How HEPA Filters Work
HEPA stands for high efficiency particulate air. The EPA calls it a type of pleated mechanical air filter. A fan pushes air through the filter. Particles get caught in it.
The EPA quotes the US Department of Energy definition. A HEPA filter can in theory remove at least 99.97% of dust, pollen, mold, bacteria and other particles that are 0.3 microns in size. Why 0.3 microns? The EPA says that size is the worst case, the size that gets through most easily. Particles that are bigger or smaller are caught even better.
The EPA also says portable air cleaners often get a high CADR (clean air delivery rate) by using a HEPA filter.
How Ionizers Work
An ionizer puts an electric charge on particles in the air. Charged particles stick to nearby surfaces, like walls, floors and furniture. Some ionizers have a collector plate that pulls them in instead.
Particles that land on a couch or a wall are out of the air, but they are still in the room. A HEPA filter holds what it catches until you change it. The EPA says ionizers were found less effective at removing particles than high efficiency particle filters.
The Ozone Problem
The EPA says air cleaners with ionizers, electrostatic precipitators, UV lights without good lamp coatings, and plasma may have the potential to give off ozone. The EPA says that when you breathe it in, ozone can damage the lungs. It also tells you to avoid air cleaners that make ozone on purpose.
California has a rule for this. The California Air Resources Board (CARB) says that since 2010, all portable indoor air cleaners sold in the state must be certified. Electronic air cleaners must also stay under an ozone limit of 0.050 parts per million, which is 50 parts per billion (ppb). CARB keeps a public list of certified devices.
Under the limit is not the same as zero. A HEPA filter is a mechanical filter, and the EPA does not list it among the types that may give off ozone.
Key Point
If a purifier has an ionizer, look for an off switch. Look for the "CARB certified" label on the box too. CARB says certified air cleaners must carry it.
Side-by-Side Comparison
| Feature | HEPA | Ionizer |
|---|---|---|
| How it works | A pleated mechanical filter. A fan pushes air through it and particles get caught. | Gives particles an electric charge so they stick to a surface or a collector plate. |
| Particle capture | At least 99.97% of particles 0.3 microns in size, by the US Department of Energy definition the EPA quotes. | No single standard. The EPA says ionizers were found less effective at removing particles than high efficiency particle filters. |
| Ozone | A HEPA filter is a mechanical filter. It is not on the EPA list of air cleaner types that may give off ozone. | The EPA lists ionizers among the types that may have the potential to give off ozone. |
| California rule | Must be CARB certified to be sold in California. | Must be CARB certified, and electronic air cleaners must stay under 0.050 ppm (50 ppb) of ozone. |
"HEPA-Type" and "HEPA-Like" Are Not HEPA
Watch the words on the box. Some purifiers say "HEPA-type," "HEPA-like" or "HEPA-style." Those words do not tell you the filter meets the HEPA definition.
The HEPA definition is a number you can check: at least 99.97% of particles 0.3 microns in size. If the maker does not say its filter meets that, do not assume it does.
What About Molekule's PECO Technology?
Molekule calls its technology PECO (photo electrochemical oxidation). It is not a HEPA filter. We found no government source that rates it against HEPA.
The EPA says research has not yet shown that plasma, photocatalytic oxidation or UV light can remove gases well in home air cleaners. For particles, HEPA is the one with a published number.
Plasma, UV-C, and Other Technologies
You will also see purifiers that list plasma, UV-C light, photocatalytic oxidation (PCO) or bipolar ionization. Here is what the EPA and CARB say:
- UV-C: The EPA says UV lights without good lamp coatings may have the potential to give off ozone. It also says research has not yet shown UV light can remove gases well in home air cleaners.
- Plasma / Bipolar Ionization:The EPA lists plasma air cleaners among the types that may give off ozone. CARB's rule covers ionizers and plasmacluster devices.
- Photocatalytic Oxidation (PCO):The EPA says research has not yet shown PCO can remove gases well in home air cleaners. CARB's rule covers PCO air cleaners too.
For gases and odors, the EPA says activated carbon works when a large amount is used. For particles, a HEPA filter is the choice with a published standard.
The Verdict
Buy a purifier with a HEPA filter. Turn off the ionizer if your unit has one. Look for the CARB certified label.
HEPA is not new and it does not sound high-tech. But it has a definition you can check: at least 99.97% of particles 0.3 microns in size.
Add activated carbon if you care about odors, smoke or VOCs. The EPA says carbon works when a large amount is used.
Common Questions
Do ionizer air purifiers make ozone?
Some can. The EPA says air cleaners with ionizers, electrostatic precipitators, some UV lights and plasma may have the potential to give off ozone. The EPA also says that when you breathe it in, ozone can damage the lungs. In California, electronic air cleaners must be certified and stay under 0.050 ppm (50 ppb) of ozone. If you want no ozone question at all, pick a purifier that uses a HEPA filter and no ionizer, or turn the ionizer off.
Do ionizers actually clean the air?
They move particles out of the air by charging them so they stick to surfaces. The EPA says ionizers were found less effective at removing particles than high efficiency particle filters. A HEPA filter holds what it catches until you change the filter.
Is Molekule PECO technology better than HEPA?
Molekule calls its technology PECO. We found no government source that rates it against HEPA. The EPA says research has not yet shown that plasma, photocatalytic oxidation or UV light can remove gases well in home air cleaners. For particles, the HEPA definition is the one with a published number.
Why do some HEPA purifiers also have an ionizer?
Some makers add one as an extra stage. For example, the Winix 5500-2 manual says the unit starts in Auto with PlasmaWave turned on, and that it meets the 0.050 ppm ozone limit. Most units with an ionizer let you switch it off. Check the manual for yours.
What about plasma, UV-C, and photocatalytic purifiers?
The EPA says research has not yet shown that plasma, photocatalytic oxidation or UV light can remove gases well in home air cleaners. It also says plasma air cleaners and UV lights without good lamp coatings may have the potential to give off ozone. California's ozone rule covers all of these types.