# Plastic Bag Death, How?
**URL:** https://boards.straightdope.com/t/plastic-bag-death-how/607458
**Category:** Factual Questions
**Created:** [December 26, 2011, 8:57pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458 "2011-12-26T20:57:04Z")
**Posts on this page:** 11
**Page:** 5
### Author:  [@am77494](https://boards.straightdope.com/u/am77494)
#### Post date: [May 10, 2022, 11:43am UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/81 "2022-05-10T11:43:58Z")
> [@Machine\_Elf](#):
>
> I’m not seeing where anybody has asserted that a blend of CO2 and air will self-stratify into a layer of pure CO2 covered by pure air; **we’re** talking about a hazard due to the release of a stream of concentrated CO2.
Bolding mine. Who is “we” in this assertion ? I am having a conversation with @engineer_comp_geek - Since when has SD changed into “we” versus “others” ? Is there a internal group, I’m not aware of ?
> [@Machine\_Elf](#):
>
> I’m not seeing where anybody has asserted that a blend of CO2 and air will self-stratify into a layer of pure CO2 covered by pure air;
> [@engineer\_comp\_geek](#):
>
> CO2 is heavier than air so it tends to collect in low lying areas in breweries.
See above . It was asserted that CO2 pools in low lying areas - This is what I am responding to.
> [@Machine\_Elf](#):
>
> Here is a demonstration in which a person literally pours CO2 out of a cup onto a flame:
Perhaps you can learn from the demonstration that firefighters use all inert gases equally. There is no preference for CO2 because it settles in low areas and smothers out liquid fuels which may be sitting in low areas too.
Here is a similar experiment with a person pouring out Helium to put out a fire. Again, nothing to do with density
[ ](https://www.youtube.com/watch?v=brnqjLwAZ9Y)
From the kinetic theory of gases, the velocity of CO2 molecule versus that of a Oxygen molecule in air is inv proportional to square root of the Mol weight.
So compared to Oxygen molecules in air, the CO2 molecules are moving at 85% of their speed, or compared to N2 molecules - they are moving at 80% of their speed.
Again, to emphasize, CO2 in pits accumulates because the source of the leak is in the pits - it does not flow down into pits without diffusing in air.
Argon has the same mol weight as CO2 - it does not accumulate in low spots either.
---
### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [May 10, 2022, 1:15pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/82 "2022-05-10T13:15:35Z")
> [@am77494](#):
>
> Bolding mine. Who is “we” in this assertion ? I am having a conversation with @engineer_comp_geek - Since when has SD changed into “we” versus “others” ? Is there a internal group, I’m not aware of ?
In this virtual space, you and @engineer_comp_geek are not the only people present. “People chatting around a table in a bar” is a decent model for the interaction here. When I say “we”, I’m not using the royal “we.”
> [@am77494](#):
>
> Perhaps you can learn from the demonstration that firefighters use all inert gases equally. There is no preference for CO2 because it settles in low areas and smothers out liquid fuels which may be sitting in low areas too.
The settling/creeping tendency of a plume of CO2 is certainly not something to be relied upon in a firefighting situation. For starters, the violent expulsion of CO2 from an extinguisher immediately contributes to turbulent atmospheric mixing. Ambient wind/HVAC convection can also help with mixing. At the same, ambient wind/convection can’t be relied upon to serendipitously move bulk quantities of CO2 to where you want it. If you see a fire, you blast it directly with CO2, you don’t hope your overspray from 20 feet away drifts over to it.
> [@am77494](#):
>
> Here is a similar experiment with a person pouring out Helium to put out a fire. Again, nothing to do with density
They did not pour out helium; the helium was confined in a flask, and the match was inserted into the flask. To the extent that helium was coming out of the flask, it was almost certainly pouring _up_ through the top half of the neck, with air entering the flask via the bottom half of the neck. If you did the same demo with a flask of CO2, I definitely would expect it to pour from the bottom half of the neck; you should be able to extinguish the match simply by holding it _near_ the flask opening, without having to actually insert it in the flask as they did in this demo.
---
### Author:  [@am77494](https://boards.straightdope.com/u/am77494)
#### Post date: [May 10, 2022, 2:32pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/83 "2022-05-10T14:32:48Z")
> [@Machine\_Elf](#):
>
> When I say “we”, I’m not using the royal “we.”
Nice try. I am not buying it. 🙂
> [@Machine\_Elf](#):
>
> The settling/creeping tendency of a plume of CO2 i
Please provide a cite for “settling/creeping tendency of a plume of CO2”
> [@Machine\_Elf](#):
>
> They did not pour out helium; the helium was confined in a flask,
They did - watch it again.
---
### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [May 10, 2022, 3:21pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/84 "2022-05-10T15:21:48Z")
> [@am77494](#):
>
> Please provide a cite for “settling/creeping tendency of a plume of CO2”
See my cite upthread re: the Lake Nyos disaster.
> [@am77494](#):
>
> They did - watch it again.
Watching it again:
1:22, flask is full of helium, stopper is removed.
1:23, lit match is inserted into the neck of the flask, where already-present helium promptly extinguishes the flame.
How/when is any kind of pouring action demonstrated in this video?
---
### Author:  [@am77494](https://boards.straightdope.com/u/am77494)
#### Post date: [May 10, 2022, 7:15pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/85 "2022-05-10T19:15:35Z")
> [@Machine\_Elf](#):
>
> See my cite upthread re: the Lake Nyos disaster.
Do not go by wikipedia. Here is a peer reviewed Journal publication of the Lake Nyos disaser.
> The initial velocity of the gas jet should have been 28 m/s, the volume of pure CO2 0.6 km3 and the gas release should occur in a very short time (a few minutes).
This a forced diffusion system with lots of factors playing into it like wind direction.
The system - @engineer_comp_geek and I were talking about was CO2 settling in low pits in a brewery.
> [@Machine\_Elf](#):
>
> How/when is any kind of pouring action demonstrated in this video?
The helium has not collected at the top of the beaker but the flame goes out right near the entrance to the flask.
---
### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [May 10, 2022, 8:05pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/86 "2022-05-10T20:05:14Z")
> [@am77494](#):
>
> Do not go by wikipedia. Here is a peer reviewed Journal publication of the Lake Nyos disaser.
You didn’t include the link, so **[I went out and found it](https://www.sciencedirect.com/science/article/abs/pii/037702739290066M)**. I’m not sure why you’re quoting that part of the abstract. The relevant part would seem to be the first few sentences:
> In order to explain the origin of the Lake Nyos Disaster, a three-dimensional model for carbon dioxide flow has been calculated. The boundary conditions were based on the field observations. Calculations show that CO2 tends to flow down slope by gravity and velocity tends to increase on steep slopes. The results of calculation are in good agreements with the effects observed in the field.
In other words, their work supports my contention that bodies of CO2 gas will flow under the influence of gravity. A large release or continuous source of CO2 need not be in a pit in order for CO2 to flow across the floor in a way analogous to water, eventually finding low spots.
> [@am77494](#):
>
> The helium has not collected at the top of the beaker but the flame goes out right near the entrance to the flask.
? The helium hasn’t “collected” anywhere. The flask was completely filled with helium before the stopper was placed. The flame doesn’t go out until the match is inserted _into_ the flask. Claiming this demonstrates pouring of helium gas is like dropping a match into a beaker of water and claiming that that demonstrates pouring of water onto the flame.
---
### Author:  [@am77494](https://boards.straightdope.com/u/am77494)
#### Post date: [May 10, 2022, 10:39pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/87 "2022-05-10T22:39:36Z")
> [@Machine\_Elf](#):
>
> In other words, their work supports my contention that bodies of CO2 gas will flow under the influence of gravity.
No it doesn’t. What it supports is my original assertion that " Low spots that have CO2 leaks will accumulate CO2. CO2 will not flow under buoyancy differential and collect in low spots.
It also proves that your wikipedia link was wrong.
---
### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [May 11, 2022, 2:00am UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/88 "2022-05-11T02:00:47Z")
> [@am77494](#):
>
> CO2 will not flow under buoyancy differential
What part of “CO2 tends to flow down slope by gravity” (from your cite) are you interpreting to mean “CO2 will not flow under buoyancy differential”?
---
### Author:  [@am77494](https://boards.straightdope.com/u/am77494)
#### Post date: [May 11, 2022, 2:09am UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/89 "2022-05-11T02:09:38Z")
> [@engineer\_comp\_geek](#):
>
> CO2 is heavier than air so it tends to collect in low lying areas in breweries.
This was the original statement, I posted about. And my link contained a link to a published paper that said -
> A very common opinion among cavers (and not only
> cavers, see (Al-Azmi, 2008)) is that dense gases tend to
> accumulate in depressions, and especially, at the bottom of
> caves. **It is widely accepted that CO2 accumulates at the**
> **\> bottom of shafts; this concept has been infrequently**
> **\> discussed, but often repeated from paper to paper. In this**
> **\> paper we show that this concept is in fact false** and may be
> regarded as an underground legend (James, 2003; Cigna,
> 2008).
This publication can be found here : [https://caves.org/pub/journal/PDF/v71/cave-71-01-100.pdf](https://caves.org/pub/journal/PDF/v71/cave-71-01-100.pdf)
As far as I am concerned, this is my final post. I believe, my assertion is adequately backed up by citation from peer reviewed literature. Yes, it is true that NIOSH and OSHA also have done the mistake of saying that "_dense gases tend to accumulate in depressions"_ but this is a myth that just keeps getting perpetuated.
---
### Author:  [@JustinC](https://boards.straightdope.com/u/JustinC)
#### Post date: [May 12, 2022, 8:29pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/90 "2022-05-12T20:29:59Z")
The _wildly feared_ problem that denser than air gases released underground or in a confined space will stay there may be a myth - I have no idea - but I’m certainly never going to test the hypothesis. If someone who’s an experienced caver or engineer tells me not to do something, I’m definitely not going to go against their advice.
---
### Author:  [@Pushkin](https://boards.straightdope.com/u/Pushkin)
#### Post date: [May 19, 2022, 9:35pm UTC](https://boards.straightdope.com/t/plastic-bag-death-how/607458/91 "2022-05-19T21:35:16Z")
We weren’t sure how seriously to take it, but at one stage as teenagers we actually popped a bag over our heads each (very briefly) to prove to ourselves it wasn’t that dangerous.
[Previous page](https://boards.straightdope.com/t/plastic-bag-death-how/607458.md?page=4)