Why “99.97% HEPA” Doesn’t Mean Your Air Gets Clean

Ninety-nine point nine-seven percent. It’s printed on almost every air purifier you’ve ever seen. It’s technically true — and on its own, almost useless. Because that number doesn’t tell you whether your air actually gets clean.

Where the number comes from

“99.97% at 0.3 microns” is the HEPA standard: in a lab, the filter media traps 99.97% of particles at 0.3µm — the hardest size to catch. Bigger and smaller particles are caught even more easily. So the filter itself is excellent. That was never the problem.

What the number hides

99.97% is a per-pass figure — how clean the air is each single time it goes through the filter. It says nothing about how much air actually goes through, or whether the machine is powerful enough for your room. A brilliant filter in a unit that pushes a trickle of air — or one that’s simply too small for your living room — leaves your air dirty. 99.97% of not-very-much is still not-very-much.

The number that actually matters: CADR

CADR — Clean Air Delivery Rate — combines filter efficiency and real-world airflow. Pair CADR with your room size and you get air changes per hour (ACH): how many times an hour the machine can genuinely clean all the air in the room. That’s the honest measure. Notice which numbers brands print in giant font, and which they bury.

What to do instead

• Find the CADR, not just the HEPA percentage.
• Match it to your room — aim for a few air changes per hour in the space you actually use. Our free ACH calculator does the maths for you.
• Check it’s true HEPA (H13), in a unit that seals the filter so air can’t sneak around it.
• Remember the filter is a consumable — budget for replacements.

That’s the whole idea of this channel: the honest version, no sales pitch. No brand pays us to pick one.