Friday, August 15, 2008

Are All Air Filter Ratings Equal?

By Dan Morrison

Don't let your lungs pay for bad air filters. Some 99% effective filters are only 10% effective when you change the sampling method. Particle size matters, and so does sampling method.

Our final speaker at Building Science Summer Camp (Day 2), Dr. Dieter Weyel, 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 that gravity affects us. 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; they just float around obeying the laws od oceanic currents. Algae are less dense than the salt water they float in, so they float. Small particles are less dense than the air they float in, so they float around moving in response to the 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 clean 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

Dr. Weytal 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 it catches. Counting particles isn’t very effective for particles over about 5 micrometers Weyal says, because larger particles tend to overlap.

According to Weyal, 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 get a percent efficiency.

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 matters too.

--Dan Morrison is managing editor of

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