Personally, the unnerving fact is not that ambient carbon dioxide is harmful in current concentrations (it almost certainly isn’t), but more that the average baseline concentration outdoors (which we have to live with and cannot really escape much) is rising seemingly drastically. It’s probably not going to be an issue in our lifetimes, but because it’s a global rise we can hardly even have a control group to test for any subtle health effects from a 100 ppm increase. Also, most advice and regulations about indoor concentrations rely on the fact that we don’t exclusively live indoors, we do get regularly exposed to baseline level outdoors, and rarely account for the fact that that level is rising.