I guess I'm confused then because in your first post you said that co2 levels above 5% concentration can be toxic. I have read more than a few threads on this forum where people were having co2 leaks from there system and I don't see how this small amount could pose a problem unless you have your tank setup in a closet that is sealed up with duct tape.
in enclosed environment, leaked co2 can not escape and concentration will rise.
about the 5% toxicity, the detail is in the provided link, very interesting article.
It is the dose that kill, any substance can be toxic if give the proper amount, so as dihydrogen oxide, a poisonous deadly substance.