What is a HEPA filter and what is not a HEPA filter?

Many people do not know what the difference between a HEPA filter and a normal furnace (media) filter. Many products on the internet or in stores claim to be "HEPA rated" or "HEPA type" filters. There are some specific definitions for HEPA that may disqualify many filters being represented as "HEPA." This article will help you in spotting true HEPA filters versus regular media filters.

Background:

HEPA stands for: high efficiency particulate air filter. The first HEPA filter that was ever introduced was in 1940 for the Manhatten Project to help reduce the spread of airborne radioactive contaminates. They did not hit the market until the 1950's for residential and commerical use. Over time HEPA filters have changed many times to meet the demands of improving air quality in medical or aeronautical applications.

There are some true HEPA products that are available for use in your home - but most of them are NOT the furnace or AC filters you change in your mechanical room:

Neither of the examples above have a true HEPA filter.

Unfortunately, HEPA filters restrict too much air to be used as a traditional furnace filter. A HEPA filter is extremely efficient, but a significant amount of air pressure is needed to force air through a HEPA filter. A typical HVAC system will not be equipped to provide enough air pressure to use a HEPA filter (which basically means that no air would blow out of your registers).

In order to use a HEPA filter for whole house filtration, you need to install a separate HEPA filtration system. A system like this will pull some of the air out of the regular air flow through your furnace, and then boost the air and pass it through a HEPA filter. The extremely filtered air is then returned back into the normal air flow.

Tests:

To get to carry the name "HEPA filter", specific guidelines that must be met as set forth by the U.S. Department of Energy. Follow the link below for factors you must have to qualify it to be a HEPA filter:

Department Of Engery Guidelines for HEPA filters

A true HEPA filter must be able to filter 99.97% of all contaminates that is at least .3 microns in diameter out of the air. To show how small a micron actually is, the human hair is about 70 microns in diameter, so you can imagine how small of particles a HEPA filter can take out. HEPA filters are composed of a mat-like material that have fibers randomly arranged. These fibers, however, do not let bigger particles pass by to a certian point as some people think. They are designed to "seek out" smaller particles and pollutants to have them stick to these fibers in three separate ways.

1. Interception, where particles following a line of flow in the air stream come within one radius of a fiber and adhere to it.
2. Impaction, where larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly; this increases with diminishing fiber separation and higher air flow velocity.
3. Diffusion, an enhancing mechanism is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 micron in diameter, which are thereby impeded and delayed in their path through the filter.

As these filters start to build up and the fibers trap more and more dust and pollutants, you will start to notice a pressure drop that can be measured with instruments that are designed to read this type of output. Here is a simple micron chart to see the size of common items' micron size:

HEPA filters today:

You probably see HEPA filters everyday when you do simple tasks around the home or out and about. Some of the most popular HEPA filters in circulation now are on vacuum cleaners. These are very popular with people that suffer from asthma or severe allergies, as a vacuum will kick up quite a bit of dust. HEPA filters on your vacuum act as a secondary filter for the air that is expelled from the vacuum itself. The only drawback with these filters is, you have to have 100% of the air going through the vacuum and then to this filter for it to work. Any cracks or points where the air can get past the filter will be defeating it's purpose as it will allow a lot of particulate matter to go right past the filter. The vacuum will usually be quite a bit bigger if it has a HEPA filter because it does take a bit bigger motor to be able to force this air through the filter.

Another place where HEPA filters are found most often is in Hospitals. Bio-Medical HEPA grade filters are a critical item to have for every hospital. A medical grade HEPA filter will be able to filter out 99.995% of all airborne contaminates. Hospitals will also usually install a UV light system above the HEPA filter to kill the other .005% that may make it past. At any time, airborne contaminates could escape a room in a hospital if they do not have these types of HVAC systems installed. They will also have multiple HEPA filters in place not only for clean air throughout the hospital but also for any air that is exhausted outside the hospital. Typical mold and bacteria have a micron size of about 1-5 microns in diameter, so almost every single one of them gets trapped in the filter very easily. There are many steps that also go into make sure no one tampers with a duct that is moving this contaminated air unless there is a medical grade HEPA filter in place.