You're mixing your units -- 0.05% of 6L is 3mL. In order for that to weigh 3g, atmospheric CO2 would have to be as dense as water. Most figures I see peg 1mL o

You're mixing your units -- 0.05% of 6L is 3mL. In order for that to weigh 3g, atmospheric CO2 would have to be as dense as water. Most figures I see peg 1mL of CO2 at closer to 2mg (it's about 50% heavier than the equivalent atmospheric volume, since that's mostly N2 with some O2). Your estimate of 240 mL / minute is about 346L per day, or about 700g of CO2. That's roughly the same order of magnitude as the cited 1 kg / person / day. edit: Another way of thinking about it: if you scale up your numbers to grams per day, you'd end up with a ludicrous 346 kg / human / day. Multiply that by 12/44 (mass of Carbon-12 vs CO2), and that's the equivalent of a human shedding 100kg of carbon every day from just breathing. Most humans don't even weigh that much.