How much CO2 does fermenting wine give off?

We have a couple of demijohns of elderberry wine bubbling away noisily in the sitting room. They were in the kitchen, but that's next to the bedroom and the bubbling was keeping us awake, so I moved them. As we sat down for the evening the other day, Ian asked me what gas it is that's bubbling out so vigorously. Carbon dioxide. Hmm. That does seem to be rather a lot of carbon dioxide. I wonder whether it's enough to be a health risk?

My attempts to find the answer to this question via Google led to one Daily Mail article about a couple of Frenchmen dying of suffocation while treading grapes, but not much else. I decided to measure it for myself, so here is what I did:

1. Find a piece of plastic tubing and bend it into an S shape using hot water to soften. This was not easy because it kept buckling.
2. Take a graduated jam jar (I marked lines at 100ml intervals) and completely fill with water, then invert in a bowl of water. This was not easy because the jar was taller than the bowl, so the whole lot had to be submerged in a bigger bowl to get all the bubbles out.
3. Faff about for ages trying to raise the jar off the bottom of the bowl so that it wouldn't squash the plastic tube, before realizing that it wouldn't squash it anyway.
4. Recognize that the jar wouldn't need to be horizontal all the time, just when the measurement was taken, and I could hold it for that bit. Considered a spirit level, then decided that would be going a bit far (!)
5. Bring a small table (for the bowl/jar), blu tac (for sealing), and mobile phone (for timing) through to the sitting room.
6. Assemble the apparatus, connecting the plastic tube to the air lock with blu tac, and start timing.
7. Notice that the tube has filled with water again, reseal the blu tac, and hope not too much gas was lost.

The tube takes the CO₂ into the water-filled jar, hopefully catching it all

Here's a closer view of the jar-in-a-bowl assembly. If you look closely, you can see a bubble rising between the two marks on the jar. Please do look closely; it took me several attempts to get this picture.

Look, a bubble! More to the point, lots of gas collecting at the top of the jar.

The idea was to see how long it would take to produce 100ml of CO₂ and then work out volume (or weight) produced per day, and compare that with what a human breathes out, or maybe a cat. I did wonder whether the extra pressure of the water might slow down the bubbles, but they were still coming out at much the same rate as the other demijohn, so that was OK. Holding the jar horizontal was a bit of a challenge, as everything else in the room, and particularly the cupboard behind, is on the skew.

And the result was... about 20 min for 100 ml CO₂. It took 22 min for the first 100 ml, then the second 100 ml, which I expected to be slower due to the extra weight of water, was actually quicker and I missed it. At 43 min, there was already more than 200 ml gas in the jar. This may be due to a bit escaping when the seal broke at the beginning, so approx. 20 min will have to do. That's 300 ml per hour, or 7.2 litres per day.

I found a few different figures for the amount of CO₂ breathed out by a human, of which this page includes the lowest estimate: An inactive person (as they would be, in a sitting room) breathes out 350 litres of CO₂ per day. That's 48.6... almost 50 times as much as my gallon of fermenting wine.

I couldn't find any information on other mammals, still less on cats, specifically.* If amount of CO₂ exhaled is proportional to body weight (which it probably isn't) and cats are about one twentieth the weight of an adult human, one cat would be breathing out about two and a half times as much CO₂ as a gallon of wine. Even with two demijohns, George is probably still breathing out more CO₂ than the wine is. Daily Mail notwithstanding, I'll stop worrying about suffocation from fermenting wine.

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* Though apparently there is a myth that cats breathe in carbon dioxide, and this is linked to the myth that they smother babies. Surely if they're breathing in the CO₂ that would be a good thing for the baby? Not only does that myth take no account of biology, it's not even internally coherent.