There are two basic ways of measuring ventilation rate – according to the number of people in the space and the size of the spaces.
Number of people (capacity)
This is the system that has been used in the UK for some time. The idea is that each person in the space should have a certain amount of fresh air supply (measured in metres3/person/hour or litres/person/second).
To find out how much air would be needed just multiply the number of occupants with the ventilation rate – so a rate of 60m3/person/hour would require a ventilation rate of 3000m3/hour for the room with 50 occupants (or half that for 25 seated occupants). This would probably represent a room of around 50m2.
Sometimes the guidance is quoted in litres/person/second which is more difficult to calculate but for reference:
30m3/person/hour = approximately 8 litres/person/second
60m3/person/hour = approximately 16 litres/person/second
Size of space (air changes)
This is a very simple calculation to understand and use. The amount of ventilation needed to fill the room once in an hour is one air change/hour. The added factor here is the ceiling height.
|For the 10m x 5m (50m2) room quoted above the volume of air required for one air change depends on the ceiling height: |
|At a height of 2.5m (50m2 x 2.5m) the room volume would be 125m3/hr|
|At a height of 5m (50m2 x 5m) the room volume would be 250m3/hr|
|In our 50m2 floor area pub: |
|a rate of 30m3/person/hour||= 12 air changes at a ceiling height of 2.5m|
|= 7.5 air changes at a ceiling height of 4m|
|a rate of 60m3/person/hour||= 24 air changes at a ceiling height of 2.5m|
|= 15 air changes at a ceiling height of 4m|
There are various and often conflicting views on ventilation rates to allow for smoking. The two major organisations who propose standards ASHRAE in the USA and CIBSE in the UK have both recently moved away from recommending ventilation rates in this area.
Previously ASHRAE (Standard 62 2001) had recommended a rate of 15+ litres per person per second (c.30m3/hr/person) and CIBSE (Guide B) 16 litres per person per second (c.60m3/hr/person) for a mix of smoking and non smoking occupants. Test work seems to suggest that this is more than enough to ensure that staff and customers are not exposed to any contamination above the recommended workplace exposure limits. Tests on this are ongoing.