Some thoughts:
1) As a first principle, if building occupants or visitors (including radon workers) are reporting any of the kinds of symptoms as described in the MMWR article for which Dan gave us the link above, it's critical to recognize this as possibly the sign of potentially even fatal carbon
monoxide (CO) poisoning, which is much more common than the relatively rare carbon
dioxide (CO2) problems described. This means that
immediate steps such as building evacuation and professional evaluation of affected parties and the building air need to be done before anyone does anything else.
2) i want to underscore Bill Brodhead's caution about oxygen levels dropping to 15% or less. Passing out isn't as harmless as entertainment often makes it out to be. It
can result in injuries or death from the fall. Bill has written a lot of papers but maybe he can point out which one he's referencing at
http://aarst-nrpp.com/wp/international-radon-symposium/symposium-proceedings/
3) Always remember that incoming soil gas, given that it is nearly always cooler than the indoor air into which it's being introduced, and especially if it is very high in CO2, is denser than the warmer indoor air and will tend to remain at floor level in the lowest area of the building near its point of introduction. Therefore, even if CO2 concentrations are not particularly high elsewhere in the building, do not assume they will remain low at the lowest points, especially if active air movement that would mix and dilute this soil gas is poor. Readers know how soil gas with radon behaves; soil gas with CO2 behaves similarly.
4) The MMWR article is worthwhile to read as a case study of how likely it is that
these problems may need several iterations before being properly diagnosed. In other words, do not expect that these "unusual soil gas" situations will be obvious to the practitioner who innocently enters a site with his or her mind solely on addressing a radon problem.
5) Regarding the "displacement of oxygen": To be clear, so readers understand the process, an inert gas such as CO2 entering the building's airspace is displacing and diluting the
air (which is mostly nitrogen), not just the oxygen that is a component of it. There is nothing preferential about the incoming CO2 displacing oxygen with respect to any other constituent of the air.
6) For some perspectives on CO2:
a) Usual situations: Current global atmospheric levels are around 420 ppm. We each breathe out about a couple of pounds a day, and in some indoor environments that will build up. Good ventilation should strive to keep concentrations in the air we breathe at less than twice that, since some cognitive detriments appear even at levels as low as 1,000 ppm. Occupational standards allow 5,000 ppm for a time-weighted average, but that doesn't mean it's a good idea to be breathing CO2 at levels that high.
b) This kind of situation: According to
https://en.wikipedia.org/wiki/Limiting_oxygen_concentration the limiting oxygen concentration for combustion of methane in an environment where CO2 is diluting air is 14.5%. This means that in order for combustion not to be sustained, no less than nearly 31% of the mixture by volume (
310,000 ppmv) would have to be CO2, so you can see we're really talking about a distinctly different beast in these very dangerous cases.
Be careful out there!