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Green Building Blog

Are All Air-Filter Ratings Equal?

Effectiveness Ratings Change, Depending on the Size of the Particles Being Filtered.

Some filters labeled as 99% effective are only 10% effective when you change the sampling method. Particle size matters and so does sampling method. Don’t let your lungs take up the slack._

Wrapping up Day 2 of building science summer camp, Dieter Weyel, Ph.D., talked about filters, filtration, and ratings. Turns out there are some pretty important differences. Particles, like everything else, have to obey the laws of physics. It’s just that our understanding of physics is skewed because we’re big enough to be affected by gravity. Gravity affects some particles, too—when they get to be about 10 micrometers. But small particles, say 1 micrometer, can fly. Sort of. They behave like algae in the ocean that just float around, obeying the laws of oceanic currents. Algae are less dense than the saltwater they float in, so they float. Small particles are less dense than the air they fly in, so they fly around in response to air currents.

Big particles fall to the ground and you have to clean them up with a damp cloth or a duster. Small particles float around and you collect them with a filter. As it turns out, lungs are excellent filters. Problem is, clean lungs work better than dirty lungs. That’s where filters come in. There are three ways to measure the effectiveness of a filter:

  1. Count the number of particles it catches.
  2. Count the area of the particles it catches.
  3. Count the weight of the particles it catches.

Weyel used 10 small particles (1 micrometer) and one large particle (10 micrometers) as an example. If the filter only catches the large particle:

  • Method #1 yields a 10% filter.
  • Method #2 yields a 92% filter.
  • Method #3 yields a 99% filter.

For ratings, particle size and sampling method matter

HEPA filters must be judged according to method 1, counting the number of particles caught. Counting particles isn’t very effective for particles over about 5 micrometers, Weyel says, because larger particles tend to overlap.

According to Weyel, ASHRAE measures “sort of by area.” They dump particles into a duct, spray them through a filter, and probe for particles before and after the filter. They shine light through the filter; if there are a lot of particles, not much light gets through. They compare one side of the filter to the other and derive an efficiency percentage.

As it turns out, particles vary in density, too—a styrofoam peanut and a cork are of similar size, and they both float on water, but they have very different densities. So density also matters.

—Dan Morrison is managing editor of


  1. Filtrete | | #1

    Some queries...
    Its is obvious that a normal home user can't use the methods mentioned above...not even the weight measurement is possible coz it is really a less amount when collected...As a scientific method the procedure may be very simple some Air filters are really good although marked to be not very efficient...

    Well what i want to say is that....a simplified review is really required...i guess...

    Regards, ResearchGuru

  2. Danny | | #2

    Counting the number of
    Counting the number of particles is quite tricky - is not the weigh a far better way to have a standardised system. Maybe its just me - but the problem with air filters is the lack of studies rather than the studies themselves. I would rather know that the air filter in myh hand has been tested then make the whole system even harder.

    Danny - blogger

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