Actually, using CO2 and N2 as laser assist gas is common
Besides my little desk top laser, my work also has a 2000W steel cutting laser. We used to use CO2 and N2 for assist gas all the time. As Doc pointed out, it gets expensive, but the amount of gas actually released is tiny in comparison to the volume of the shop. The nozzle is just slightly only about .06" dia. It's a lot when you're paying for it, but tiny when you're breathing it. We still keep the gasses in stock for when we need them, but 99% of what we cut is 304 stainless steel, and it cut's adequately with compressed air, so we mostly use that now. Look up "laser assist gas" and you'll see how common those are with big lasers, and I've never heard of any sort of asphyxiation issues.