When you design a ventilation system for an office that should last many decades, what assumption do you make for atmospheric CO2 ?
The ventilation system in our new office will be the only source of fresh air for its 50-140 occupants (windows can be opened, but there will be no need to do so). Sizing and designing that ventilation system is critical if you want to make sure the air quality inside is top notch. Research has shown that a CO2 concentration of 800ppm (parts per million) is the maximum value of CO2 to strive for in an office.
Obviously CO2 is not the only pollutant in the air, but it is much easier to measure than other values and serves as a good indicator of air quality in general. So our ventilation system will measure CO2 levels (plus relative humidity, temperature and pressure) in all rooms and increase and decrease air volume to ensure the concentration stays between 600 and 800ppm. In order to do that it will pull out "dirty" air, send it over a big rotary heat exchanger (which recovers heat and moisture) and replace it with the same quantity of fresh air.
When you decide how big the ventilation system should be to keep CO2 inside below 800ppm, you need to know how much CO2 is in the fresh air outside, or you don't know how much "refreshing" capacity you are installing.
And this is where it gets quite interesting... Nearly all building guidelines prescribe a concentration of 350ppm to be used for atmospheric CO2. Which was the atmospheric CO2 concentration in the 1990's....
But why were we building an ultra sustainable Energy Plus Office again ? Right, because atmospheric CO2 is rising way too fast, causing climate change and threatening the survival of our species on this planet. In fact, in my lifetime it has risen from 320ppm to 400ppm, the highest value in over 800.000 years.
Now, if we assume 350ppm for atmospheric CO2 as prescribed, this means that the outside air has more "refreshing" potential because it can absorb more CO2 from our office before it needs to be replaced. But since atmospheric CO2 has already reached 400ppm, we will in fact need more air volume for the same amount of CO2 absorption.
If you think about it, all buildings that have been designed based on 350ppm in the last 20 years either have to start accepting higher and higher internal CO2 levels, which is not ideal for health and productivity, or open their windows more, which is very bad for energy efficiency.
So what did we decide? Despite the fact that we hope the world will radically change course over the next few years to limit its CO2 emissions, we want to make sure the office stays healthy and efficient, so we designed our ventilation system for an atmospheric CO2 concentration of 450ppm, which is what is expected in about 20 years from now in the "business as usual" scenario.
I sure hope that in 20 years time we can look back and conclude that the human species was clever enough to avert disaster by switching quickly from coal, oil and gas to renewable fuels and by stopping deforestation. The technology is already available and the economy will be much better off.
In that case we can then all be very happy that we oversized our ventilation system and that we will never need to run it at full capacity:-)