HEPA vs ionizers vs ozone generators

These three sit next to each other in shops and in search results, and they are not variants of one thing. Understanding the differences takes about four minutes and it is the single most useful thing on this site if somebody in your house has a respiratory condition.

A HEPA filter is a dense mat of fine fibers. Air is pushed through it and particles are physically captured — intercepted, impacted, or caught by diffusion depending on their size. Nothing is created, nothing is emitted, and the captured material is removed from the room when you replace the filter.

The only meaningful variables are how efficient the media is and how much air passes through it, and Clean Air Delivery Rate captures both together. That is why CADR is the number to buy on and why filter grade alone is a poor proxy. What CADR measures.

An ionizer emits charged ions into the room. Airborne particles pick up that charge, become attracted to each other and to surfaces, clump, and settle out of the air faster than they otherwise would. Particles are removed from suspension, which is genuinely something — but note where they go. They land on surfaces in the room rather than being captured in a filter you can throw away.

The relevant concern is that generating ions can also generate ozone. EPA states that air cleaners containing electrostatic precipitators, ionizers, UV lights without adequate lamp coatings and plasma air cleaners may have the potential to emit ozone.

How much, in practice? This is where the ENERGY STAR register earns its keep, because it publishes a measured figure per model.

These are not stronger air purifiers. They work by generating a reactive gas, and the concentrations required to affect airborne contaminants are not concentrations that are appropriate in a room with people in it. They are marketed hardest for exactly the situations where people are most desperate — smoke damage, mold, persistent odors. The full comparison.

• No respiratory conditions in the house: buy on certified CADR and running cost. A 2 ppb ionizing machine is not something to worry about.
• Asthma, COPD, or a small child: choose a machine the register records as fan-and-filter with 0.00 ppb. You give up nothing — our overall pick across this site is in that group.
• You already own an ionizing machine: if the ionizer is switchable, switch it off. Certified CADR does not depend on it.
• Never run an ozone generator in an occupied space.

Quick picks

Certified machines that the register records as fan-and-filter with zero measured ozone.

#	Product	Best for	Certified CADR	Price
1	No photo Zero ozone, best efficiency Honeywell InSight HPA4300 Fan and filter only, 0.00 ppb measured, 7.4 CFM per watt.	True HEPA on the lowest power draw	229CFM smoke AHAM VERIFIDE	Check price on Amazon→
2	Zero ozone, more air Winix AT41 265 certified CFM with no ion generator at all.	Pet allergies, whole living room	265CFM smoke AHAM VERIFIDE	Check price on Amazon→
3	No photo Zero ozone, large rooms LEVOIT Core 600S 391 certified CFM across 606 sq ft, 0.00 ppb measured.	Large open-plan rooms	391CFM smoke ENERGY STAR verified	Check price on Amazon→

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