What Efficiency of Air Purifiers Means Q&A Air purifier efficiency measurement explained - hepa filters technology Answer

From: Carol C
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.

The 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. The EFFICIENCY rating of the machine, is the ratio between the dirty air going in as measured by particles per cubic foot, and the measured particles per cubic foot coming out as clean air into the room. 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.
The best filter over 90% of the air that goes into the unite. A couple are over 95%.One is nearly 100%.Many 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.
Now then imagine if an 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 filter out allergens of viruses as fast as possible, then actual efficiency is important.
This is the air filter industry's little secret- all hepa air purifiers are not made equal even when they all have hepa filters in them. Many factors affect the efficiency of the machine-the filter size itself- some hepa filters are huge and other's teeny. Some hepa filters are thrown together and the pleating on them is uneven so the filter media is not so reliable. Some manufacturers talk about hepa -"something" which is a sure sign that the filter is substandard. 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).
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 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%). Not what I would want to trust my health to.
Good Luck- it is a very complex subject and I truly have tried to keep it simple!
Mercia Tapping
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