Fascinating!!! Sounds like a Symposium Paper Winner!!!

There have been a couple of reports of extremely high carbon dioxide in homes. Here's one:
https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5350a5.htm

There was another incident in 2007 of high carbon dioxide levels (>6000 ppm or 0.6%) in northern
Illinois homes that were hit with heavy rains. The source of the
carbon dioxide appeared to be limestone present in the soil that was lifted
up by rainwater. Carbon dioxide gas seems have entered three homes
through the sump hole. Homeowners were unable to light their pilot because
oxygen was displaced by the carbon dioxide, and they called the fire
department. Their carbon dioxide measurement devices could only read up to
6000 ppm, so levels were at least 6000 and could be considerably higher.

There was an incident in Kansas as well. Unfortunately, the news stories of the IL and KS incidents are no longer available online, but I'd kept some notes with the weblinks. You could contact the newspapers to get more info:

https://www.beloitdailynews.com/articles/2007/08/23/news/news02.txt
http://www.rrstar.com/homepage/x1359806021
http://www.kansas.com/188/story/529039.html

I had some friends in Wichita that experienced extremely high levels of some soil gas ( I don't recall that it was Co2) after heavy rain. It was probably the story Dan is referring to and I'm guessing it happened 10 or 12 years ago. I don't have any idea what would cause high Co2 concentrations in the soil, but I do know heavy rain will increase soil gas entry significantly, many times even in buildings with mitigation systems installed. This spring has been especially bad for rain induced radon concentration spikes.

Rain induced radon spikes happen in all types of soil conditions (permeable vs tight). I work in an area with mostly very tight soils, but I have seen rain induced radon spikes in a house with very permeable soils (not sure if it was karst or glacial moraine). A little off topic but interesting, atmospheric pressure actually played a pretty big roll in radon concentrations in that house. There will likely be a presentation dealing with that house at the symposium.

Angela
I have worked on a half dozen homes with elevated co2 in Pennsylvania. These homes had co2 levels so high they displaced the oxygen and often caused the pilot light to go out on the water heater in the basement. There may have been one death from co2 that i think was misdiagnosed. In all the cases we think the co2 was produced by rotting wood in the ground. In one case it may have been the builder who buried trees. In most of the cases I worked on it was due to rogue coal mining shafts done years before that had rotting timbers.

We were able to fix these houses using standard ASD systems. Be careful in these homes. If oxygen levels go below 15 percent you can pass out. The fire department was called in to one of the houses when the daughter passed out and one of them passed out.

I did publish an aarst paper about the work.

Bill

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!

The paper I presented at the 2003 AARST conference that includes remediating high CO2 levels with ASD system is the following.

http://www.wpb-radon.com/pdf/Vapor%20Intrusion%20Mitigation%20VOC.pdf

Bill, very interesting paper, thank you for sharing it!
Was there any speculation on what caused the high CO2 in the Conshohoken example, or why the neighboring house cleared up without mitigation?
Jim