Air Purifiers: HEPA Filters, CADR, and Room Size Explained

A portable air purifier can help clean the air in the room where you spend the most time, whether that is a bedroom, a home office, or a living room shared with pets. Many people look at one during pollen season, when wildfire smoke drifts in, or when cooking smells and pet dander linger longer than they would like.

The labels can be confusing, with terms like True HEPA, CADR, ACH, and carbon filters. This guide explains what those terms mean, how to size a purifier for your room, what the U.S. Environmental Protection Agency says about air cleaners, and a range of picks from several stores.

Top Air Purifiers

Start with the source

Before you buy, it helps to know where an air purifier fits. The EPA says the most effective ways to improve indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air. It adds that filtration can be an effective supplement to source control and ventilation. In other words, a purifier works best alongside habits like running the kitchen vent fan, fixing moisture problems, and opening windows when outdoor air is clean.

The EPA also notes that no air cleaner or filter will eliminate all of the air pollutants in your home. A good purifier can make a real difference in one room, but it is one tool, not a cure-all.

Key terms explained

• HEPA filter: A dense filter that captures fine particles like dust, pollen, and pet dander. Most models in this guide use one.
• CADR: Clean air delivery rate, a measure of how much filtered air a purifier delivers. The EPA says the higher the CADR, the more particles it can filter and the larger the area it can serve.
• Carbon filter: Activated carbon targets gases and odors. The EPA notes that the CADR rating system is for particles only, and there is no widely used rating system for gas filtering.
• ACH: Air changes per hour, how many times the purifier can filter the air in a room of a given size in an hour.
• Auto mode: A built-in sensor adjusts fan speed based on air quality readings.

How to size a purifier for your room

The EPA advises choosing a portable air cleaner with a CADR large enough for the size of the room where you will use it. Most packaging lists the largest room size a model is meant for. The EPA's sizing chart is based on an 8-foot ceiling, so if your ceilings are higher, it suggests picking a purifier with a higher CADR.

When in doubt, size up. A bigger purifier running on a quiet low setting often cleans more air, and sounds better, than a small one running at full blast. Room size claims on product listings vary in how they are measured, so compare CADR numbers when they are available rather than room size alone.

Features worth having

• Air quality display: Shows how the air is doing, often with a color light.
• Sleep mode: Dims the lights and runs the fan quietly at night.
• Timer: Lets the purifier run for a set time and shut off.
• Filter indicator: Tells you when it is time for a new filter.
• Carbon layer: Helpful if cooking, pet, or smoke odors are a concern.
• Easy filter access: A filter you can swap in a minute makes upkeep more likely.

Amazon picks

The COWAY Mighty AP-1512HH pairs a HEPA filter with an air quality monitor, auto and eco modes, a timer, and a filter indicator.

The WINIX A230 combines a True HEPA filter with a carbon filter, an air quality monitor, and an auto mode, a good choice when odors are part of the problem.

The BLUEAIR Blue 311i Max is listed for rooms up to 2,107 square feet, which makes it a fit for open living areas.

The Levoit bedroom purifier is a quiet model designed for bedrooms and homes with pets.

The smaller Levoit is sized for bedrooms, dorm rooms, and home offices where floor space is tight.

Picks from Target and Walmart

At Target, the Honeywell Allergen Plus HPA120 is a True HEPA purifier at a friendly price.

Also at Target, the Shark HP152 Never Change purifier lists a five-year filter life and five settings, for anyone who dreads remembering filter swaps.

At Walmart, the DREO purifier offers HEPA filtration, a sleep mode, a PM2.5 display, and a listed 116 CFM airflow.

Where to put it

Place the purifier in the room you use most, often the bedroom, since you spend many hours there each night. Keep it a little away from walls, curtains, and furniture so air can flow freely in and out. Close the door to that room to let the purifier keep up. If you want cleaner air in several rooms, one purifier per room usually works better than one moved around the house.

Run time and fan speed

The EPA says higher fan speeds and longer run times generally increase the amount of air filtered. Many people run a purifier on auto mode during the day and on sleep mode at night. Running it all the time on a low or auto setting tends to work better than turning it on only when the air seems bad.

Filters and upkeep

The EPA points out that all filters need regular replacement, and a dirty, overloaded filter will not work well. Check the maker's schedule, and budget for replacement filters when you compare prices, since some cost far more than others. Many models also have a washable pre-filter that catches hair and lint; rinse or vacuum it every few weeks to protect the main filter.

A note on ozone

Some devices sold as air cleaners, called ozone generators, intentionally produce the gas ozone. The EPA warns that ozone can cause health problems at high concentrations, and that no agency of the federal government has approved these devices for use in occupied spaces. Check the product details before you buy. The picks in this guide rely on filters to clean the air.

Allergies and pets

If pollen or pet dander is your main concern, a HEPA filter is the feature to look for, since it targets fine particles. Put the purifier in the bedroom, keep pets out of that room if you can, and wash bedding often. Vacuuming with a HEPA vacuum and brushing pets outdoors cuts down on what the purifier has to catch. If you have asthma or serious allergies, talk with your doctor about the right plan; a purifier can help, but it does not replace medical advice.

Noise and energy use

Purifiers are loudest on their top setting, so check listed noise levels if you are a light sleeper. A larger model on low is often quieter than a small one on high. Since a purifier may run for many hours a day, look at the listed wattage too; models with efficient motors cost less to run over a year.

Common mistakes

The most common mistake is buying a purifier that is too small for the room and expecting it to clean the whole house. Another is pushing it into a corner behind a couch, which blocks airflow. People also forget to replace filters, or run the purifier with windows wide open next to a busy road. Finally, skipping the basics, such as vacuuming, dusting, and using kitchen and bath fans, leaves the purifier working harder than it needs to.

Questions to ask before you buy

• How big is the room? Match the CADR and listed room size.
• Are odors a concern? Look for an activated carbon filter.
• Will it run while you sleep? Check for a sleep mode and dimmable lights.
• What do filters cost? Add yearly filter costs to the price.

The short answer

For most rooms, a HEPA purifier with a CADR sized for the space, an auto mode, and easy filter changes is a solid choice. Add a carbon filter if odors bother you. Keep it in the room you use most, run it often, change the filters on time, and pair it with good ventilation and source control.

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