Well first is should be around 1500 ppm ,to help the plants grow, and the temp should be about 5 d.f. higher to help with the up take of co2.
NOW IF you go in the room /space/hole/etc. co2 displaces O2 , thats what kills people.
Wonder why people spend money on metering out co2 , it works. Now if it goes over 3000 ppm it will help kill the plants . If you starting out use lots of fresh air. ITS free and earth has lots of it at about 300 ppm.
SAFETY IF NUMBER ONE. If a tank has a small leak in a small room it can easly fill up the room. Always check all fittings with a solution of soap, and look for any bubbles.