HEPA Air Purifiers

High Efficiency Particulate Arresting (HEPA) filters have been around for quite some time.  In the 1950s the US Atomic Energy Commission needed something to remove small particulates that are radioactive. Since then HEPA air purifiers have been using this technology to clean the indoors in cleanrooms to beauty parlors. HEPA products are also one of the many techniques and products used by Allergy and Asthma doctors to help alleviate the symptoms of those conditions. First we will discuss what HEPA really is.

Imagine a filter that only allows very tiny particles to pass through. If we placed a single sheet of this HEPA paper in front of a fan constrained as it would be in air purifiers, very little air would be able to pass because of the small size of the holes. In addition, air purifiers made like this would need to have the filter changed often because the holes would plug rapidly. Now if we double the size of the sheet we would get twice the airflow and filter life. You can not keep growing the sheet larger because it would be impractical. Instead let us try folding the paper back and forth so that we can present a very large surface area to the airflow and thus the HEPA made this way would be efficient. This is how real-world HEPA filters are made in real HEPA systems. Some have as much as 40 square feet of the filter material folded into the HEPA section. As a side note the material from which HEPA is constructed is either fiber or paper-like on one hand or a polymer on the other.

HEPA material does not look like a screen or a colander. Instead it looks like a very thin bail of fibers.  Thus the air has to find a route through this maze of fibers.  There are three ways the HEPA filter stops particulates.  First and the easiest to understand is that a particle runs into a fiber and sticks. Secondly, the particle gets within one diameter of a fiber of the HEPA filter and while it tries to "skid by" the fiber it is gets stuck on the fiber.  Third, as a very small, about 0.1 micron, particle moves in the gas flow it dithers about due to collisions with molecules (Brownian motion)  and again happen to slide close to a fiber and get caught.

Therefore, HEPA air purifiers stop mold spores as well as many bacteria and viruses and of course larger items such as dust. Most air purifiers claim to be 99.97% efficient at removing particles 0.3 microns from the air that passes through the HEPA filter.  The operative phrase is "pass through"!  If the airflow does not have an opportunity to pass through the HEPA filter it will not be cleaned.  Therefore the claim of 99.97% of all .3 and larger particles being removed is not accurate in poorly designed HEPA air purifiers where some of the airflow may pass around the HEPA filter and return to the room not cleaned. The IQAir HealthPro series is the only line that specifies that over 95% of the airflow that enters the air purifier goes through the HEPA filter.

As good as that sounds pure HEPA air purifiers do not remove odors, chemicals or gasses. Since these are molecular level substances the 0.3 micron holes are large compared to the gas molecules. Therefore typical HEPA air purifiers have some level of activated carbon based material to absorb odors and chemicals. The activated carbon that is included with all of these units comes in a number of varieties.  This varies from a thin mat in an Air-O-Swiss unit, to pounds of activated carbon in an Austin Air Purifier.  In any form the carbon absorbs gasses that will not be caught by the HEPA element.  These chemicals are either harmful gasses or those that cause odors. 

As a side note the Blueair product line uses a special approach to HEPA.  In addition to the providing a large surface air in their products the Blueair products have larger openings in their polymer based filter media. This allows a much higher airflow and thus requires a small, less noisy fan.  The trick in these air purifiers is the addition of electrostatic brushes in the airflow prior to the HEPA element.  This charges the pollution particles and gets them to "stick" electronically to the filter media.

HEPA air purifiers are straight forward, a fan forces airflow through a filter.  The more times the airflow goes through the filter in an hour the cleaner the room. So, the bigger the room, the bigger the product, or our preference, the more small air purifiers.  Why?  Think about a long narrow room.  If you put the product at one end how often is the air from the other end going to get through the HEPA filter?  Not often.  If you put smaller air purifiers at either end of the room the air will only have to move half as far to get through the HEPA and get clean.

Ted Zajac, Jr.
MSE Johns Hopkins University