Editorial · June 5, 2017
Air Purifier Buying Guide: CADR and Filters Explained
Air purifier buying guide basics start with one number: CADR, short for clean air delivery rate. It tells you how much clean air a unit delivers each minute. A higher number means faster cleaning. Look for this number before you look at anything else on the box.
Two other checks matter after that. Does the filter fit your needs? And can you live with the noise and the upkeep? The sections below walk through each step in plain terms.
An air purifier cleans the air that passes through it. It does not remove the source of dust or smoke. The EPA says the best first step is usually to control or remove the sources of pollution and to bring in clean outdoor air. A purifier may help if those steps are not enough. This is general information, not medical advice.
What CADR tells you
CADR was developed by the Association of Home Appliance Manufacturers, known as AHAM. It is measured in cubic feet of clean air per minute. The EPA gives an example. A unit with a CADR of 250 for dust may bring dust down to the same level as adding 250 cubic feet of clean air each minute.
The AHAM standard for portable cleaners is called ANSI/AHAM AC-1. It gives a separate CADR for each of three particle types: tobacco smoke, dust and pollen. So a good box shows three numbers, not one. Compare the number that matches your problem. A home near a busy road or with a smoker needs a good smoke score.
CADR only counts particles. It does not tell you how well a unit handles gases or odors. The EPA also notes that many portable units on the market do not have a high enough CADR to remove large particles such as pollen, dust mite and cockroach allergens from a typical room. Large particles settle quickly, so a filter often never sees them.
Match the unit to your room
Start with the size of your room. Multiply length by width by ceiling height to get the volume in cubic feet. A room that is 10 feet by 12 feet with an 8 foot ceiling holds 960 cubic feet.
Now compare that to the CADR. A unit rated at 250 delivers 250 cubic feet of clean air a minute. That is about four minutes for one full room of air at the highest speed. A unit rated at 50 would need close to 20 minutes. The bigger the number, the more often the room air passes through the cleaner.
Portable units are meant for one room or one area. The EPA says they are not intended for whole house filtration. If you want to clean a large open living space, pick a unit with a high CADR. Better still, check what room size the maker states and treat that as a limit, not a target.
Buying a stronger unit than you need is fine. Buying one that is too small is the common mistake. The unit runs all day and the air still feels the same.
Filters and what they remove
The EPA lists several ways to clean particles from air. Mechanical filters trap particles on filter material. High efficiency particulate air filters, called HEPA, belong to this group. Electronic cleaners, such as electrostatic precipitators and ionizers, use an electric charge instead.
Gases and odors need a different kind of part. Gas filters use a material such as activated carbon. The EPA explains that these filters are made for one gas or a short list of gases. They will not reduce gases they were not made for. They may last only a short time. None are expected to remove every gas in a typical home.
That has a real consequence for shoppers. A claim that a unit removes "all odors" or "all chemicals" is too broad. Ask which gases the filter is made for and how long the carbon lasts. If the seller cannot say, assume the effect is small.
Plan for filter upkeep. A clogged filter moves less air. Check how often the maker says to replace it, and find the price of a new one before you buy the unit.
Skip ozone makers
Some devices make ozone on purpose. Ozone is a lung irritant that can cause adverse health effects. The EPA says that at levels that stay within public health limits, ozone does little to remove most indoor pollutants.
Some sellers say the federal government has approved ozone devices for use in occupied rooms. The EPA states the opposite: no agency of the federal government has approved these devices for use in occupied spaces.
California has a state rule for this. The California Air Resources Board runs an air cleaner regulation with certification requirements for makers. If a seller brags about ozone, walk away.
Noise, running cost and what to compare
A purifier that is too loud gets switched off. Check the noise rating in decibels, if the box lists one. Try to hear the unit on its low and medium settings, since that is where you will use it at night.
Look at the full cost, not only the price tag. Add the price of a replacement filter and how often you need one. Check the power draw in watts too, because the unit may run for many hours a day. The EPA notes that some portable cleaners are ENERGY STAR qualified, which is one more point to compare. The same note says EPA does not endorse any maker's claims of healthier indoor air.
Here is a short checklist for your shopping trip:
- Find the three CADR numbers and pick the one that fits your problem.
- Work out your room volume and compare it with the CADR.
- Ask what the filter removes and how often it needs replacing.
- Read the noise rating and the replacement filter price.
- Avoid any unit that makes ozone on purpose.
One more point. EPA neither certifies nor recommends particular brands of home air cleaning devices. No air cleaner removes every pollutant, and results vary from home to home. Use the unit as one tool among several. This is general information, not medical advice.