseous by-products from ozone-generating units,[32] noise level, frequency of filter replacement, electrical consumption, and visual appeal. Ozone production is typical for air ionizing purifiers. A high concentration of ozone is dangerous, although most air ionizers produce low amounts, low rates of ozone reduce the effectiveness. A build up can cause detrimental health effects especially for vulnerable people.[33] The noise level of a purifier can often be obtained through a customer service department and is usually reported in decibels (dB). The noise levels for most purifiers can vary and may be dependent on fan speed.[34] Frequency of filter replacement and electrical consumption are the major operation costs for any purifier. There are many types of filters; some can be cleaned by water, by hand or by vacuum cleaner, while others need to be replaced every few months or years.[35] Sometimes suitable filters are only sold by the manufacturer for a high cost, some have DRM control so only replacement filters authorised by the manufactuere can be used.[36] In the United States, some purifiers are certified as Energy Star and are energy efficient.

HEPA technology is used in portable air purifiers as it removes common airborne allergens. The US Department of Energy has requirements manufacturers must pass to meet HEPA requirements. The HEPA specification requires removal of at least 99.97% of 0.3 micrometers airborne pollutants.[37] Products that claim to be "HEPA-type", "HEPA-like", or "99% HEPA" do not satisfy these requirements and may not have been tested in independent laboratories.[38]

Air purifiers may be rated on a variety of factors, including Clean Air Delivery Rate (which determines how well air has been purified); efficient area coverage; air changes per hour; energy usage; and the cost of the replacement filters. Two other important factors to consider are the length that the filters are expected to last (measured in months or years) and the noise produced (measured in decibels) by the various settings that the purifier runs on. This information is available from most manufacturers.

Potential ozone hazards

As with other health-related appliances, there is controversy surrounding the claims of certain companies, especially involving ionic air purifiers. Many air purifiers generate some ozone, an energetic allotrope of three oxygen atoms, and in the presence of humidity, small amounts of NOx. Because of the nature of the ionization process, ionic air purifiers tend to generate the most ozone.[citation needed] This is a serious concern because ozone is a criteria air pollutant regulated by health-related US federal and state standards. In a controlled experiment, in many cases, ozone concentrations were well in excess of public and/or industrial safety levels established by US Environmental Protection Agency, particularly in poorly ventilated rooms.[39]

Ozone can damage the lungs, causing chest pain, coughing, shortness of breath and throat irritation. It can also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections—even in healthy people. People who have asthma and allergy are most prone to the adverse effects of high levels of ozone. For example, increasing ozone concentrations to unsafe levels can increase the risk of asthma attacks.[40]

Due to the below average performance and potential health risks, Consumer Reports has advised against using ozone producing air purifiers.[41] Some manufacturers falsely claim outdoor and indoor ozone are different.[42] Claims that these devices restore a hypothesized ionic balance are unsupported by science.[42]

Ozone generators are used by cleanup contractors on unoccupied rooms to oxidize and permanently remove smoke, mold, and odor damage, and are considered a valuable and effective industrial tool.[43] However, these machines can produce undesirable by-products.[41]

In September 2007, the California Air Resources Board announced a ban of indoor air cleaning devices which produce ozone above a legal limit. This law, which took effect in 2010, requires testing and certification of all types of indoor air cleaning devices to verify that they do not emit excessive ozone.[44][45]

Industry and markets

As of 2015, the United States residential air purifier total addressable market was estimated at around $2 billion per year.[46]

See also


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