Buying a Used Air Purifier? The Filter Is the Real Product

The number to look for on a portable air purifier is its CADR, short for clean air delivery rate. The U.S. Environmental Protection Agency (EPA) gives a quick chart to match it to a room: at least 65 cfm for 100 square feet, 130 for 200, 195 for 300, 260 for 400, 325 for 500 and 390 for 600. Those figures assume an 8-foot ceiling. If you are looking at a used purifier, that number and the filter inside it matter more than the brand.

With a used air purifier, the machine is the cheap part and the filter is the part that does the work.

This guide covers what the EPA and ENERGY STAR say about sizing, filters, ozone and running costs, then gives a short checklist for a secondhand unit.

Match the CADR to the room

The EPA's method is simple. Multiply the room's length by its width, then compare that to the maximum room size on the packaging.

For higher ceilings, pick a higher CADR. For an open floor plan, think about the whole connected space. To target fine particles, the EPA says to look for a high CADR for tobacco smoke, because that rating reflects the smallest particles.

Particles are one thing, gases another

The CADR rating covers particles only. HEPA filters and a high CADR address particles. For gases such as volatile organic compounds, the EPA says to look for an activated carbon or other absorbent filter, and that it works best with a large amount of material, so thicker is better. It adds that no widely used performance rating exists for gas-removal filters, and that research has not shown plasma, photocatalytic oxidation or UV to remove gases effectively in portable home units.

The ozone warning

Some purifiers make ozone on purpose, and ozone irritates the lungs. The EPA says to avoid air cleaners that intentionally produce ozone, and notes that devices using electrostatic precipitators, ionizers, UV lights without adequate lamp coatings or plasma technology may emit ozone. It points to lists kept by the California Air Resources Board and the Association of Home Appliance Manufacturers of devices that emit little or no ozone. For a used unit, check the model number against those lists before you buy.

Filters: the part that wears out

The EPA says to replace filters according to the manufacturer's recommendations and to change them more often if they look heavily soiled, because a dirty, overloaded filter will not work well. For furnace filters, it gives a typical replacement interval of every 60 to 90 days. I would not assume the same for a portable unit. Look up the manual.

That has two practical consequences for a used purifier:

1. Ask when the filter was last replaced. If the seller does not know, assume it needs a new one.
2. Check that the replacement filter is still sold and what it costs. A purifier with a discontinued filter is a paperweight. (This is my advice, not the EPA's.)

Running costs and what the label means

ENERGY STAR says a standard room air cleaner running continuously uses about 394 kWh per year, and certified models are more than 50% more energy efficient, saving about 211 kWh per year. It estimates savings of roughly $18 to $40 per year, depending on the size of the unit. The page also warns that the label certifies energy efficiency, not that the unit will improve your health, and that the EPA does not endorse manufacturer claims about how much a specific product will improve indoor air. Larger models use more energy.

The EPA adds a note about furnace and HVAC filters: they only work while the system runs, which is usually less than 25% of the heating and cooling season, and running it longer raises electricity costs.

A checklist for a used air purifier

1. Find the model number and look up its CADR and recommended room size.
2. Check it against the table for the room you plan to use it in.
3. Check it against the ozone lists the EPA mentions.
4. Ask about the filter's age and whether replacements are available.
5. Open it and look at the filter. Heavily soiled filters are a bad sign.
6. Run it and listen. A rattling fan is a sign of wear.
7. Run a recall check using our five-minute guide.

Where this fits in a DC-area home

Many homes in the DC, Maryland and Virginia area are sealed up tight in winter, which makes dust and cooking fumes linger. Moisture is a separate issue; see the dehumidifier guide. If you are browsing for appliances, check Buy Now and read the help page for how DealsDMV handles pickup.

The bottom line

Match the CADR to your room, avoid ozone-producing designs, and treat the filter as the main cost. A used purifier can be a good buy when the unit is clean, the replacement filter is available and you have checked its ozone record.

If you know someone shopping for an air purifier this season, send this along.