A HEPA filter, as those used in hepa air purifiers, is a high efficiency air filter to trap microscopic particles and is used in different devices, such as: hepa air purifiers, vacuum cleaners, etc
What is a HEPA Filter for?
A HEPA air purifier is used to clean the air in the home of impurities that are in suspension. It is ideal to almost completely reduce the presence of allergens such as pollen, spores, mould, dander and pet hair, etc. and to improve the lives of people with respiratory allergies or asthma.
They are also capable of trapping most microscopic particles in the air, which is why the best purifiers use HEPA filters and are recommended by experts.
How Does a HEPA Filter Work?
It consists of a skein of glass fiber filaments (such as borosilicates) arranged at random, like a net, with diameters between 0.2 and 2 microns.
The characteristics that determine the type of HEPA filter are
- Fiber diameter.
- Thickness of the filter.
- Velocity of the contaminating particles in the air flow that passes through it.
In a home hepa air purifier, when the contaminating particles are sucked into the device, they are trapped (stuck to a HEPA filter fiber) by:
By interception: the smallest particles (>0.4 µm) travelling with the aspirated airflow pass near a fibre, graze it and stick to it.
By Impact: in this case, the larger particles (>0.4 µm) traveling with the aspirated airflow do not fit through the gaps in the HEPA filter’s fiber network (due to their size) and crash into them.
By Diffusion: The smaller particles (<0.1 µm), collide with the gas molecules which prevents and delays their passage through the filter. This is the most common mechanism when the air flow is slow.
As we have seen, interception and impact occur in particles larger than 0.4 microns and diffusion occurs in the most microscopic particles, smaller than 0.1 microns.
In the case of medium sized particles, i.e. 0.3 microns, it is called “Most Penetrating Particle Size”. For this size of contaminating particle, diffusion and interception are not very efficient.
For this reason, a good HEPA filter must include in its technical specifications, the retention of the medium size particles that define the type of filter.
What does HEPA filter mean?
HEPA stands for High Efficiency Particulate Air.
Types of HEPA filters
Since their invention in the 1950s, HEPA filters have been governed by a standard. They must meet a number of technical requirements, which are applied by various international organizations.
In the European Union, a HEPA filter is regulated by the standard UNE-EN 1822-1:2010 “Absolute filters (EPA, HEPA and ULPA). Part 1: Classification, general principles of testing and marking” in force since June 2, 2010.
In this regulation, HEPA filters are called very high efficiency filters and are subdivided into two classes of HEPA filters:
HEPA filter class H13, trapping at least 99.95% of the contaminating particles (only allows 50 out of 100,000 particles to pass).
Class H14 HEPA filter, traps at least 99.995% of the contaminating particles (only allows 5 out of 100,000 particles to pass).
If you are looking to buy an air purifier, you may have noticed that in the current market, there are several types of HEPA filters, commercially labeled as True HEPA, HEPA-like and HEPA-type.
It is important to pay close attention to this detail, because they are not the same as each other, nor are they equally effective. For example, a HEPA-type or simply HEPA filter is effective in trapping 99% of particles up to 2 microns in size, but a True HEPA filter can remove 99.97% of particles as small as 0.3 microns. As you will see the difference is very large and its effectiveness for some allergens as well.
To choose a good air purifier with a HEPA filter, we must always look in the technical specifications that the filter is True HEPA (real or true HEPA).
A filter rated as True HEPA meets the requirements of the U.S. Department of Energy, which states that a HEPA filter must trap at least 99.97% of particles at least 0.3 microns in size. To give you an idea, a human hair is 5 microns in diameter (almost 7 times the size of the smallest allergen).
Buying a hepa air purifier with a True HEPA filter gives you the guarantee and security that it meets the minimum requirements set by a regulation, whereas with the other types of HEPA filters we would need to trust what the manufacturer promises.