HEPA stand for "high efficiency particulate air". Put simply a HEPA filter is a type of filter that can catch a large amount of very particulate matter. HEPA air filters stop mold spores as well as many bacteria and viruses and of course larger items such as dust. (HEPA filters on their own do not remove odours, chemicals or gasses - therefore typical tools that utilise HEPA virus usually have an activated carbon based filter as well to absorb odours and chemicals).

History of HEPA air filters

HEPA air filters were originally developed by the Atomic Energy Commission (AEC) during the second World War to efficiently remove and hold onto radioactive dust from the air inside the laboratories. The first HEPA air filters were very large compared to the HEPA air filters that are produced today. HEPA filter technology was declassified after World War 2. Since the war, HEPA filters have been used in laboratory and medical environments where the cleansing of the air of contaminants is critical for human health and safety or the success of an experiment.

HEPA air filters and tools that contain them (air purifiers and vacuum cleaners) have only recently become available for domestic use. They are very effective in removing harmful micro-organisms and other particulate matter from the air.

The Environmental Protection Agency (EPA) as well as the Occupational Safety and Health Administration have endorsed the HEPA filter as being the best filter type for removing particles from the air.

Types of HEPA filters

There are two types of HEPA filters - true and HEPA type.

True HEPA filters (absolute HEPA filters)  

These types of filters must pass a test to prove they are HEPA filters. If the filter passes the test, it will have a serial number allocated to it – the results of the test will be printed on the filter. True or Absolute HEPA filters must be able to catch at least 99.97 percent of particulate matter of 0.3 microns in size, almost 300 times smaller than the width of a human hair. These types of filters tend to be more expensive the normal HEPA filters, but they perform better. 

HEPA type Filters

There is another category of HEPA filters that are less expensive than True or Absolute HEPA filters. These are usually known as HEPA type filters. Although they may be made in a similar way to True or Absolute filters, and may even look like them, they do not trap the same amount of particulate matter – often only  85 to 90 percent of particulate matter or less for particles of 1 micron and below.

How tell the difference

How do you tell the difference between HEPA type filters and True HEPA filters? The key is the presence of the serial number and test results. They should be printed on a True or Absolute HEPA filters. HEPA type filters do not have this data. When you read the data, make sure that the test results at .3 microns are 99.97 percent or above.

HEPA filters should data will state the following:

Direction of flow

% penetration at rated flow

Size of the filter

Separator type

Resistance at rated flow

How do HEPA filters work?

HEPA filters are made up of a mesh or network of randomly arranged fibre glass fibres pleated in a “V” arrangement resembling a folded paper fan. This is attached to a sturdy base, forming the core of the filter. Fibre density and diameter, and filter thickness are the two key characteristics of the filter that affect its efficiency. HEPA filters are designed to trap very small particles and contaminants by these methods:

Sieving

This the most simple form of filtation in a HEPA air filter. Sieving prevents large particles from passing through the filter. This includes all particles 5 μm and above.

Interception

Put simply, this where particles in an air stream flowing through the filter get close to a fibre in the network and stick to it.

Impaction

Put simply, this is where particles in an air stream flowing through the filter fly into a fibre and embed on it.

Impaction and interception are the dominant methods for particles above 0.4 μm.

Diffusion

This is where very small particles, especially those below 0.1 µm in diameter, are impeded and delayed in their path through the filter because they crash into gas molecules. Diffusion predominates below the 0.1 μm diameter particulate matter size.

In between 0.4 μm and 0.1 μm, in the region of 0.3 μm, diffusion and interception methods of filtration tend to dominate.

The nature of the filter mesh means that HEPA restricts air flow and a separate fan is needed to handle the restriction. The fan usually draws in air through a louvered grill.