All you need to know about HEPA filters in air purifiers

Used in many air purifiers sold in Australia, HEPA filters play an important role in air purification by trapping airborne bacteria and viruses.

All you need to know about HEPA filters in air purifiers

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High-efficiency particulate air filters, more commonly known as HEPA filters, are a standard air purification feature found in many air purifier models sold in the Australian market. They remove microscopic particles such as dust, allergens, and even germs from the air through mechanical means, forcing air to pass through the extremely fine mesh of the filters. Under normal use, these filters need to be replaced once every 6 to 12 months when the fine mesh is clogged up by microscopic particles, rendering the air purification process inefficient.

How efficient is your HEPA air purifier?

To qualify as a HEPA filter, the filter has to be able to remove at least 99.97% of airborne particles as small as 0.3μm in size, as defined in the Australian Standard AS4260:1997. This Australian Standard classifies HEPA filters into several grades based on their efficiency.

Performance grade Efficiency
Grade 1 At least 99.97% filtration rate for particles with 0.3μm in size.
Grade 2 At least 99.99% filtration rate for particles with 0.3μm in size.
Grade 3 At least 99.999% filtration rate for particles with 0.3μm in size.
Grade 4 At least 99.999% filtration rate for particles with 0.12μm in size.

*The unit μm is known as micrometre or micron. 1μm = 0.001mm.

The size 0.3μm is universally used in measuring the filtration efficiency of HEPA filters because it is considered the most penetrating particle size (MPPS) for HEPA filters, the size of particles that are most able to pass through the filter. As airborne allergens and germs come in various sizes and most of them are bigger than 0.3μm, this way of measuring filtration efficiency gives the assurance that the HEPA filters can capture most airborne particles.

Size chart of common indoor air pollutants

That said, HEPA filters can get rid of airborne particles greater than 0.3μm in size efficiently. Common air pollutants found indoors are able to be filtered out from the air by HEPA filters.

Indoor air pollutants Size Sources of information
SARS-CoV-2 virus 0.1µm A study published in eLife
Viruses 0.02 - 1µm Encyclopædia Britannica
Bacteria 1 - 10µm Arizona State University
Mould spores 1 - 63µm A study on long-distance dispersal of fungi published in Microbiology Spectrum
Allergens from cats 2.5 - 5µm A comparative study published in American Review of Respiratory Disease
Cough aerosol < 5µm World Health Organization
Particulate matters
(dust, smoke particles, etc.)
< 10μm United States Environmental Protection Agency
Pollens 10 - 70µm The University of Western Australia
Dust mites 250 - 330µm Asthma and Allergy Foundation of America

*Viruses can be way smaller than 0.3microns. However, during airborne transmission, they are attached to relatively large aerosols. HEPA filters will be able to effectively filter out cough aerosol and, by the same token, prevent the transmission of airborne diseases.

Do “true” HEPA filters exist?

To differentiate HEPA filters that conform to the Australian Standard from other air filters which do not have the same air filtration efficiency, air purifier manufacturers often label their HEPA filters as “true” HEPA filters. While there are many ways of grading HEPA filters according to regulations of different countries, the distinction between “true” HEPA filters and “normal” HEPA filters is an arbitrary one.

The best way to identify whether the air purifiers use actual HEPA filters is to check out the filtration efficiency label. It should be able to filter out at least 99.97% of airborne particles of the size 0.3μm.

The efficiency of HEPA filter in preventing the transmission of airborne germs

Viruses and bacteria are transmitted through the air by attaching to tiny airborne droplets in the form of aerosol particles. These aerosol particles are microscopic and can suspend in the air for an extended period of time. Inhaling these infectious aerosol particles introduces the viruses and bacteria into our body, potentially causing an infection.

It is well known that a HEPA air purifier can reduce the amount of infectious aerosols suspended in the air. In 2021, a study conducted by Centers for Disease Control and Prevention in the United States with SARS-CoV-2 aerosols showed that using 2 HEPA air purifiers close to the aerosol source reduced aerosol exposure to the others by up to 65%. The exposure towards the viruses can be further reduced by up to 90% by wearing face masks while having HEPA air purifiers running.


Even though standard HEPA filters are able to trap most bacteria and viruses, they have some limitations. HEPA filters are not able to destroy the trapped germs or capture gaseous indoor air pollutants such as volatile organic compounds (VOCs). To ensure that your air purifier provides all-around protection, opt for a model that includes additional features aside from the HEPA filter, such as an activated carbon filter that deals with VOCs and germicidal technology like UV light. Choosing an air purifier with the right combination of air filters and features helps you tackle specific indoor air quality issues that you are facing.

Disclaimer: The information provided in this article is for general reference only. Please seek advice from professionals according to your needs.

Ionmax Australia

Ionmax Australia

Founded in 2003, Ionmax Australia has received numerous awards and accolades for its innovative products and commitment to customer satisfaction. Ionmax dehumidifiers and air purifiers have been recognised for their effectiveness in reducing indoor air pollutants and improving air quality. The company is committed to researching and developing new and improved products that meet the needs and expectations of its customers, making it a trusted name in households across Australia and beyond.

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