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I was reading this article about balancing a draft system, and came across this assertion:

Do not place your kegerator in your basement. A catastrophic failure could cause the tank to drain, flooding your basement with CO2

As I understand it, CO2 is not toxic itself, but will displace atmosphere (and hence oxygen) and so can create an asphyxiation risk in confined spaces. CO2 asphyxiation in fermentation tanks has been recognized as a workplace hazard in the winemaking industry.

Surely the risk is inversely proportional to the size of the basement, and directly proportional to the the quantity of CO2 released. My basement is around 800 square feet (and not very well sealed -- it's an old house), and my CO2 tank contains 5 pounds of gas when full. Does this combination present any risks to my family?

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No, at homebrew scale and in household settings leaking CO2 is not a serious concern, and is probably very low on the list of "occupational" hazards that homebrewers are at risk of and fail to recognize.

Before getting into the math, you must realize that almost all houses are leaky in the sense that inside air is mixing with outside air, either because they are actually leaky or because the newer, "tight" houses have efficient air exchange systems. Secondly, CO2 gas does not "push out" air, nor does it sink, but rather it will mix because all gases want to be dispersed evenly throughout the available volume, subject to other things that inhibit mixing (see Dalton's Law, the combined gas law, etc.) Even your walking through some leaking CO2 gas will mix it, so you will never have a "blanket" of CO2 waiting to suffocate you (nor sitting only in the bottom half of the headspace of your fermenter).

Let's assume your basement is sealed and air tight.

So the volume of one mole of gas at STP (standard temperature and pressure) occupies 22.4 liters, and one one mole of CO2 has a mass of 44.1g. This means that 5 lbs. of CO2 comprises 51.4276644 moles and 5 lbs. of CO2 occupies a volume of 1151.979683 liters or 40.68 cubic feet. Your basement is (800 sq. feet x 8 feet high), if I assume an eight foot ceiling, or 6,400 cubic feet. Thus, if you released all of the CO2 in your tank at once, it would occupy 0.64% of the volume of the room, and will mix freely with the other gases.

So is this level toxic? Let's see how much CO2 would be mixed in the air.

Normal atmosphere has approximately 78.09% nitrogen, 20.95% oxygen, 0.93% argon, and 0.04% CO2, as well as some small amounts of other gases and around 1% water vapor. If you mixed in 40.68 cubic feet of CO2 into 6,400 cubic feet of normal atmosphere, you would end up with 77.59% nitrogen, 20.82% oxygen, 0.92% argon, and 0.67% CO2. As you can see, this is not a huge increase in CO2.

Further, Wikipedia says that "in concentrations up to 1% (10,000 ppm), it will make some people feel drowsy. Concentrations of 7% to 10% may cause suffocation, even in the presence of sufficient oxygen.".

Furthermore, CO2 is not like CO in that it binds to your hemoglobin and asphyxiates you -- you will respirate excess CO2 as soon as you get back into normal air.

So, I think that there is little risk of asphyxiation from your ordinary 5 or even 10-lb. CO2 tank in a household environment. Yet, it is always good to be cautious anyway, especially in especially confined spaces, and if children and pets are around.

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From this page we get the conversion of 1kg of CO2 to 509.1 liters. Scaling up to your 5 pounds = 2.26796 kilograms, and thus 509.1 liters * 2.26796 = 1155 liters, which is about 40.8 cubic feet. Spreading this throughout your basement, (assuming an 8 foot ceiling, for a volume of 6400 cubic feet), only adds 40 / 6400 = 0.625% CO2 to the entire room. While this is a significant jump from the normal 0.0397%, it shouldn't present a problem, even if you lived down there the rest of your life.

I think you're safe.

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    CO2 does not form a layer. There is nothing in physics to suggest that CO2 precipitates out of the atmosphere; otherwise we would be walking around in a several hundred feet of CO2 along the ground as the CO2 dropped out of the atmosphere. Instead, you need to figure out the mixed concentration of CO2 in the basement's air volume, and then whether it is toxic. – Chino Brews Sep 9 '14 at 15:25
  • @ChinoBrews Since CO2 is heavier than normal air, wouldn't it tend to settle on the floor if the leak was slow enough and the air still enough? Isn't this the basis of open fermentation? – CDspace Sep 9 '14 at 15:36
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    Gases don't work like that, however. Otherwise logically if you sealed a room, you would end up with strata of the different gases, and you would suffocate if you entered the room and you were the wrong height. The force that causes gases to expand to fit the volume exceeds the force of gravity on them. So in that sealed room you will eventually end up with a fully mixed atmosphere even if you started with a vacuum and added gas one molecule at a time so as not to create any air currents. – Chino Brews Sep 9 '14 at 17:42

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