How do air change rate, ventilation rate and maximum carbon dioxide concentration in a room relate? Its complicated. But in general they relate like this:*
*Example classroom, the shape remains the same, while actual y-axis value change with room/age/activity. 1/7
*Example classroom, the shape remains the same, while actual y-axis value change with room/age/activity. 1/7
If the maximum concentration in a room is above ~2,000 ppm we know that the ventilation rate/air change is BAD (flat part of curve). Small increases in ventilation will drop the carbon dioxide concentration quickly. 2/7
If the maximum concentration in a room is below 700 ppm we know the ventilation probably good (steep part of curve). Big increases in ventilation will only slightly drop the carbon dioxide concentration. 3/7
What about 700 ppm to 2000 ppm? That is where it is hard to say from maximum carbon dioxide if the ventilation rate needs to be improved based solely on pre-pandemic guidelines (ASHRAE etc. It depends on the room/use/size/age). 4/7
But we can say at 2000 ppm roughly 4% of the air you inhale has been in other people lungs. At 800 ppm roughly 1% of the air you inhale has been in other people lungs. Remember #COVIDisAirborne. 5/7
So lower carbon dioxide concentrations between 700 -2000 ppm are better in general. It will reduce the amount air inhaled that has been emitted from other people in the room and hence the risk associated with airborne viruses. 6/7
In summary, maximum carbon dioxide concentrations in classrooms (measuring all day with kids present) can be used to
1) identify poorly ventilated classrooms.
2) identify well ventilated classrooms.
3) give idea of the relative amount of risk from a rebreathe fraction. 7/7
1) identify poorly ventilated classrooms.
2) identify well ventilated classrooms.
3) give idea of the relative amount of risk from a rebreathe fraction. 7/7
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