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What is HEPA?

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.

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 filter efficiency at 0.3 microns is based on scientific tests that show that this is the point where it is most difficult for a filter to capture airborne particulates. 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 from 0.3 microns 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 they have the lowest efficiency is around 0.3 microns. As seen in the example graph below, diffusion provides higher efficiencies below 0.3 microns and interception provides higher efficiencies above 0.3 microns.

 

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The other aspect of HEPA is the 99.97% or more removal rate. This is the standard set by the US Dept 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 pas-s 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 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. 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 which is much lower standard.

So what should you do?

What is HEPA

If you are sensitive to allergies or have asthma, COPD, etc you will want to use a HEPA filter and in most cases you get what you pay for. If you are highly asthmatic or allergic or suffer from pollution you may want to consider a filter that is more efficient than HEPA.

One way to gauge the quality of the filter is to find out how much filter media it has in sq feet. The more filter media there is the better the filter generally performs and the longer it will last.

HEPA Class (American Satndard) 0.3 – 1 µm 1 – 10 µm HEPA class (Europen Standard) retention (total) retention (local)
Merv 15 85%-95% >90% E10 > 85%
Merv 16 >95% >95% E11 > 95%
Merv 17 >99.97% >99.97% H13 > 99.95% > 99.75%
Merv 18 >99.99% >99.99% H14 > 99.995% > 99.975%

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