Air Change Rate (ACH) Tests
Air Change Rate (ACH) tests measure how many times the air in a room or enclosed space is completely replaced within an hour. This is critical for ventilation efficiency, air quality control, and regulatory compliance in settings such as hospitals, cleanrooms, laboratories, offices, and industrial spaces.

1. Purpose of Air Change Rate Tests
Ensures proper ventilation for fresh air circulation and contaminant removal.
Verifies compliance with industry standards (e.g., ASHRAE, ISO 14644, CDC, WHO).
Prevents airborne disease spread (e.g., in hospitals and isolation rooms).
Maintains air purity in cleanrooms and pharmaceutical environments.
Optimizes HVAC efficiency to balance energy use and air quality.

2. Air Change Rate Formula
The Air Change Rate per Hour (ACH) is calculated using:
A
C
H
=
Q
V
×
60
ACH = \frac{Q}{V} \times 60 where:
Q
Q = Airflow rate in cubic feet per minute (CFM) or cubic meters per hour (m³/h).
V
V = Room volume in cubic feet (ft³) or cubic meters (m³).
60
60 = Converts minutes to hours.

3. Air Change Rate Testing Methods
a. Direct Measurement (Ventilation System-Based)
Measures the supply and exhaust airflow at air diffusers using an anemometer, balometer, or airflow hood.
Total airflow is used to calculate ACH using the formula above.
b. Tracer Gas Decay Method (for Existing Spaces)
Uses a tracer gas (e.g., CO₂, SF₆) introduced into the room.
Measures the decay rate of gas concentration over time using gas sensors.
ACH is determined from the exponential decay equation:
A
C
H
=
−
ln
⁡
(
C
/
C
0
)
t
×
60
ACH = \frac{-\ln(C/C_0)}{t} \times 60 where:
C
0
C_0 = Initial tracer gas concentration.
C
C = Final concentration after time t
t (in minutes).
c. Smoke Visualization Testing
Uses smoke or fog generators to observe air mixing and clearance time.
Not precise for calculations but helps visualize airflow patterns.

4. Recommended Air Change Rates (ACH Standards)
a. General Ventilation Guidelines
Space Type
Recommended ACH (Air Changes per Hour)
Office Spaces
4 – 10
Classrooms
5 – 6
Restaurants
8 – 12
Residential Homes
0.35 – 1 (per ASHRAE 62.2)
b. Healthcare & Cleanroom ACH Requirements
Environment
Recommended ACH (Air Changes per Hour)
Operating Rooms
20 – 25
Isolation Rooms
≥12 (CDC & WHO)
Laboratories
6 – 12
ISO 7 Cleanroom
30 – 60
ISO 5 Cleanroom
≥240

5. Interpretation of ACH Results
Too low ACH → Insufficient ventilation, leading to poor air quality, high CO₂ levels, or contamination risks.
Too high ACH → Energy waste due to excessive ventilation.
Balanced ACH → Ensures healthy indoor air quality (IAQ) and compliance with safety standards.

6. Applications of Air Change Rate Testing
Hospitals & Isolation Rooms: Ensures safe removal of airborne pathogens.
Cleanrooms & Pharmaceutical Labs: Maintains contamination control.
Data Centers & Server Rooms: Prevents overheating by maintaining airflow.
Industrial Facilities: Ensures proper exhaust of fumes and gases.