my friend, CO2 is a natural part of the atmosphere and is emitted from your mouth with every breath you take. Carbon monoxide (CO) is the dangerous gas that you are thinking of.
Yes, CO2 is perfectly natural, as is Arsenic. Yet, a high concentration of CO2 implies, in most non-contrived situations, a high concentration of CO. Which is also about as good for you as a high concentration of Arsenic. Make of that what you will, and do consider that if you think it's a perfect cure for (whatever), you might be a tad wrong...
It's incredibly easy for CO2 levels to get to uncomfy levels in an enclosed living space such as my office at home with no central air and a closed door, especially in the heat of local summer where I will try very hard to not open the windows, or a shitty small meeting room at the overpriced coworking space. It's emitted from my mouth with every breath I take yeah that's exactly the problem. :(
It's definitely not as bad as carbon monoxide poisoning but it's also much easier to inflict on yourself at least to the point where it can make you drowsy and worsen your headaches.