Best for
- People confused by room-size marketing
- Buyers comparing AHAM-style sizing with brand claims
- Anyone using a CADR calculator
Watch for
- Treating all square-foot ratings as equivalent
- Assuming 1 ACH means the room becomes perfectly clean once per hour
- Ignoring mixing and pollution sources
ACH is a rate, not a guarantee
One air change per hour means the purifier moves a room-volume-equivalent amount of clean air in an hour. It does not mean every molecule in the room passes through the machine exactly once.
Real rooms have imperfect mixing, open doors and continuing pollution sources.
Why brands show two room sizes
Manufacturers increasingly publish a large 1-ACH coverage number and a smaller 4.8-ACH or similar number. Both may be mathematically valid but answer different questions.
For comparison shopping, use the same ACH basis across every model.
CADR makes cross-brand comparison easier
CADR is an airflow-style performance metric tied to particle removal. Once you know your target CFM, it is easier to compare products even when marketing room sizes use different assumptions.
Use room size, ceiling height and desired ACH to set the target first.
Use the same basis across brands. Room-size claims, operating-cost assumptions and moisture ratings can use different methods. Normalize the inputs before comparing products.