Frequently asked questions about air cleaners

"Air cleaner", "air scrubber" and "air purifier" are terms used interchangeably for the devices that remove particles and some gases from the air. An air cleaner helps to reduce risks of COVID-19 transmission in your indoor environment where ventilation is inadequate.

We know that airborne viral transmission between people can happen much further than the arbitrary six feet (US), 2 metre (UK) or 1.5 metre (Australia) social distances. We also know that COVID-19 viral particles can remain airborne for hours, particularly in environments where there is poor ventilation. When there is no fresh air circulated through an indoor space, the risk of infection is much greater as infectious aerosols accumulate and increase exposure risks.

That means, if an infectious person has been breathing, talking or coughing in a poorly-ventilated space, there is a risk of susceptible people in that space inhaling airborne infectious particles and becoming infected – even if these people are not standing near each other. Wearing a well-fitted mask also known as a respirator reduces the amount of respiratory particles in the space, protecting others and the wearer.

Factors that increase the risk of COVID-19 transmission include high-traffic / congregating areas, poor ventilation, lots of people, the quality of masking and whether high aerosol generating behaviours are occurring (such as intense exercise, singing or playing of wind instruments).

You may also consider purchasing an air cleaner to reduce the effects of respiratory allergens triggered by particulate matter.

Yes. Air cleaners remove the small airborne particles (called aerosols) that carry respiratory viruses such as SARS-CoV-2, the virus that causes COVID-19, by capturing these particles on a High Efficiency Particulate Air (HEPA) filter. The highest tolerable fan setting should be used to remove respiratory particles in the air and never the AUTO setting.

Air cleaners work by drawing in air through a filter using a fan - thereby trapping the aerosols within that air stream onto the filters. Clean air is then returned into the room. Air cleaners are required when ventilation is insufficient (see "How do I know if my ventilation is sufficient?").

Respiratory viruses exist in the air as the virus surrounded by proteins and salts in an aerosol. Respiratory viruses are airborne as aerosols are generated from coughing, sneezing, breathing, talking, singing and shouting. Most respiratory aerosols are small (<5 microns) and remain airborne for several hours, even aerosols of 40 microns in size remain in air currents for > 20 minutes. Aerosols greater than 100 microns can be inhaled and infect a susceptible person.

Air cleaners work to remove airborne SARS-CoV2 virus that causes COVID-19 from the air. While the naked SARS-CoV-2 virus is ~0.12 microns, as an infectious respiratory particle the virus is surrounded by proteins and salts, so is >0.3 microns. Air cleaners efficiently remove the particles larger than 10 nm or 0.01 microns via HEPA filtration. Air-cleaners that feature a HEPA 13 or higher (High Efficiency Particulate Air) filter trap at least 99.97% of airborne particles at 0.3 microns. 0.3 microns is the minimum in filtration efficiency due to the two different processes that filter particles from the air. Diffusion (Brownian random motion) acts below 0.3 microns to capture particles within the HEPA fibre matrix and impaction/interception acts like a sieve to stop particles larger than 0.3 microns.

Some air cleaners have a particle sensor which controls the fan if on AUTO settings, and gives the particulate number count (or coloured lights). Only if it is dusty or smoky are there really lots of particles in the air and the AUTO fan will be running on the highest fan setting. Using the AUTO setting is not helpful if you wish to remove respiratory particles containing viruses such as SARS-CoV2 - because the particle sensor can not detect people breathing in a room. Do not use AUTO if you wish to clean the air for COVID-Safe purposes - use the highest tolerable fan setting.

Yes. Air cleaners remove aerosols that are the size of smoke, mould and pollen. Some air cleaners that feature a filter (such as activated carbon) can additionally absorb Volatile Organic Compounds (VOCs).

Air cleaners work by drawing in air through a filter using a fan, thereby trapping the aerosols within that air stream onto the filters. Clean air is then returned into the room.
Common particulate environmental risks within this size range include; bushfire smoke (<2.5µm), cigarette smoke (0.1–1µm, medium of 0.25 µm), mould spores (3–40µm), pollen grains (10–1000 µm, medium of 30µm) and household dust (0.5–100µm).

Some air cleaners are also able to trap harmful gases, such as Volatile Organic Compounds (VOCs). VOCs are associated with adverse health effects and are emitted from a variety of sources such as paints, solvents, aerosol sprays, disinfectants and air fresheners. Air cleaners remove VOCs using an activated carbon filter – these are the filters found on bench-top compost bins to absorb odours. Activated carbon has a large surface area that allows molecules to become adhered to the carbon. Pay attention to the amount of activated carbon in each air cleaner which will determine the amount of VOCs the cleaner can remove. Some filters may only feature a coating of activated carbon as opposed to being composed of activated carbon (e.g: in packed granular activated carbon pellets). In some air cleaners the activated carbon pellets are not tightly packed.

Activated carbon filters are regarded as an efficient method of removing VOCs, perfume and odour, and has no dangerous by-products of the removal process. It is important to note that some cleaners which use ionisation or photolytic oxidation technologies with ultraviolet light can contribute further hazardous oxidized VOCs to the atmosphere (see "How do electronic cleaners work and why are they dangerous?” below).

Formaldehyde, a known carcinogen and common indoor air pollutant, doesn’t adsorb efficiently to activated carbon. Some air cleaners use activated aluminium oxide combined with potassium permanganate filter media to remove formaldehyde from the air using filtration, chemical adsorption and oxidation.

To ‘know’ that a space is well ventilated or establish whether 6 Air Changes per Hour (ACH) is being achieved requires a ventilation audit. Here’s how to do a self-assessment of your business. If you don’t know how much fresh air is being delivered to your space (L/second/person or ACH) professional advice can be sought via your building manager or ventilation engineers. Measuring carbon dioxide is a quick way to determine if a space is poorly ventilated for the occupancy and monitoring is continuous so it enables high risk ‘events’ to be found.

By measuring CO₂ we can know whether enough fresh air is being supplied for the room’s volume, occupancy and activity. CO₂ is a proxy for the amount of respiratory aerosol build up in a space. Each person breathes 8 litres a minute and each exhaled breath contains 37,000 ppm CO₂. As a rule of thumb for each 400ppm CO₂ above the background of 420ppm 1 % of the air in the space has been breathed out by someone else or is the ‘rebreathed fraction’.

If you’d like to monitor your indoor space here is a great guide. For a CO₂ monitor we advise NDIR (Non Dispersive Infrared) sensors and a device that logs continuously. One device that we’ve found to be really versatile is this one from Aranet4.

This graphic from New Zealand Ministry of Education provides a useful translation of ACH, supply rate (L/s/person), and CO₂ levels. Also shown is productivity and cognitive improvements as we know fresh indoor air provides many health benefits.

No. Gases such as O₂, N₂ and CO₂ pass through the HEPA and activated carbon filters. The HEPA filter removes particles (such as virus containing respiratory aerosols, smoke particles and dust) from the air. The activated carbon (charcoal) filter removes volatile organic compound (VOC) gases and odours.

Carbon dioxide (CO₂) levels are often used when describing ventilation quality in indoor environments (see “Do I need one?” and “How do I know if my ventilation is sufficient?” above). Air cleaners do not remove CO₂ from indoor environments, although some are advertised as removing other gases (see “Can an air cleaners reduce environmental risks?” above).

Because CO₂ is not removed by the air cleaner, when air cleaners are running, the risk of respiratory particles is reduced but CO₂ levels can remain high – only ventilation can lower CO₂ levels.

Particle sensors are integrated in many air cleaners, usually with an associated red or orange light when PM_2.5 and PM₁₀ number concentrations are at unhealthy levels in the air (i.e. it is smoky outside). These ‘particulate levels’ sensors are not usually aligned with the Australian air quality categories so you should check the outdoor pollution levels from your state’s EPA website for advice.

You may notice the air cleaner will show a red or orange light in an indoor environment with a high particle count. The air cleaner screen may temporarily change to orange or red due to a short-lived source of particles such as an aerosol spray in a small room or smoke from cooking activities. A continual orange or red light indicates unhealthily high levels of aerosols or VOCs (gases) in the air.

Note - the particle sensor is what controls the fan speed when an AUTO mode is selected. This works well for dust removal, but for respiratory aerosols AUTO will not provide sufficient air cleaning therefore the AUTO setting should be avoided.

Important: Keep your air cleaner running on a high fan setting until the lights return to green – the particulates in the air will be removed, protecting everyone inside the room.

Maintenance schedules will vary for each air purifier; if used constantly, you should expect to clean the pre-filter regularly (every 2-4 weeks) and replace the HEPA filter every 6–12 months.

Maintenance schedules for air cleaners will vary depending on use; filters need to be changed more frequently for higher use/dusty situations. Some air cleaners have a washable pre-filter and a separate filter that needs to be replaced. Many air cleaners will be fitted with a sensor and notification system for when the HEPA filter should be changed or cleaned. For example, Samsung Air Purifiers recommend the entire filter will need to be changed every 6–12 months and the pre-filter cleaned every 2 weeks if the cleaner was used 24 hours per day.

The pre-filter captures dust, ensuring air flows through the device and its maintenance is important.

There is low risk of COVID-resuspension from the filter, but gloves and a mask should be worn when changing filters to minimise any risk. If filters can be changed after several hours of low occupancy, this will further reduce risk.

Studies have shown that the SARS-CoV-2 virus is able to survive outside of a body from hours to several days in ideal environments. The HEPA filter of an air purifier provides an inhospitable environment for viruses as aerosols are become trapped in the filter media and subjected to high air flow. To minimise any resuspension risk, appropriate PPE should be worn when changing air purifier filters and done outside if possible.

Air cleaners work within a room as they improve the air’s circulation and deliver clean air. They need power so having a power point and avoiding a trip hazard is important. Other things to consider are being away from doors and windows (ie placing the air cleaner where air stagnation may occur). In public/community settings, air cleaners should be placed in the room with the highest risk (common areas etc).

Air cleaners remove aerosols from the air which may carry COVID-19. They are only able to clean the air that they draw in, so the placement of the air cleaner is important.

• When the source is known (in household settings): it is most efficient to place the air cleaner in the room of the COVID-positive person (with the door closed). Placing the air cleaner at the source of the COVID-carrying aerosols will enable a larger amount of the infectious material to be removed from the air in the house.
• When the source is unknown (in public/community settings): it is recommended to place the air cleaner in the room with the highest risk. This includes environments with poor ventilation (see “How do I know if my ventilation is sufficient?" above), high room occupancy and the presence of people not wearing masks, particularly when engaging in activities such as talking, eating, projecting the voice and playing brass/woodwind instruments. For example, the highest risk environment in an office is likely to be the tea-room, where people gather to talk and eat without masks.

If someone in your house is COVID-positive, to protect other household members you should place the air cleaner in their bedroom with the door closed and, if safe, open/crack a window in their room.

Within a room, place the air cleaner at the area of highest risk. For example, an air cleaner will have more impact when placed in a “stagnant air zone” (where the air is still), as opposed to next to an open window where air is already moving.

Air cleaners should never be placed near heat sources or flammable objects. Be careful about trip hazards with power cables when placing an air cleaner in a commonly-used area. More information on placement can be found on this flyer produced by the Victorian government.

The number or size of air cleaner/(s) will depend on the volume of the room and existing ventilation.

Air cleaners are often made in different sizes and effectiveness (the rate at which they deliver clean air) with varying price points to match. In order to work out which air cleaner you require you will need to know the volume of your room (in m³). Each air cleaner usually has a recommended floor area labelled on the packaging (or on the associated website). Ideally you wish to achieve a clean air delivery rate (CADR) that achieves 6 air changes per hour.

See our online calculator for calculating how many air purifiers you need for your space here and ACH desired.

To work out the effective air changes per hour (ACH_e) achieved by a given air cleaner (and fan setting) for your room’s volume (V):

\[ACH_e=\frac{CADR[\frac{m^3}{h}] } {V [m^3]}\]

Air cleaners range in noise. They are quietest at lowest fan speeds, but most effective at high fan speeds. Some air cleaners have a “sleep” setting of a low noise and quiet fan speed.

Noise is an important consideration when running in a public space or running in household settings for sleep. Each air cleaner operates at a different noise level, depending on the fan level. Noise levels for each fan level should be advertised on the packaging or website of the air cleaner. When running an air cleaner with the aim of reducing COVID transmission risk, it should be run at the highest fan setting.

The World Health Organisation recommends an average night noise level of under 40 dB(A) to avoid any sleep disturbance [PDF], with a guideline upper limit of 45 dB(A) (WHO, 2018) for night-time road noise. To avoid health impacts day-time road noise is recommended not to exceed 53-55 dB(A). Some air cleaners have “sleep” modes which often dim indicator lights and reduce fan speeds to achieve low noise. See our comparison of air cleaner noises in dB(A).

You can download the US National Institute for Occupational Safety and Health (NIOSH) App to measure the ambient noise levels in your environment. You don’t want air cleaners that raise the noise significantly. Noise is measured on a logarithmic scale, and ranges from high to low frequencies. Filter weighted average (A, B or C) are applied to come up with one number for noise -  dB(A), dB(B) or dB(C). Air cleaner noise is reported usually as dB(A) (aka dBA) which means low frequency noise is weighted less in its calculation – ie how humans hear sounds.

Sones is an alternative measure of loudness, but works as a linear scale whereas dB is a log scale. Sones means a doubling of the number represents a doubling in loudness. To obtain Sones from dB the following equation is used:

Sones = 10^{((dB-28)/33.2)}

ie. these are doublings of loudness: 1 Sone = 28dB, 2 Sone = 38dB, 4 Sone=48dB, 8 Sone =58dB.

Yes. Although an air-cleaner reduces the risk of COVID-transmission in a room, it is only one part of the layered mitigation strategy against COVID transmission and infection.

Running an air cleaner will reduce the risk of COVID transmission between people in an environment, however it takes time to ‘scrub’ the air of COVID-carrying aerosols. It can only clean what it draws in (see “What is CADR?” below for more information). The source of COVID-aerosols can also be continuous, so an air cleaner is continuously ‘keeping up’ with an ongoing introduction of COVID-aerosols. To reduce the risk of COVID infection, it is recommended to wear masks even when an air cleaner is running.

ACH describes the number of Air Changes per Hour. It provides a measurement of how many times the air is entirely replaced within a defined space.

The number of Air Changes per Hour (ACH) is defined as the ratio of the ventilation rate (Q) to the volume of the room (V), as follows:

\[ACH=\frac{Q[\frac{m^3}{h}] } {V [m^3]}\]

Certain environments have a regulated minimum ACH. Workplaces are recommended to have at least 6 ACH, deemed “Excellent”, with 4-6 ACH rated as “Good”, and 2-4 ACH as the “Bare minimum” (Department of Health, Victoria State Government) .

Clearance time is the time it takes to completely remove/replace a certain percentage of the air in the room.

The Centre for Disease Control defines ‘safe’ as when 99% or 99.9% of the air in the room is replaced (CDC, 2003), assuming a well mixed-room.

The clearance time is specified as follows:

\[t_2-t_1={{-ln\frac{(C_2-C_0)}{(C_1-C_0)}} \over {Q/V}}={{-ln\frac{(C_2-C_0)}{(C_1-C_0)}} \over {ACH}}\]

Where:

• t₁, t₂ are the initial and final times
• C₁, C₂ are the initial and final concentrations and (1- C₂/C₁) is the removal efficiency
• C₀ is the background concentration - often zero, but ~420ppm for CO₂ for example
• Q/V is air changes per hour (ACH) which is the ventilation rate (Q) divided by the volume (V) of the space

Clearance times are often defined as the amount of time taken for 99% of the air in the room to be replaced, which is dependent on the number of air changes per hour. A 99% removal efficiency is deemed “virtually certain” and a 63% efficiency is termed as “likely” that a contaminant has been removed. At 6 ACH (recommended for office settings), it takes 46 minutes to remove contaminants at a 99% efficiency. At the bare minimum ACH in office environments (2 ACH), it takes 138 minutes. The disparity in clearance times between well-ventilated and poorly-ventilated environments demonstrates the importance of proper ventilation for reducing infection risk in public spaces.

The Clean Air Delivery Rate describes how quickly an air cleaner can clean the air within a particular room.  It is an established standard that can be used to compare air cleaners. More effective air cleaners will have a higher CADR.

The Clean Air Delivery Rate describes the volume of air (in m³) that is able to be effectively cleaned per minute or hour by an air cleaner.  The CADR describes the fraction of aerosols for a particular size distribution that have been removed from the air sucked into the device, multiplied by the air flow rate through the device.

Measured CO₂ provides an accessible proxy for ventilation, as high CO₂ is indicative of a high respiratory aerosol presence.

CO₂ accumulation indoors is primarily from exhaled breath. High CO₂ is indicative of a high respiratory aerosol presence (i.e. stagnation of air), therefore CO₂ can be used as a proxy for respiratory aerosol load and the fraction of exhaled air in indoor air (re-breathed air) can be established:

\[f={{C-C_0} \over {C_a}}\]

Where C₀ is the outside CO₂ concentration ~410 ppm, C is the monitored indoor concentration and C_a is the concentration of CO₂ in exhaled breath ~37,500 ppm. For example, when indoor CO₂ levels are 600 (healthy indoor levels of CO₂), 0.51% of air is rebreathed. At CO₂ levels of 1500 ppm (poor), 2.91% of air has been breathed by someone else. At higher fractions of rebreathed air, the risk of infection is greater.

There are several types of electronic air purifiers on the market - these can be divided into ionizers, plasma, photocatalytic oxidation, or UV. These technologies are considered unproven/untested technologies, and in some cases dangerous technologies. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has prepared a Position Document on Filtration and Air Cleaning, finding only limited data documenting the effectiveness of gas-phase air cleaning as an alternative to ventilation. Marwa Zaatari and Marcel Harmon penned an open letter to school district facility managers and administration leadership to address the use of electronic air cleaning equipment in school facilities.

Ionisation or Plasma

Corona discharge (also labelled: ionizing, negative ion, bipolar ionizing, activated oxygen, plasma, mountain fresh air, etc.) works by creating a lightning-like charge inside the devices. The aim is to ionize oxygen in the air, creating oxygen radicals which react quickly in air to form hydroxyl, super-oxide anions, nitrogen oxides and ozone - basically facilitating air radical chemistry. This air oxidation chemistry causes particles in the air to become negatively charged and deposit on surfaces, or ideally within the devices: be attracted to positive cathodes, where the particles can be washed/removed (i.e. this is how electrostatic precipitators in power-stations work for particulate matter pollutant control).

Ideally, a highly oxidative environment would oxidize gaseous volatile organic compounds (VOCs) through to CO2 and water, but in reality oxidative radical air chemistry forms all sorts of things, like formaldehyde (a class I carcinogen), higher order aldehydes and acetone. Creating a highly oxidative air environment leads to a degradation of indoor air quality as detailed in ASHRAE Journal; New York Vol. 60, Iss 12 (Dec 2018): 64–67.

Photocatalytic oxidation (PCO)

By shining UV-C on a coated surface such as TiO2 catalyses oxygen radical formation from the oxygen in air, thereby forming hydroxyl and super-oxide anions (but requiring less energy) than ionization process (above). The same highly oxidative environment is therefore created and as outlined above ozone and VOC oxidation products readily form. [Note: TiO2 is widely found in paints as it provides excellent light scattering properties].

Real-Time Laboratory Measurements of VOC Emissions, Removal Rates, and Byproduct Formation from Consumer-Grade Oxidation-Based Air Cleaners, by Qing Ye et al., highlights the fact that ozone, formaldehyde and VOCs are emitted continuously from plasma, UV-C + TiO2, UV-C + anions and photoelectrochemical oxidation.

UV-C and Germicidal UV

Ultra-violet (UV) light from the Sun is largely filtered by oxygen and ozone in our atmosphere. UV light is subdivided into the following bands (WHO):

• UV-A (315-400 nm)
• UV-B (280-315 nm)
• UV-C (100-280 nm)

UV light destroys DNA but because our atmosphere has 21% oxygen and a stratospheric ozone layer, no UV-C reaches the surface naturally and this enabled life on land to evolve ~2.4–2.0 billion years ago. When UV-C light from the Sun reaches the stratosphere it splits oxygen in the air (oxygen dissociation is <240 nm), creating atomic oxygen (O(1D)) these oxygen atoms then react with oxygen molecules to form ozone. Ozone also is photolysed by UV light returning atomic oxygen and oxygen molecules (this happens between 200-310 nm).

As UV-C light destroys DNA and it has been employed as UV germicidal radiation indoors to denature viruses and bacteria. Most commonly two wavelengths are used: 254 nm (mercury lamps) or 222 nm (KrCl lamps), with doped glass to ensure very narrow wavelength band of light is produced, with the aim that unintentional ozone formation is minimised.

Note: Microbe Shield™ uses UV-C light within their air cleaners.

Adding chemicals to the air for the purpose of disinfecting the air, i.e. spraying bleach, constitutes a pesticide product. Pesticide products are regulated in the US.

While regulatory reporting standards exist for outdoor air quality, there doesn't exist the same for indoor air in Australia. Outdoor air standards are set by the Commonwealth government under the National Environment Protection Council Act as National Environmental Protection Measures (NEPMs).  Implementation of the NEPMs is then determined by each state government, usually reporting and compliance of outdoor air standards is overseen by the state EPAs.

Health is central to the setting of NEPMs and the WHO guidelines for outdoor and indoor air are closely followed for setting the threshold levels. Particulate matter (PM₁₀ and PM_2.5), ozone (O₃), nitrogen dioxide (NO₂) and sulfur dioxide (SO₂) are all known as criteria air pollutants - these are regulated outdoors. Lead (Pb) is also a criteria air pollutant regulated under NEPMs in Australia.

Indoor air within buildings has limited regulatory compliance in Australia. Safe Work Australia's Workplace Exposure  Standards for Airborne Contaminants updated in 2022 provide Time Weighted Average (TWA - 8 hours) and Short Term Exposure (STEL - 15 minute) exposure limits to air pollutants. These are designed for work places where PPE is available to worker not public health based like the WHO guideline standards. For example -  STEL for 15 minutes for NO₂ is 5 ppm (5000 ppb) whereas the NEPMs for 1 hour is 80 ppb (62 times smaller) - i.e. the threshold above which health effects are experienced is much lower than the threshold that will cause disability or death.

The Australian Building Codes Board is updating their Indoor Air Quality handbook as the pandemic has highlighted that poor ventilation (and hence IAQ) has driven much of the spread of the SARS-CoV-2 virus. Here is my response to that consultation (PDF 384.0 KB) - where the standards for IAQ are summarized around the globe, and recommendations for the IAQ handbook update are made. The recommendations made are for thresholds below which indoor air is considered healthy - the benefits of which are immense as we spend 90% of our time indoors, and poor indoor air was estimated in 1998 to cost the Australian economy $12 billion a year.