AHAM AC-1 Standard: How Clean Air Delivery Rate (CADR) Is Empirically Measured in 1,008 Cu Ft Chambers

The Clean Air Delivery Rate (CADR) is the single most authoritative engineering metric for evaluating portable air cleaners. Standardized under ANSI/AHAM AC-1, it quantifies the volume of contaminant-free air delivered per minute by isolating mechanical filtration from natural particle deposition.

1. The 1,008 Cubic Foot Sealed Test Chamber

Under AHAM AC-1-2020 protocol, testing occurs inside a rigid, airtight chamber measuring exactly 10.5 ft × 12.0 ft × 8.0 ft (1,008 cu ft / 28.5 m³). The chamber is lined with electropolished stainless steel or fluoropolymer sheeting to minimize electrostatic wall losses. Temperature is regulated to 70°F ± 5°F (21°C ± 3°C) and relative humidity is locked at 40% ± 5% RH.

2. Differential Decay Physics (k_total - k_natural)

CADR is calculated using first-order aerosol decay differential equations. First, the test chamber is injected with challenge aerosol to reach peak concentrations (e.g., 200,000 particles/cc). Natural settling, surface adsorption, and wall deposition are tracked over 20 minutes to establish the natural decay constant (k_n):

C(t) = C_0 × e^(-k_n × t)

Next, identical aerosol levels are regenerated, and the test air cleaner is operated at maximum blower speed. The total decay constant (k_t) is logged by high-speed condensation particle counters (CPC) and optical particle sizers (OPS). The CADR is calculated by multiplying the chamber volume (V = 1,008 ft³) by the differential rate:

CADR = V × (k_total - k_natural)

Because natural settling of 10-micron pollen is much faster than 0.2-micron smoke, subtracting k_natural ensures that the CADR metric isolates only the air volume purified by the machine.

3. The AHAM Two-Thirds (2/3) Rule for Room Sizing

To achieve the standard 4.8 air changes per hour (ACH) in a room with 8-foot ceilings (an 80% particle reduction in 30 minutes), the Smoke CADR rating must equal at least two-thirds of the floor area in square feet:

Minimum Smoke CADR = Room Area (sq ft) × 0.667

For a typical 300 sq ft master bedroom, an air purifier must carry an AHAM Smoke CADR of at least 200 CFM. Running an undersized 90 CADR unit in that same space will fail to outpace ambient indoor particle infiltration from open doors and HVAC air handlers.