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Why the room size on the box is about five times too big
"Covers 1,500 sq ft" usually means the purifier can pass that room's air through the filter once in an hour. That is not enough to keep up with dust, pollen or smoke that keeps coming in.
The number that matters: CADR
CADR (clean air delivery rate) is how many cubic feet of clean air the purifier delivers per minute. The industry group AHAM verifies it independently for members, shown as "AHAM Verifide". There are separate scores for smoke, dust and pollen. Smoke is the smallest particle and the most useful number.
The two-thirds rule
AHAM's guidance is that the smoke CADR should be at least two-thirds of the room's area in square feet. Flip it around: room size = smoke CADR times 1.55. That works out to about 4.8 air changes per hour with 8 ft ceilings.
| Smoke CADR | Real room size |
|---|---|
| 100 | 155 sq ft |
| 143 (Levoit Core 300) | 220 sq ft |
| 200 | 310 sq ft |
| 250 | 390 sq ft |
Decoding the box
- "X sq ft in 1 hour": divide by 4.8.
- "X sq ft in 30 minutes": divide by 2.4.
- "4.8 air changes per hour in X sq ft": that number is already realistic. The GermGuardian listing does this.
- No CADR and no time frame at all: be careful.
When to size up
For wildfire smoke, size up or run the purifier on high. Smoke keeps leaking in, and a purifier on its quiet setting delivers only a fraction of its rated CADR. Buying one size bigger and running it on medium is quieter than running a small one flat out.