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The real efficiency of air purifiers with HEPA filters!

This article is the first in a series on air purification technologies. It focuses on air purifiers with HEPA filters whose efficiency must be questioned.

What is an air purifier with filters?

A filtered air purifier is a device designed to remove domestic and industrial pollution. It is generally made up of several superimposed filters and covers an area of 5 to 150m².

  • The pre-filter stops visible particles larger than a micron in size.
  • The activated carbon filter captures volatile organic compounds and odours.
  • The HEPA filter, which stands for High Efficiency Particulate Air, filters out at least 99.97% of particles with a diameter of 0.3 µm or more.

The HEPA filters and their theoretical efficiency

HEPA filters and their theoretical efficiency. The name "HEPA" is often presented to consumers as a guarantee of efficiency. Highly controlled, it is only awarded to filters that have passed the DOP (Dispersed Oil Particulate) test. This test, developed in the 1950s, measures the filtration efficiency of particles larger than 0.3 microns. In the purifier market, many products are presented as "HEPA-type", "HEPA-like" or "99% HEPA" and have not been tested in the laboratory. Their manufacturers would like to take advantage of the confidence that consumers place in HEPA-certified air purifiers. Nine, this one has indeed an extreme efficiency on the particles that pass through them. Envious enough to make people envious!

Their efficiency in practice

The DOP test does not measure the filtration efficiency of particles smaller than 0.3 micron, which represent 90% of suspended particles, including bacteria and viruses.


There are many factors that limit the effectiveness of HEPA :

  • The nature of the filtering layer (weight, thickness and compactness)
  • The quality of the filter pleating
  • Leaks between the filter and the filter holder
  • Leaks between the filter support and the air purifier
  • The number of associated filters: the more filters there are, the lower the flow rate of purified air, or the noisy and energy-consuming purifier
  • Air temperature
  • Air velocity through the filter: slow, the air in the room is not purified in its entirety. Quickly, the device becomes noisy and annoying due to the draughts it generates.
  • Wear and tear: a HEPA loses 25% of its efficiency over a period of 6 months in a low-pollution environment.
  • The level of dirt in the filter by particle deposition
  • The electrostatic treatment which allows the high efficiency of new filters disappears in a few hours or even days of use.


According to a recent Canadian study, purifiers with HEPA filters actually filter 68% of ultrafine particles and 63% of fines. Let's make a distinction between theory and practice!

A very demanding interview

Depending on the type of appliance and the filters that make it up, maintenance is a delicate but essential operation. It is closely related to the pollution rate and the quality of the prefilters. Manufacturers are adamant that if the purifier is not regularly serviced, it becomes a source of air pollution itself.


  • The filter should be cleaned every 3 – 4 weeks.
  • The filter should be replaced at least once a year. Chinese bloggers recommend to replace it several times a month, during pollution peaks.
  • Allergy and asthma sufferers are strongly advised not to handle filters.
  • Manufacturers recommend wearing a mask and gloves when changing the filter and washing your hands thoroughly after the filter change. Pollutants disperse very easily. Changing the filter, which is very demanding, is essential.
When you pay for an air purifier with HEPA filter, you know what you are committing yourself to: noise, drafts, maintenance costs ... and an efficiency less practical than theoretical, unlike Teqoya ionizer air purifiers.

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