HEPA filters were first developed during World War II by the US government as a way to create an effective gas mask. Little has changed with HEPA filter media since then and it remains the most effective way to remove particulates from the air.
What is a HEPA filter?
By definition, a HEPA filter removes 99.97% or more of particles that are sized 0.3 microns and larger. Allow me a moment to get nerdy and explain why the standard is based on 0.3 microns.
The reason for measuring the HEPA filter efficiency at 0.3 microns is based on the original test equipment that showed this as the point most difficult for a filter to capture airborne particulates. More recent research has revealed the most penetrating particle size (MPPS) to be in the range of 0.12 to 0.17 microns. It’s the weakest point. It makes sense that performance will increase as particles get bigger but what is not commonly understood is that performance also improves when you move below this point to smaller sized particles.
There are different principles of physics at work that influence particle removal rates and they perform very differently. Taken together, the point where HEPA filters have the lowest efficiency is at the most penetrating particle size. As seen in the example graph below, diffusion provides higher efficiencies below the MPPS and interception provides higher efficiencies above this point.
The other aspect of HEPA filters is the 99.97% or more removal rate. This is the standard set by the Department of Energy. What’s important here is that while these numbers all sound impressive there is a huge difference in performance between 99%, 99.97% and 99.99%+. A 99.97% performer compared to a 99% will remove much more of the tiniest of particles and those happen to be the most dangerous since they are too small to be captured by your nasal system and can pass directly to your lungs. These tiny particles stay suspended in the air the longest and are more likely to be breathed in.
True HEPA filters are fairly expensive to manufacture and with a push to provide affordable air purifiers, many are sadly marketed as HEPA or true HEPA filters when in fact they are not. These filters are probably made with HEPA filter media however when they are bent to form the small filter (ie less than 2” thick) and very little filter media is used there can be a significant drop in performance. These products are almost always made in China and would venture to guess that these companies cannot provide independent test results that verify their performance claims. There is no HEPA filter police to confirm the product claims so nearly every product claims to be HEPA.
Another trick that some marketers use is to claim 99.97% effectiveness but in the fine print they say that is for particles 2 microns and larger. Really? That’s like saying you can run 100 meters in 10 seconds and then footnote that you only went 60 meters.
So what should you do?
What you need depends upon your concerns. If you are sensitive to allergies or have asthma, COPD, etc you will want to use a HEPA air purifier filter and in most cases you get what you pay for. If you are highly asthmatic or allergic you may want to consider a filter that is more efficient than HEPA.
One way to gauge the quality of the HEPA filter is to find out how much filter media it has in square feet. The more filter media there is the better the HEPA filter generally performs and the longer it will last. A filter with very little filter media will experience increased air flow resistance much more quickly and as a result will give you get less air flow (air cleaning) more quickly.
Other informational articles related to HEPA filters.