Air change rate: a vital measurement in the fight against COVID-19-KIMO instruments China

Recommendations from specialist ventilation and air-conditioning bodies all come to the same conclusion: that maintaining indoor air quality is vital in the fight against COVID-19 and other viruses, and that a ventilation system’s air change rate is of paramount importance.

Air flow meter and capture hood: how a measurement instrument helps protect against viruses

Air change rate is the speed at which all the air inside a room is replaced by the ventilation system. It is calculated as the air flow from each ventilation system inlet relative to the total volume of the room in question, and is expressed per hour. Some busy or sensitive rooms, such as hospital wards, open-plan offices and classrooms, require a constant air change rate.

Our DBM 620 air flow meter is the first measurement instrument in its class capable of quickly calculating air change rate. The app automatically combines air flow measurements from each ventilation inlet and works out the rate according to the volume of the room in question.

What air change rate is required to protect against COVID-19?

As this table indicates, the recommended air range change differs across various scenarios. The values were published before the COVID-19 pandemic. Even higher rates are now needed to prevent the virus from spreading indoors. In a hair salon, for instance, the sustained use of aerosols means that the ventilation system needs to complete between 10 and 15 air change cycles each hour.

Since viruses are often compared to aerosols on account of their physical behaviour, it makes sense that a high air change rate is required to reduce contamination risk. As the second table below shows, increasing the number of air changes per hour (ACH) from two to four decontaminates the room twice as quickly. But it still takes at least 69 minutes to remove all viral particles from the air. In order to achieve 99.9% removal efficiency in eight minutes, the air in the room would need to be changed no fewer than 50 times per hour!

ACH	Time (mins) required for removal 99% efficiency	Time (mins) required for removal 99.9% efficiency
2	138	207
4	69	104
6	46	69
8	35	52
10	28	41
12	23	35
15	18	28
20	14	21
50	6	8

Other ways to keep indoor air clean

Close the ventilation system’s indoor air recycling circuit: the incoming air should come exclusively from outside the building.
Maintain vertical laminar air flow: ideally, the ventilation system should move air from the ceiling down towards the ground, vertically and in a uniform manner, and the air outlets in the room should be located near to the floor.
Purify the air within the ventilation system: install HEPA filters to trap tiny particles like bacteria and viruses, fit UV purifiers behind the pipes to kill micro-organisms, use a high heat setting (not always feasible for HVAC systems), and employ ionising and ozone-based purifiers.

Changing the air regularly helps to reduce indoor air pollution – especially important given that indoor air is often ten times as polluted as outdoor air. Air change rate is therefore an essential measurement in all aspects of indoor air quality.

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