Appendix 17. Air change per hour (ACH) measurement recommendations


The ACH in a mechanically ventilated room should remain more or less constant, whereas natural ventilation will vary according to:
– Whether the doors/windows/vents in that room are open or not;
– Wind speed and direction;
– Temperature and humidity differential between inside and outside.

The ACH rate is one tool among others to assess if:
– The efficiency of the system in delivering the outdoor air and in removing the pollutants to each location in the room;
– The overall airflow direction is from clean to dirty zones.

To calculate the ACH in a given room:
– Start by drawing a sketch of the room;
– Measure the dimensions of the room and calculate the volume (in m3);
– Measure the surface (in m2) of all the openings/vents in the room and air direction across the openings/vents;
– Measure the air speed (in meters per second) using an anemometer.

             ACH = 0.65 x air speed (m/s) x opening area (m2) x 3600
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                                          Room volume (m3)


Summary of proposed specifications:


Surface
(m²)

Height
(m)

ACH

Opening window
surface area (m²)

Single rooms

> 7.5 (2.5 x 3)

> 3

> 12

> 25%

Wards

4.5 m²/patient

> 3.5

> 12

> 15%

Waiting rooms
(preferably outside)

3 m²/patient

> 3.5

> 12

> 15%

Sputum collection areas
(preferably outside)

> 1.5

> 2.5

> 20

> 50%

Toilets

> 1.2

> 2.5

> 12

> 25%

Consultation rooms

> 7.5 (2.5 x 3)

> 3

> 12

> 25%

Central corridors
(avoid in new buildings)

> 2

> 3

> 12

> 25%

There are two main techniques to measure the ventilation. The most commonly used is the anemometer that measures the velocity (speed) of air (see manufacturer’s recommendations for various types of anemometers). The technique using the gas analyser is difficult and should only be used by trained staff.