Tecnobosque Wildfire Knowledge Graph · Smoke & Indoor Air
Reference Concept · v1.0Definition → relationship → calculation → evidence context
Air changes per hour (ACH) is a volumetric rate normalized by room volume: the equivalent number of room-air volumes supplied, removed or cleaned in one hour.
ACH
What is air changes per hour (ach)?
ACH is a normalization tool. It says how large a flow is relative to the volume of the space being considered.
In smoke filtration, the relevant quantity is often equivalent or clean-air ACH based on CADR, not simply outdoor ventilation. Bringing in more outdoor air during heavy smoke could increase indoor particles unless that air is properly filtered.
Why it matters
For wildfire smoke, clean-air ACH makes CADR comparable across rooms of different sizes and helps translate purifier capacity into a room-level planning target.
ACH turns an airflow into a room-scale number. The same 300 cfm means much more cleaning in a small bedroom than in a large open-plan space.
ACH = Q × 60 ÷ VFor Q in cubic feet per minute and V in cubic feet. In SI units, ACH = Q(m³/h) ÷ V(m³). Use “clean-air ACH” only when Q is CADR or another equivalent clean-air flow.
Flow normalized by room volume.
The flow being normalized.
Air volume of the space.
Used when Q is supplied in cfm.
CADR to clean-air ACH
A purifier has a CADR of 300 cfm in a 2,400 ft³ room.
ACH = 300 × 60 ÷ 2,400ACH = 7.5 h⁻¹This does not mean 7.5 complete physical replacements of every air molecule; it is an equivalent well-mixed flow metric.
What controls or changes this quantity?
Larger rooms reduce ACH for the same airflow.
Higher effective clean-air flow increases ACH.
Poor distribution can reduce practical effectiveness even when the arithmetic ACH is high.
Outdoor smoke entry can offset clean-air delivery and influence actual indoor concentration.
Engineering context
Outdoor air intentionally supplied to the room.
Uncontrolled outdoor-air exchange through leakage.
Equivalent clean-air flow from filtration or air cleaners.
Some models combine multiple first-order removal mechanisms, but the terms should remain distinguishable.
How to interpret it correctly
- Always state what airflow the ACH represents.
- Use actual room volume, including ceiling height.
- Do not assume higher outdoor ventilation is beneficial during severe smoke without filtration.
- Use PM2.5 measurements or a mass-balance model when concentration—not just capacity—is the question.
Use the calculator as a transparent technical aid and keep its assumptions explicit.Open Wildfire Smoke CADR Calculator →
Why it matters in wildfire analysis
ACH is useful because it makes a purifier selection portable across differently sized rooms. A device that provides adequate clean-air ACH in a bedroom may be undersized in a large living space.
The Tecnobosque CADR Calculator uses this relationship to move between room geometry, desired ACH and required clean-air delivery.
Limitations and boundaries
- ACH assumes a well-mixed space for simple interpretation.
- It does not directly predict PM2.5 concentration without source and removal information.
- Airflow ratings may differ from real installed performance.
Air Changes per Hour (ACH) FAQ
What does 5 ACH mean?
It means the relevant airflow is equivalent to five room volumes per hour.
How do I calculate ACH from cfm?
Multiply cfm by 60 and divide by room volume in cubic feet.
Is ACH the same as CADR?
No. CADR is a flow rate; ACH normalizes that flow by room volume.
Does more ACH always mean cleaner air?
Not necessarily. It depends on whether the added flow is clean or polluted, how the room is mixed and what pollutant sources are present.