How to choose an air purifier

​Indoor Air Pollution Fact Checks

[1] HEPA Filter

High Efficiency Particulate Arrestance or HEPA is a mechanical filter, designed to remove fine and ultra-fine particulate matter by the mechanisms interception, impaction and diffusion.

HEPA filters are specified by particle removal efficiency near the most penetrating particle sizes (MPPS), for example 99.97% at 0.3 μm, as between 0.1 and 0.4 μm are the most penetrating particle sizes, hence specification is at 0.3 μm.

There is a common misconception that HEPA filters are not effective at removing particles smaller than 0.3 microns. In fact, the removal efficiency of particles 0.01 to 0.1 microns is also close to 100%.  Read more about HEPA filters on our FAQ page...

[2] Coarse Particle Filter

They are mechanical filters that have a more porous structure relative to HEPA. They are commonly used in HVAC systems such as air conditioners and ducted gas heaters because of their low cost and low air resistance. A coarse particle filter is usually placed before a HEPA filter of an air purifier to extend the life of a HEPA filter.

[3] Carbon Filter (or Molecular Filter)

Molecular filters remove odours, gases and VOCs (such as Formaldehyde) by a process called adsorption, where molecules of gases, liquids or solids adhere to the surface of an adsorbent material.

Activated carbon is the most commonly used adsorbent because:

  • It is completely safe and is sometimes used as oral medication;
  • It has an extremely large surface area in the form of microscopic cracks and fissures that can adsorb the widest variety of chemicals. One gram of activated carbon typically has a surface area between 500sqm and 1500sqm (2 – 6 tennis courts), and can adsorb up to 50% of its own weight in harmful airborne gases or VOCs.

Air purifying carbon filters usually come in the form of carbon impregnated mats for light duty or in a bed of granulated pellets for heavier duty.




  • HEPA filters;
  • Coarse particle filters; ​
  • Electrostatic filters
  • Electronic filters, such as electrostatic precipitators
  •  negative Ion generators, or ionisers
  • Ozone generators;
  • Ultraviolet germicidal irradiation (UVGI) purifiers
  • Carbon filters;

  • TiO2 Photo catalytic Oxidation (PCO) purifiers;
  • Ozone generators





There are no chemical reactions that take place to generate by-products.


Negative ion generators

and electronic filters involve high voltages to charge the particles and produce a small amount of ozone.


Pollutants are destroyed in the process of oxidation and or decomposition producing by-products such as formaldehyde and ozone.



Pollutants are locked in the filter media (fibers, screen mesh and activated carbon etc.)


Pollutants are attracted to objects with opposite electronic charge. Some will attach to the filter's charged plates, others escape and may fall from suspension, attaching to nearby walls, furniture and floors.


Products and by-products, both harmless and harmful remain in the air.

There are three must-have filters in an effective air purifier:

  • A genuine medical grade High-Efficiency Particulate Arrestance (HEPA) filter to remove particulate matter of ALL sizes, including ultra-fine particles that can bypass the lungs defences.

  • An activated carbon filter for broad spectrum removal of harmful gases, VOCs and odours.

  • A pre-filter (coarse particle filter) to extend HEPA life.

If a purifier has these three filters and is well designed to allow unimpeded air flow, it will remove pollutants in the most reliable and effective manner and will not need any  electronic enhancements.

Summary of air purification devices available on the market​​

The combination of HEPA and carbon filters efficiently removes the full spectrum of particles and targets common gaseous pollutants without generating

harmful by-products. 

​​Sanctuary Air SA-250 Series

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What filters should my purifier have?

[4] Electrostatic filter

Electrostatic filters are mechanical filters manufactured from electret fibres which have permanently charged surfaces. The advantage of these filters is that electret fibres can be coarser in diameter and spaced with greater porosity, meaning they are cheaper to manufacture and there is less air flow resistance.

Electrostatic filters are safe and effective at collecting small and fine particles. However, as dust loads up in the filter, the ability to trap fine particles diminishes quickly because the electrostatic action stops once the charged sites in the electret media are used up. For this reason, electrostatic filters need to be changed very frequently to maintain high collection efficiency of fine particles.

[5] Electronic filter

Electronic filters, such as electrostatic precipitators, negative Ion generators, or ionisers were invented as a low cost alternative to traditional mechanical filters.

The advantages are: smaller size and lower energy consumption in comparison to mechanical filters with the same air flow rating, and that the filter is re-usable. 

The disadvantage is that high voltages are used to charge the particles, some of which may deposit on the filter’s plates, and some of which may electrostatically deposit on nearby surfaces such as walls,  furniture and ceilings giving them a soiled appearance.

Maintenance is also higher for reusable electronic filters because the filter’s plates need to be cleaned regularly in order to maintain good dust collection efficiency. Some people report that these units work well when new but become difficult to clean as they age. 

All electronic filters or electrostatic precipitators produce some ozone.

[6] Ozone generator

Ozone generators use UV lamps or electrical discharges to produce ozone that reacts with chemical and biological pollutants eventually transforming them into harmless substances. Ozone is a potent lung irritant, which in concentrations that do not exceed public health standards, has little potential to remove indoor air contaminants. Thus ozone generators are not always safe and effective in controlling indoor air pollutants.

Electronic air filters  may generate harmful by-products including ozone.​

"Residential indoor ozone concentrations may be affected by the amount of ozone emitted by electronic air cleaners, which varies among models.Even at concentrations below public health standards, ozone reacts with chemicals emitted by such common indoor sources as household cleaning products, air fresheners, deodorizers, certain paints, polishes, wood flooring, carpets, and linoleum. The chemical reactions produce harmful by-products that may be associated with adverse health effects in some sensitive populations. The ozone reaction by-products that may result include ultrafine particles (smaller than 0.1 µm in diameter), formaldehyde, ketones, and organic acids."

EPA -  Residential Air Cleaners (Second Edition): A Summary of Available Information} 

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How to choose an air purifier