The Air Purifier Numbers
Air purifier shopping runs on a handful of numbers most listings never explain: CADR (and which of its three figures actually matters), the AHAM 2/3 rule for room sizing, the ACH math behind it, the True HEPA threshold versus unregulated "HEPA-type" marketing, the European H13/H14 filter classes, and CARB's ozone limit for ionizing purifiers. This page is the normalized decoder table — a reference dataset, not a buying guide.
The one thing to know: CADR (Clean Air Delivery Rate) is an AHAM-administered test measured in CFM (cubic feet per minute) for three separate particle types — smoke, dust, and pollen — because smaller particles are harder to capture. AHAM's sizing guidance (the '2/3 rule') says the unit's Smoke CADR should be at least two-thirds of the room's square footage, e.g. a 300 sq ft room wants a Smoke CADR of at least ~200. Always size by the Smoke number, the smallest and hardest-to-catch particle class — a high Pollen CADR on the box can mask a mediocre Smoke CADR.
The decoder table
| Metric / Standard | Authority | What it measures | Key figure |
|---|---|---|---|
| Smoke CADR | AHAM | Clean Air Delivery Rate for smoke particles (0.1-1.0 microns), in CFM | Use this number for sizing — the smallest, hardest-to-capture particle class |
| Dust CADR | AHAM | Clean Air Delivery Rate for dust particles (0.5-3.0 microns), in CFM | Medium-particle removal rate |
| Pollen CADR | AHAM | Clean Air Delivery Rate for pollen particles (5.0-11.0 microns), in CFM | Large-particle removal rate — typically the highest of the three numbers on the box |
| AHAM 2/3 Rule | AHAM | Room-sizing guideline relating Smoke CADR to room square footage | Smoke CADR ≥ 0.67 × room sq ft |
| Wildfire Smoke Sizing | AHAM | Stricter sizing guidance for wildfire-smoke events | Smoke CADR ≥ 1.0 × room sq ft (vs 0.67× normal) |
| ACH (Air Changes per Hour) | Standard engineering formula | How many times the unit filters the room's full air volume per hour | ACH = (CADR in CFM × 60) ÷ room volume in cubic feet |
| Recommended ACH — allergy/asthma | General allergist / public-health guidance | Commonly recommended air-change rate for allergen-sensitive households | 4–5 ACH |
| Recommended ACH — wildfire smoke | General public-health guidance | Commonly recommended air-change rate during wildfire smoke events | 5–6 ACH |
| True HEPA (US) | U.S. DOE / IEST | Minimum particle-removal efficiency at the most-penetrating particle size | ≥99.97% at 0.3 microns |
| "HEPA-type" / "HEPA-like" | None — unregulated marketing term | No verified or required efficiency threshold | Not a certified or standardized claim |
| EN 1822 Class H13 | European EN 1822 standard | Minimum particle-removal efficiency at MPPS | ≥99.95% at MPPS |
| EN 1822 Class H14 | European EN 1822 standard | Minimum particle-removal efficiency at MPPS | ≥99.995% at MPPS |
| CARB Ozone Limit | California Air Resources Board | Max allowed ozone emission for electronic/ionizing air cleaners sold in CA | ≤0.050 ppm (50 ppb) |
| CARB Certification Requirement | California Air Resources Board | Mandatory certification for portable air cleaners sold in California since 2010 | Electrical safety test + ozone test (electronic types only) |
| Filter Replacement Interval — True HEPA | Aggregated manufacturer specs | Typical service life of a True HEPA cartridge under normal household use | 6–12 months |
| CADR-to-Noise Tradeoff | General industry engineering behavior | Relationship between higher CADR/fan speed and higher decibel output | Roughly linear — noise rises with the fan speed needed to hit higher CADR |
How to use these numbers together
Start with a room's square footage and 8 ft-ceiling assumption to get cubic feet. Apply the AHAM 2/3 rule (Smoke CADR ≥ 0.67 × sq ft) to find a purifier that's at least adequately sized, then run the ACH formula (CADR × 60 ÷ room volume) to see the actual air-change rate you'll get — 4-5 ACH is a common allergy-household target, 5-6 ACH for wildfire smoke. Separately, verify the filter is True HEPA (not "HEPA-type"), and if it's an ionizing/electronic model, check it against CARB's certified-devices list for its ozone status. See What CADR Do I Need for My Room? for the full walkthrough.
Caveats — read before citing a figure
- CADR is model-specific. These are standard definitions, not a specific product's rating — verify any product's actual CADR against the AHAM Verifide public directory.
- ACH targets are general guidance, not a single binding regulation. 4-5 ACH and 5-6 ACH figures reflect common allergist and public-health guidance, not one universally mandated number.
- CARB listing is not a zero-ozone guarantee. It confirms the device tested at or below the 0.050 ppm ceiling, not that it emits none.
- This table is general reference, not a health or safety verdict for any specific product, room, or respiratory condition. It reports what a standard or metric measures, not medical outcomes.
- Standards can be revised. AHAM, DOE/IEST, EN 1822, and CARB periodically update thresholds; this decoder reflects the figures as understood as of the version date below.
Methodology & versioning
Version 1.0 — published July 30, 2026. Facts are drawn from AHAM's public CADR and sizing guidance (AHAM Verifide program), the U.S. Department of Energy / IEST True HEPA specification, the European EN 1822 filter classification standard, and the California Air Resources Board's public regulation and certified-devices materials on indoor air-cleaning device ozone emissions — cross-checked against our standard-by-standard guides. We do not test purifiers ourselves; this is a specification-facts reference, not a lab result. Corrections: if a standard's figure changes or we find an error, this page and the CSV below are updated in place and the version/date bumped — see How We Evaluate for our full editorial policy.
Machine-readable download: the full table is available as CSV at /air-purifier-numbers.csv, and a condensed digest is maintained in /llms-full.txt.
License: This dataset is published under CC BY 4.0 — reuse and redistribution are welcome with attribution.
How to cite: Pure Air Lab. "The Air Purifier Numbers." Version 1.0, July 30, 2026. https://pure-air-lab.pages.dev/air-purifier-numbers/
This page is general reference information about specification standards, not a health or safety determination for any specific product, room, or respiratory condition.
Frequently Asked Questions
A normalized reference table of the core specification numbers behind air purifier shopping — AHAM's CADR system and 2/3 sizing rule, the ACH math, the True HEPA efficiency threshold, European H13/H14 filter classes, and CARB's ozone emission limit for ionizing purifiers. It exists because most 'best air purifier' content repeats marketing figures without the underlying standard behind them.
The Smoke CADR. AHAM publishes three numbers — smoke, dust, and pollen — and smoke particles are the smallest and hardest to capture, making that number the most conservative and realistic sizing basis. A high Pollen CADR on the box can mask a much lower, less flattering Smoke CADR.
No. True HEPA is a specific U.S. DOE/IEST threshold — at least 99.97% removal at 0.3 microns. 'HEPA-type,' 'HEPA-like,' and 'HEPA-grade' are unregulated marketing phrases with no efficiency requirement behind them at all, and a filter carrying that label may perform well below certified HEPA media.
No — it means the device was tested and found to emit no more than 0.050 ppm (50 ppb) of ozone, CARB's regulatory ceiling for electronic/ionizing air cleaners sold in California, not a zero-ozone guarantee. A device that isn't CARB-listed simply hasn't been verified against that limit.
No. This table normalizes what each standard or metric means in general. For any specific purifier, verify its actual CADR numbers against the AHAM Verifide public directory and its ozone status against CARB's certified-devices list before buying.