Comment by jefftk

The article explains it well:

Here’s a thought experiment: Take a 1000 cubic feet room and a purifier that processes 100 cubic feet of air per minute. (I follow Wirecutter in using vulgar imperial units.) Assume pessimistically that all particles are the worst-case size. If you run that purifier with an E12 filter, the fraction of particles that will remain after one minute is

     .1 × (1-.995) + .9 = 0.9005.

That’s because 10% of the air goes through the purifier and has 99.5% of particles removed, while 90% of the air doesn’t go through the purifier at all.

Meanwhile, if you run that purifier with an H13 filter instead then the fraction of particles that remain will be

     .1 × (1-.9995) + .9 = 0.90005.

If you noticed that 0.9005 and 0.90005 are almost identical then congratulations—you understand air filters better than the Wirecutter. Both 99.5% and 99.95% are close enough to 100% that performance is almost entirely determined by the volume of air they process.

Almost all of these review sites, not understanding the physics involved, believe a HEPA filter sieves particles down to a size of 0.3 microns, which implies that anything smaller passes on through.

This is utterly false. HEPA filters are measured at the efficiency of what’s known as the MPP (the Most Penetrating Particle size). It’s the hardest particle size to capture as it can get by the two methods used to capture large particles (impaction), and smaller particles (diffusion).

Considering almost none of the air in a room is passing through the filter at a given moment, the efficiency of the filter is less important than how much air it moves through the filter media per minute, which IKEA have favoured here.

Essentially this filter performs close to par with more expensive units, while using less energy, and having dramatically lower costs for filter replacements when due.

What they don’t do is give reviewers either kickbacks or basic physics lessons.

HEPA makes sense if you filter all the air, ie. the filter is inline like in a laminar flow cabinet/cleanroom or directly inserted in an air stream filtering 100% of the downstream air. In those cases you care a lot about how many particles make it through since they will cause yield loss or contamination in the processes.

For EU standards, a filter removes X% of the hardest particle size to remove (called the Most Penetrating Particle Size or MPPS). That size varies between filters, but is often around 0.3 microns. Filters will do better than X% for particles both larger and smaller than the MPPS. So for the EU standard, we're saying "it'll do X% worst case, and better than X% for all other cases, meaning it'll always filter out more than X% of all particles, regardless of size".

If X% is 99.95% or higher and below 99.9995%, it's technically HEPA; 99.9995% and above it's ULPA, and below 99.95% (but 85% or higher) it's EPA.

So let's say you've got an E12 filter that removes 99.6% of particles. Technically not HEPA, but after one pass through the filter and you've got 0.4% of the particles left. Two passes and you've got 0.0016% left, three passes and it's 0.0000064% left.

An H13 filter might remove 99.97% of particles. One pass and you've got 0.04% left, two passes and you've got 0.000016% left.

Or in other words, one pass through a technically HEPA filter might leave you with 25 times more particles than two passes through a technically non-HEPA filter. So if the air is cirulating back through the filter (as it would in a closed room), what matters is both how good the filter is and how much air it can filter. And since at the high end the filters are all so good, the volume of air processed dominates. A filter that processes twice as much air is vastly better than one that filters out an extra fraction of a percent of particles.

(US standards are similar, but the cutoff for HEPA is 99.97% of 0.3 micron particles, not 99.95% of "whatever the filter is worst at". But the difference is generally irrelevant.)