As long as theres plenty of O2 in the room, it's unlikely that breathing slightly elevated levels of CO2 will bother you.

The level would have to be fairly significant, 2%, or, 20,000ppm before you or your buddy would begin having minor issues with breathing. And somewhere on the order of 80,000ppm for seriously dangerous levels. I won't bothering mentioning that 80k ppm is hard to make happen unless theres a big time drop in the available breathable O2 levels.

If you happen to notice your buddies lips turning blue...get out!

Optimum levels of CO2 for your plants can't even begin to come close to being a bother to a healthy human.