Do HEPA filters remove pet dander?

HEPA filters are rated at 0.3 microns because that is the most difficult particle size to capture, not the smallest one they handle. Larger particles are caught by simple interception and impaction — they cannot follow the air around a fiber. Much smaller particles move by Brownian diffusion, jittering randomly enough that they collide with fibers more often than a straight-line path would predict.

Between those two regimes sits a minimum, at roughly 0.3 microns. Testing at the worst point means a filter meeting the standard performs better on both sides of it. So “captures particles as small as 0.3 microns” is the exact opposite of the limitation it sounds like — and marketing that boasts about capturing 0.003 microns is describing a regime where filtration is easier, not harder.

Pet allergen is a protein, not a particle size. Fel d 1 and Can f 1 attach to whatever is available — skin flakes, dried saliva, dust — and end up distributed across a wide range. Custovic and colleagues found airborne cat allergen concentrated on relatively large particles, with a persistent smaller fraction. All of it is within HEPA media’s competence.

Which is why filter grade is rarely the thing that decides whether a purifier works for you. Particle sizes, laid out properly.

“True HEPA” is a marketing phrase. No body certifies it in the consumer purifier market. What does exist is the filter classification recorded in the certification registers, and it is not always what the box says.

The practical reading: a HEPA-Type machine with high certified CADR delivers more clean air than a HEPA machine with low certified CADR. The classification is a real distinction and it is not the whole picture — which is exactly why CADR, being a combined measure of filtration and airflow, is the better number to buy on. The full explainer on filter grades.

Not the media. Three things, in order:

• Airflow through the filter. A superb filter with a weak fan cleans very little air. CADR captures both together, which is why it resists gaming.
• Room size versus certified coverage. The commonest failure by a wide margin.
• Whether it is switched on. Which is a noise and running cost question, not a filtration one.

Filter grade is real and worth checking. It is fourth on that list, and the category talks about it first because it is the easiest thing to print on a box.