HEPA vs carbon air purifier filters, explained

Key takeaways

• A true HEPA filter, as defined by the U.S. Department of Energy, removes at least 99.97% of airborne particles 0.3 microns in size, and catches larger and smaller particles even more efficiently.

• Most filters are built for either particles or gases, so many air purifiers pair a particle filter with a second filter for gases and odors, according to the EPA.

• Activated carbon is the filter the EPA points to for gases, and it works best when the filter holds a large amount of carbon.

• HEPA and carbon filters can't be washed and have to be replaced, while many pre-filters can be vacuumed or washed, per the Washington State Department of Health.

• Every portable air cleaner sold in California must be certified by the state's Air Resources Board, and electronic models must stay under an ozone limit of 0.050 parts per million.

The layers inside a typical air purifier

Open most portable air purifiers and you'll find a stack of filters, each with its own job. The Washington State Department of Health describes a common setup as an optional pre-filter, a main HEPA filter, and an optional carbon filter. The main filter handles particles like dust, pollen, and smoke, and the other two support it.

Particles and gases are different problems. The EPA's guide to air cleaners in the home notes that most filters are designed for one or the other, which is why many units carry two filters, one for particles and one for gases. Some are also aimed at specific gases or VOCs, and the packaging says which.

Pre-filters: the first catch

The pre-filter is the outer layer, the first thing incoming air hits. Montana's state health department says pre-filters remove larger particles such as pollen and dust. Washington's health department adds that they help extend the life of the HEPA filter behind them, since less coarse material reaches the finer media.

Pre-filters are also the layer you're most likely to clean rather than replace. Washington's guidance is that pre-filters may be washable, and King County, Washington, says to vacuum or wash a reusable pre-filter as the manufacturer indicates. Some pre-filters are meant to be replaced instead, so the manual is the place to check which kind you have.

What makes a filter true HEPA

HEPA stands for high efficiency particulate air. The EPA's explainer on HEPA filters describes it as a pleated mechanical filter, officially defined by the U.S. Department of Energy, that can remove at least 99.97% of dust, pollen, mold, bacteria, and other airborne particles 0.3 microns in size.

That 0.3-micron figure is the hardest size to catch, which the EPA calls the most penetrating particle size. Particles larger or smaller than that are trapped with even higher efficiency. HEPA filters get there with very deep pleats that give them far more surface area than an ordinary pleated filter, according to the EPA's technical summary of residential air cleaners.

In a home, that covers the usual particles. Montana's health department lists dust, mold, pet dander, and particles from smoke and cooking among the indoor pollutants a HEPA filter traps. King County's HEPA cleaner guidance adds smog, soot, and allergens.

Reading the HEPA claim on a spec sheet

The words on the box can vary from brand to brand. The EPA's technical summary notes that there is no widely accepted definition of HEPA performance for consumer products, so the name on a filter tells you less than the numbers behind it.

The useful figures are the share of particles a filter captures and the particle size that share applies to. A spec sheet that lists both, such as 99.97% at 0.3 microns, lets you compare filters directly, and Washington's health department suggests asking the manufacturer when the technical specifications don't say.

Activated carbon for odors and gases

A particle filter is built for particles, so gases and odors are a job for a second layer. To filter gases, the EPA recommends an air cleaner with an activated carbon filter or another filter made for gases. Its guide adds that carbon filters work best with a large amount of material, and that thicker is better.

Montana's health department puts the division of labor in plain terms: a HEPA filter removes visible smoke, while an activated carbon filter gets rid of the smoke smell. If smoke or cooking smells are the problem, look for both layers.

Carbon works by adsorption, in which gas molecules stick to the carbon's huge internal surface. The EPA's technical summary says it can remove most hydrocarbons, many aldehydes, and organic acids. That surface fills up over time, so carbon filters have a finite capacity and must be replaced regularly. King County notes they usually need changing more often than HEPA filters.

What each filter type targets

Here's how the three common layers compare.

Why CADR matters as much as the filter

A great filter still needs enough airflow to clean a room. The number to compare is CADR, the clean air delivery rate. The EPA says the higher the CADR, the more particles a unit removes and the larger the area it can serve. CADR is usually measured at the highest fan speed, and it covers particles only.

For fine particles like smoke, the EPA suggests looking at the tobacco smoke CADR, which represents the smallest particles of the three ratings. Our guide to what CADR means and how to size an air purifier walks through the math for your room.

Ozone generators and ionizers that emit ozone

Some devices sold as air cleaners release ozone on purpose. The EPA's page on ozone generators sold as air cleaners states that no agency of the federal government has approved these devices for use in occupied spaces. It also says that when inhaled, ozone can damage the lungs, and that ozone does not remove particles such as dust and pollen.

The California Air Resources Board (CARB) sorts air cleaners into groups. Units that use only physical filtration, such as pleated or HEPA filters, are classed as "mechanical," meaning they don't generate ozone or ions. For units with electronic features such as ionizers, electrostatic precipitators, or UV lamps, the EPA's guide says some may emit ozone and points to CARB's list of models tested and shown to emit little or none.

California requires every portable indoor air cleaner sold in the state to be certified. On the CARB certified air cleaning devices list, all models are tested for electrical safety, and electronic ones must stay at or below 0.050 parts per million of ozone. Certified packaging carries a label stating that the unit meets California's ozone emissions limit.

How often to replace air purifier filters

There's no single schedule, because filter life depends on the filter and on how hard the unit works. The EPA's rule is that all filters need regular replacement, since a dirty, overloaded filter won't work well. Washington's health department says to replace filters according to the unit's indicator or the manufacturer's recommendations.

Montana's health department gives an example of how the timing can differ inside one machine: the HEPA filter might last one year, the carbon filter six months, and the pre-filter three months. The filter life for each layer is usually listed in the product's instructions.

How you use the purifier shifts those numbers. King County says a unit in a space that lots of people use may need cleaning more often than the manufacturer recommends. Washington suggests considering a filter change after wildfire smoke or after days when the Air Quality Index reaches "Unhealthy for Sensitive Groups" or higher. A filter that has changed color, or weaker airflow from the unit, is another sign it's due.

Reading replacement indicators

Many purifiers include a change-filter light, and others have to be checked by hand, Montana's guide notes. The EPA's technical summary adds that some units can monitor filter status, and some can be controlled from a smartphone. Either way, the indicator and the manual are the best guide to your model's timing.

Don't skip the reset. King County's checklist is to turn the unit on after a filter change, make sure air is flowing, and confirm that any timers or replacement indicators have been reset. Jotting down the date of the change makes the next one easier to plan.

How to change a filter

Washington's guidance on using a portable air cleaner and King County's checklist cover the routine:

1. Unplug the purifier before opening it.

2. Take out the used filter outdoors or in a well-ventilated room, and put it straight into a bag or outdoor bin.

3. Vacuum or wipe the inside and outside of the unit, and clean a reusable pre-filter as the manual directs.

4. Fit the new filter snugly, with no air gaps around its edges.

5. Close the panels, power the unit on, and reset the filter indicator.

Keeping a spare filter on hand helps during wildfire season, when Washington's health department recommends having extras available.

Which filter setup fits your home

Start with what's in the air. The EPA says the most effective steps are reducing pollution sources and ventilating with clean outdoor air, with filtration as a supplement, and our guide on how to clean the air in your home covers those first steps. After that, match the filters to the problem:

• For dust, pollen, and pet dander, a true HEPA filter with a pre-filter, sized by its smoke CADR.

• For cooking smells and smoke odors, HEPA plus a substantial activated carbon filter.

• For wildfire smoke, HEPA with a high smoke CADR, run on a higher speed, with a spare filter ready.

• For units with electronic features, a model on CARB's certified list.

Frequently asked questions

What's the difference between a HEPA filter and a carbon filter?

They do different jobs. A HEPA filter traps particles such as dust, pollen, and smoke, and an activated carbon filter adsorbs gases and odors, so many purifiers use both.

How can I tell if a filter meets the HEPA standard?

Check the numbers. The Department of Energy definition is at least 99.97% of 0.3-micron particles, so look for a capture rate and a particle size on the spec sheet, and ask the manufacturer if they aren't listed.

Can I wash a HEPA filter?

No. Washing a HEPA or carbon filter can make it ineffective, so both are replaced, though you can vacuum loose debris off the outer surface.

How often should I replace air purifier filters?

Follow the manufacturer's schedule and the unit's indicator. As one state example, a HEPA filter might last a year, a carbon filter six months, and a pre-filter three months.

Does a HEPA filter remove odors?

Not on its own. HEPA handles particles, so for smoke smells or cooking odors look for a purifier that adds an activated carbon filter.

How do I know an air purifier meets ozone limits?

Look for CARB certification. Every portable air cleaner sold in California must be certified, and electronic models on CARB's list must stay at or below 0.050 parts per million of ozone.

Should I replace filters after wildfire smoke?

Consider it. Washington's health department suggests a change after poor air quality events, and a filter that has changed color or weaker airflow means it's due.