What is the Best Air Cleaner

If you have allergies or asthma, live with pets, deal with wildfire smoke, or just want cleaner air at home, you have probably thought about buying an air cleaner.

There are a lot of makes and models, and prices run from inexpensive to extremely expensive, so choosing one can feel overwhelming. Some people give up. Others buy the wrong type or the wrong size and end up disappointed with the results.

We've been helping people with allergies and asthma since 1989. This guide covers what we've learned about choosing a HEPA air cleaner: what "HEPA" really means, what air cleaners can and can't do about viruses, how to size one for your room, and what else to check before you buy.

What Is a HEPA Air Cleaner?

HEPA stands for High-Efficiency Particulate Air. To be a true HEPA filter, it must capture at least 99.97% of particles that are 0.3 microns in size. That size is close to the hardest for any filter to catch, which is why it is the test size.

The technology traces back to the 1940s and the Manhattan Project, where scientists needed to keep very small, very hazardous radioactive particles out of the air. That is the kind of job HEPA was built for.

Tip: Look for the number, not the label. "HEPA-type," "HEPA-like," and "hospital-quality" are marketing terms and do not mean the filter meets the HEPA standard. When in doubt, ask whether it is true HEPA and look for 99.97% at 0.3 microns.

Do HEPA Air Cleaners Work Against Viruses?

Respiratory viruses, including the flu and COVID-19, spread when an infected person coughs, sneezes, talks, or breathes out virus-containing particles. Those particles can land directly on another person, settle on surfaces, or stay in the air where someone else can breathe them in.

The EPA says that, used properly, air cleaners can help reduce airborne contaminants, including viruses, in a building or small space. It is just as clear that filtration is not enough by itself. It works best as one layer alongside ventilation and the other steps public health agencies recommend.

To capture virus-sized particles, a unit has to remove particles in the 0.1 to 1 micron range. The EPA's advice for choosing a home air cleaner is to pick one sized for the room that is designated HEPA, has a high CADR for smoke, or states that it removes most particles below 1 micron.

Engineering research backs up HEPA's track record with tiny particles. A NASA technical report on spacecraft cabin air reviewed how HEPA media filters remove fine particles (under 2.5 microns) and ultrafine particles (under 0.01 microns). A CDC/NIOSH guide to building filtration notes that biological particles and radioactive particulates are efficiently removed by HEPA filters, as long as air cannot leak around the filter and the fragile media is not damaged. That guide was written for large buildings, not homes, so treat it as evidence of how the filters work rather than a guarantee for any one room.

Ventilation matters too. The CDC recommends aiming for at least 5 air changes per hour of clean air in buildings, and says you can reach that with a mix of fresh air, filtration, and air treatment. We will show you how to check that number for your own room below.

Important note: An air cleaner can help reduce airborne particles, but it does not prevent or treat any illness. Talk to your healthcare provider about how to protect yourself.

What Is the Difference Between a Filter and a Purifier?

"Air cleaner" and "air purifier" are catch-all terms that cover very different technologies. A filter removes particles by trapping them so they cannot become airborne again. Other devices clean the air in other ways, and some of those ways deserve a closer look before you buy.

A note on ozone

Ozone is a reactive gas made of three oxygen atoms, instead of the two in the oxygen we breathe. It is very effective at breaking down some odors, but it is an airway irritant. The EPA is direct about it: do not use ozone generators in occupied spaces. At concentrations that stay within public health limits, ozone does not effectively remove viruses, bacteria, mold, or other biological pollutants.

Unfortunately, some companies still market ozone generators as air purifiers, sometimes calling the gas "activated oxygen" or "super-oxygenated." These units are not HEPA air cleaners.

Tip: California requires every indoor air cleaner sold there to be tested and certified to emit less than 0.050 parts per million of ozone, and the box must say "ARB Certified." Even if you live elsewhere, that label, or CARB's certified list, is a handy way to screen a model. Read CARB's fact sheet.

How Do I Pick the Best Air Cleaner?

The most important factor is the size of the room you need to clean. Large rooms need machines with stronger motors and larger filters. A dorm room or office cubicle can be handled by a smaller unit.

Next, look at the motor. A filter can only trap particles that actually reach it, and moving air takes a motor and a fan. Those are mechanical parts, and they make some noise. A good air cleaner keeps the noise down, but a "silent" air purifier is not moving much air. Even the wind makes noise when it blows.

The faster an air cleaner can cycle the room's air through its filter, the faster it removes particles.

Two numbers tell you how much air a unit moves:

• CFM (cubic feet per minute) is the airflow of the motor, listed in the specifications. The higher the CFM, the more air passes through the filter.
• CADR (Clean Air Delivery Rate) is a standardized rating of the volume of clean air a unit delivers, in cubic feet per minute, at its highest speed setting. The EPA suggests favoring a high CADR for smoke, rather than pollen or dust, when you are after virus-sized particles.

Both numbers drop at lower fan speeds, and fan speed matters a lot. The Austin Air HealthMate HM400, for example, is rated at 75 CFM on speed 1, 200 CFM on speed 2, and 400 CFM on speed 3, and its noise rating climbs from 50 dB to 66 dB across that range. That is why it pays to size a unit so it hits your target on a speed you can live with, not only on high.

How Do I Figure Out What Size Air Cleaner I Need?

You need two pieces of information: how much air is in the room, and how much air the machine can move.

Step 1: Find the room's volume. Multiply length by width by ceiling height.

Room volume (cubic feet) = length x width x ceiling height

A 10 x 15 bedroom with standard 8-foot ceilings holds 1,200 cubic feet (10 x 15 = 150 square feet, and 150 x 8 = 1,200). That is worth knowing. If you only looked at square footage, you would think you had 150 to clean, which is 8 times less than the air you really have.

Step 2: Find the machine's airflow. Check the specifications for the CFM (or the CADR, if listed).

Step 3: Calculate air changes per hour. This tells you how many times an hour the machine moves all of the room's air through its filter.

Air changes per hour = (CFM x 60) / room volume

Using the Austin Air HM400 in our 10 x 15 bedroom: on speed 3 (400 CFM), the air changes 20 times per hour. On speed 2 (200 CFM), it changes 10 times per hour. Even on the quietest speed 1 (75 CFM), it changes about 3.75 times per hour.

Step 4: Compare the result to your target. Aim for at least 5 air changes per hour, which matches the CDC's target for clean air in occupied spaces. We like to see 10 or more when the room and budget allow it, and more is better.

Here is a quick reference so you do not have to do the math. It works backward from the formula: CFM needed = room volume x air changes per hour / 60.

Important note: These numbers assume the room's air mixes well and use the manufacturer's CFM at the fan speed listed. Furniture, closed doors, and a unit tucked into a corner can all lower real-world performance. If you are between sizes, go up.

As a worked example, here is the unit we used above:

Austin Air HealthMate HM400 Air Cleaner

True medical grade HEPA with 4-stage filtration, including about 15 lbs of activated carbon and zeolite for chemicals, gases, and odors. Three fan speeds (75, 200, and 400 CFM). Filter life is rated at 5 years, and the machine carries a 5-year parts and labor warranty. Made in the USA.

Shop Austin Air HM400

For a bedroom, office, or kitchen, Austin also makes the smaller HealthMate Junior HM200.

What Else Should I Check Before I Buy?

Filter size and life

Machines with small filters need new filters more often. Frequent changes are a chore, and they add to the cost of running the machine. Look for a filter large enough to last several years, unless you do not mind the time and expense of frequent changes. Ask what replacement filters cost before you buy.

Washable filters

Be careful with washable filters. They are generally pre-filters, and if one is not thoroughly cleaned, rinsed, and dried before it goes back in, it can harbor mold and bacteria. Follow the manufacturer's cleaning instructions exactly.

Gases and odors

A HEPA filter catches particles, but not gases. The CDC/NIOSH filtration guide points out that particulate filters remove particles but not gases and vapors, while sorbents such as activated carbon collect gases and vapors but not particles. If you also want to reduce odors or chemicals, look for a HEPA unit with a substantial carbon bed.

For example, Airpura makes a HEPA air cleaner designed specifically for tobacco smoke:

Airpura T600 Air Purifier

HEPA filtration plus a TarBarrier pre-filter and a deep carbon bed built to absorb the chemicals, tars, and gases in tobacco smoke. Variable-speed 560 CFM motor. Warranty is 5 years on parts and 10 years on labor, excluding replacement filters.

Shop Airpura T600

Placement and use

Put the air cleaner in the room where you spend the most time, usually a bedroom or living room. If someone in your household is especially vulnerable to infection, or is isolating because they are sick, put it where that person is. Keep the air intake and outlet clear of furniture, and do not run it in a closet. Run it whenever the room is occupied, because the longer it runs, the more particles it removes.

Warranty

A quality HEPA air cleaner will not be inexpensive, but it should last for many years. As with any long-lived appliance, understand the warranty before you buy: what it covers (parts, labor, filters) and for how long.

The Bottom Line

Choosing an air cleaner comes down to a few decisions. Make sure it is true HEPA. Size it for your room using CFM or CADR, and aim for at least 5 air changes per hour. Add carbon if gases and odors are a concern. Skip anything that intentionally makes ozone. And remember that an air cleaner works best alongside ventilation and regular cleaning, not instead of them.

Getting the right one the first time is worth the extra homework, because a well-chosen HEPA air cleaner should serve you for many years.

Additional reading

• Air purifier buying guide for allergies and asthma
• Are Air Fresheners Bad for Your Health?
• How Protective Bedding and Air Purification Turn Your Bedroom Into a Safe Space
• Study Shows HEPA Air Filtration Reduces the Risk of COVID in Schools

Sources
U.S. Environmental Protection Agency. Air Cleaners, HVAC Filters, and Coronavirus (COVID-19).
U.S. Environmental Protection Agency. Air Cleaners and Air Filters in the Home.
Centers for Disease Control and Prevention. Improving Ventilation in Buildings.
CDC/NIOSH (2003). Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks.
NASA (2016). Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials.
California Air Resources Board. California's Regulation to Limit Ozone Emissions from Indoor Air Cleaning Devices.

Medical disclaimer: This content is for educational and informational purposes only and is not intended as medical advice. Always consult a qualified healthcare provider regarding allergies, asthma, infections, or other medical conditions.

Want the complete picture? This page is part of our free Practical Guide to Allergy-Free Living, which draws on decades of allergy expertise and covers dust mites, pet dander, air quality, laundry, and more. Start at the beginning or jump to the section that matters most to you.