What does air purifiers efficiency mean?

What does HEPA classification of Air Purifiers Means Q&A

QuietPure Home Air Purifier
QuietPure Home Air Purifier


Hi Mercia, Sorry to bother you again.

What does efficiency mean, whether it’s 99 percent or 15 percent?

Does it clean X percent of some array of particles over X volume of room over X amount of time?

I need a simple, accurate way to say it.


Carol C


Dear Carol,

Firstly, it’s not a bother at all, we’re always happy to help!

“What does efficiency mean, whether it’s 99 percent or 15 percent?”

The EFFICIENCY rating of the air purifier, is the ratio between the dirty air going in as measured particles per cubic foot, over the measured particles per cubic foot coming out as clean air into the room.


There is a relationship like you suggested but it’s a ratio or proportional relationship. What is important  is dirty air in a room is sucked into the HEPA filter by a fan. There are millions of particles per cubic foot as the dirty air gets sucked into the machine. After the air has passed through the filter, clean air gets expelled out into the room. I hope these additional questions are helpful Carol.

Are all HEPA air filters are the same?

In other words, some machines suck dirty air into the filter and blow a huge proportion of still dirty air out the other side. How could that possibly happen if all the machines have HEPA filters in them? Answer NOT ALL the air actually hits the HEPA filter in a badly designed machine. It’s like having a leaky drafty house. In the best designed machines of which there are few, ALL the air gets forced through the filter so the clean air has zero or near zero particles coming out the machine.

What filtering level should I be looking for?

The best air purifiers filter have over 90% of the air that goes into the unit. A couple are over 95%. One is nearly 100%. Many air purifiers, sadly, are like a sieve; air goes in and very little ever hits the HEPA filter and it just comes out the other side as dirty as ever.

Is the percentage of air purifier filtering the only thing I should be looking at?

Now then imagine if an air purifier takes several passes to get the same air clean because it is inefficient- it will take longer to clean the air in the room, but it might get the room clean eventually. So if your allergies are not bad, or it is not so important to you, you might well decide to go with a cheaper, less efficient machine- you have time on your side. On the other hand, if you are really sick or really need to actually filter out the allergens or air pollution, as fast as possible, then actual efficiency is important.

What does HEPA mean? Aren’t all HEPA filters the same?

This is the air filter industry’s little secret- all HEPA air purifiers are not the same, even when they all have HEPA filters. Many factors affect the efficiency of the machine:

  • Filter size – some HEPA filters are huge and other’s teeny.
  • HEPA quality filters – some HEPA air purifier filters are thrown together and the pleating on them is uneven so the filter media is not so reliable.
  • Low quality filter – some manufacturers talk about HEPA -“something” which is a sure sign that the filter is substandard.
  • Depend on fan speed – some HEPA filters are fine when the fan speed is low (and therefore smaller room coverage) and some are still efficient when the fan speed is up (larger coverage).

What kind size HEPA air purifier should I be looking at?

Association for Home Appliance Manufacturers (AHAM) rates air purifiers for room size based on high fan speed – problem is no one wants to run them on high fan speed because they are noisy. So most consumers buy an air purifier with HEPA filter AHAM rated for a room size and then run it on low fan speed (usually)1/4 ration to high fan speed and then wonder why they do not work. The issue is they have bought an undersized machine. They ought to buy a machine which they think is too big, then run it on lower and quieter fan speeds and then they will get close to what they want.

Ok…I am going to stop now before I start confusing you…there is way more to this story. What is important to understand is HEPA is the best technology and this is what the government at Ready.gov is recommending for homeland defense security and also gave FEMA grants to citizens after 9/11 in NYC. There are other air purifier filter technologies that compete with HEPA; their advantage is they are often quieter and cheaper to run, but their efficiency is terrible (15 to 30%) and not something I would trust with my health.

Good Luck- it is a very complex subject and I truly have tried to keep it simple!

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