What does CADR mean and why does it matter more than square footage claims?

CADR stands for Clean Air Delivery Rate, measured in cubic feet per minute, and it is certified by an independent testing body under a standardized protocol. A purifier gets three CADR numbers: smoke (the smallest particles, 0.09 to 1 micron), dust (0.5 to 3 microns), and pollen (5 to 11 microns). Smoke CADR is the most demanding and the one worth comparing. Square footage coverage claims, by contrast, are calculated by the manufacturer using whatever air-change assumption they choose. A unit quoting two air changes per hour can legally claim roughly two and a half times the coverage of the same machine quoted at the 4.8 air changes per hour that allergy guidance uses. CADR is a measurement; coverage is an interpretation. Compare the measurement.

How often do air purifier filters need replacing and what does it cost?

True HEPA filters typically last 6 to 12 months of continuous operation, and carbon layers saturate faster, usually 3 to 6 months. Manufacturer intervals assume average conditions; a home with pets, smokers, or an active construction site nearby will need replacements sooner. Real annual costs vary widely: Coway AP-1512HH runs about 60 dollars a year, Levoit Core 300 about 50, Winix 5500-2 about 45 to 60 with its washable carbon pre-filter, and Blueair 211+ about 60 to 80. Some budget units with proprietary cartridges run 150 to 200 dollars a year, which exceeds the machine's purchase price. Before buying any purifier, look up the exact replacement filter part number, note its price and interval, and multiply by five years. Add that to the sticker price to get the number that actually distinguishes these products.

Do air purifiers produce ozone and is that dangerous?

Pure mechanical filtration purifiers, meaning a fan pulling air through HEPA and carbon media, produce no ozone. Ozone comes from electronic air-cleaning features: ionizers, plasma generators, electrostatic precipitators, and some UV lamps produce it as a byproduct. Ozone is a respiratory irritant that can worsen asthma and reduce lung function, and there is no established concentration at which it usefully cleans indoor air without also being an irritant. California's Air Resources Board maintains a certification list for indoor air cleaners sold in the state precisely because of this. The practical advice: prefer mechanical filtration. If a purifier you want includes an ionizer or plasma feature, verify it can be switched off independently of the fan, then switch it off and leave it off. The HEPA filter is doing the work regardless.

Is one large air purifier better than two small ones?

For a single enclosed room, one correctly sized unit is better: a large fan moving a given volume of air is quieter and more efficient than a small fan straining to move the same volume. For an open-plan space or multiple rooms, two smaller units placed apart usually outperform one large unit in a corner, because air distribution matters as much as raw CADR. Purifiers clean the air that reaches them, and a single unit in a 900 sq ft open-plan space creates a well-filtered zone near itself and a poorly filtered far end. Two units at opposite ends produce a more uniform result even at a lower combined CADR. The rule: match one unit per enclosed room, and split coverage across large or irregular spaces.

Air Purifier Buying Guide: What to Know Before You Buy

Shopping for a new air cleaner means sorting through CADR numbers, filter marketing claims, and coverage estimates that rarely mean what they seem to mean. This buying guide breaks the decision down into the specifications that actually predict performance: certified smoke CADR against your room's square footage, true HEPA versus HEPA-type media, activated carbon quantity for odors, and the noise level at the fan speed you will realistically run day to day. It also covers the cost most shoppers overlook, replacement filters, where models like the Coway AP-1512HH, Levoit Core 300, Winix 5500-2, and Blueair 211+ run anywhere from roughly 45 to 80 dollars a year depending on filter design. Ozone-generating features such as ionizers and plasma generators get a plain explanation of what they do and why mechanical filtration is the safer default. Read through before comparing models, since the two-thirds CADR-to-square-footage rule and the five-year filter cost math here apply to almost every purifier on the market.

How to Use This Guide

Four specifications determine whether an air purifier changes the air in your room: CADR against room size, filter type, carbon quantity, and noise at the speed you will actually run it. Everything else is secondary, and some of it is worth switching off.

CADR and Room Sizing

CADR is the certified measure of clean air output in cubic feet per minute, tested independently against smoke, dust, and pollen. Smoke CADR is the hardest test and the number worth comparing across products.

The two-thirds rule: your purifier's smoke CADR should be at least two-thirds of the room's square footage, assuming standard 8-foot ceilings. This delivers about 4.8 air changes per hour.

Room size	Minimum smoke CADR	Allergy target
150 sq ft bedroom	100	150
250 sq ft bedroom	167	250
350 sq ft room	233	350
500 sq ft living area	333	500

For allergy or asthma households, target smoke CADR equal to square footage rather than two-thirds. The practical benefit is not just faster cleaning: a larger unit reaches the same result on a lower fan speed, which is quieter and extends the fan's life.

Ignore the coverage claim printed on the box unless it also states the air changes per hour it assumes. Two manufacturers can quote coverage numbers that differ by a factor of two and a half for machines with identical CADR.

Filter Types: What Actually Matters

True HEPA (H13 / H14). A defined standard: 99.97 percent capture at 0.3 microns, the most-penetrating particle size. This is the filtration layer that does the work. Insist on the explicit 99.97 percent at 0.3 microns language.

HEPA-type, HEPA-like, 99 percent HEPA. Marketing terms with no standard behind them, typically 85 to 95 percent at 0.3 microns. Adequate for pollen and pet dander, meaningfully worse for smoke and ultrafine particulate.

Pre-filter. A coarse mesh or fabric layer capturing hair, lint, and large dust before the HEPA media. A washable pre-filter is the single most cost-effective feature in a home with pets, because it can double HEPA filter life.

Activated carbon. Handles gases and odors, which HEPA does not touch at all. Quantity is everything: a carbon-dusted mesh saturates in weeks, while a granular bed measured in pounds works continuously. The Austin Air HealthMate carries 15 pounds; most purifiers under 300 dollars carry a fraction of a pound.

Ionizers, UV, and Ozone

Mechanical filtration produces no ozone. Ionizers, plasma generators, and some UV implementations do produce it as a byproduct.

Ionizers charge airborne particles so they clump and fall out of the air faster. The airborne count drops, which looks good on a sensor readout, but the allergen lands on surfaces rather than being trapped in a filter. UV-C lamps in a purifier have air contact times of a fraction of a second, far below germicidal requirements.

Neither adds meaningful benefit over a correctly sized HEPA unit, and ozone is a respiratory irritant that is specifically counterproductive for asthma. Choose mechanical filtration. If your preferred model includes these features, confirm they switch off independently of the fan, then leave them off.

Noise: The Spec That Determines Whether You Use It

A purifier you turn off because it is too loud filters nothing. Noise figures worth knowing:

20 to 30 dB: genuinely quiet, usable while sleeping
30 to 40 dB: noticeable but acceptable background, like a soft fan
40 to 50 dB: intrusive in a bedroom, fine in a living room
50 dB and above: loud enough that most people run it only when away

Manufacturers quote the lowest speed's dB figure. What matters is the noise at the speed that delivers the CADR your room needs. This is the strongest argument for buying a larger unit than the minimum: an oversized purifier at speed one is quiet and effective, while a correctly-sized-to-the-minimum purifier must run at maximum to hit the same number.

Total Cost of Ownership

Purchase price is roughly half the five-year cost. Work it out before buying:

Find the exact replacement filter part number for the model
Note its current price and the manufacturer's stated interval
Multiply out five years
Add to the purchase price

A 230-dollar Coway at 60 dollars a year is 530 dollars over five years. A 100-dollar purifier with 45-dollar proprietary cartridges every three months is 1,000 dollars over the same period. The cheaper machine is the more expensive purchase.

For specific model comparisons see the best air purifiers list and the Coway Airmega review.