CO2 in caves

[thehungrytroglobite]

On a 2 day drive back to UK so lots of time to ask questions on UK Caving.
We encountered quite bad CO2 in Aven de Noel this week.

The metre at the bottom of the entrance pitches read 1.42% but we reckon it was a lot higher than this in other parts of the cave. We were a group of 7, 6 of us felt effects to varying degrees and one didn't feel it at all. Of those 6, about 3 of us felt it more severely. The main symptoms we experienced were just being really out of breath; at rest I had to breathe heavily through my mouth and while walking I was breathing as heavily as if I were sprinting uphill. This was accompanied by some general grogginess / feeling a bit 'hazy' but nothing much else. Another member of our group experienced a brief headache after lying on the floor pretending to be a shrimp. I noticed that after we ascended a bit again on some in situ ropes it got somewhat better and I was almost able to comfortably breathe through my nose at rest, but down in the lower main passageway and also the sideways crawls it was much worse.

We were wondering why some members of the group felt it more than others? What impacts the extent of the effect that high CO2 will have on someone? Merci

 

[Edwardov]

Might be something similar to how altitude sickness affects people differently.

 

[mrodoc]

The effects vary widely. My brother in law gave up digging at one site because he got a persistent headache after the trip. We have done series of measurements at the site which draughts and levels are partly seasonal. Readings got up to around 3.5% and at that level I certainly was puffing hard coming out of the cave. The most common effect is a sensation of breathlessness producing hyperventilation.

 

[Bob Mehew]

We were wondering why some members of the group felt it more than others?

I have not come across an explanation of why different people have different sensitivities to a given level of CO2. But I presume it is due to the effect of the level of CO2 in the blood which impacts on the function of the brain aside from the most significant function to make you breath faster. But it is real and observable. Rather like canaries.

What impacts the extent of the effect that high CO2 will have on someone?

From https://www.hse.gov.uk/carboncapture/assets/docs/major-hazard-potential-carbon-dioxide.pdf

CO2 is commonly thought of as posing a threat to life through asphyxiation when it displaces the oxygen in air down to dangerously low levels. For CO2 to reduce the oxygen concentration in air down to a level that is immediately dangerous to life, the CO2 concentration would need to be in the order of 50% v/v. Evidence shows, however, that CO2 does create an immediate threat to life at a concentration of only 15% in air due to the toxicological impact it has on the body when inhaled at this concentration.

In humans, CO2 is a normal component of blood gases at low concentrations, however, at high exposure inhalation levels it is lethal. In humans one of the most powerful stimuli known to affect the respiration is CO2 and this results in humans being very sensitive to changes in CO2 concentrations. The inhalation of elevated concentrations of CO2 can increase the acidity of the blood triggering adverse effects on the respiratory, cardiovascular and central nervous systems. Depending on the CO2 concentration inhaled and exposure duration, toxicological symptoms in humans range from headaches (in the order of 3% for 1 hour), increased respiratory and heart rate, dizziness, muscle twitching, confusion, unconsciousness, coma and death (in the order of >15% for 1 minute).

At CO2 concentrations in excess of 50% in air whether a person dies due to the toxicological effect of CO2 inhalation or due to oxygen depletion is not clear and arguably immaterial. In both cases death would be the outcome.

 

[mikem]

Smokers certainly are more immune to the effects.

A data logger at the base of the p90 in that cave shows levels varying from at least 0.8 to 3.3, with higher results at this time of the year (CO2 dissolves in water, so expect lower levels when water high - although you probably don't want to go in then either!):
"CO2 study
It is well known that in the Ardèche the basements can have extraordinary CO2 levels.
Instead of the usual 0.04% found in the ambient air outdoors, in the Ardèche cavities it is common to find rates >1% but even going up to 3% or beyond at rates that the descent forbids us to mention."

Etude de CO2

Le CO2 dans l'Aven de Noël. Le Dioxyde de Carbone en Ardèche dans les grottes.

a-r-s-p-a-n-fr.blogspot.com

 

[mikem]

Previous discussions:

Cuckoo Cleeves Closed

A Wessex party visited Cuckoo Cleeves last night with Nick Williams gas tester to check the reports of bad air in the cave. Our intention was to record the level of oxygen (and by deduction the level of carbon dioxide) at various points in the cave. The tester showed an oxygen level of 20.9% at...

ukcaving.com

Discussion on elevated CO2 levels and how to recognise them

I've started this in a new topic because it was an interesting discussion point but hidden in the Shatter Pot thread (sorry for only chopping out the serious bit of your comments Joel! Readers can follow the quote link below for the original post in full): If you recognise the signs then you...

ukcaving.com

Carbon dioxide - again

Did a trip to Swildons Sump 2 on Saturday incidentally meeting Whitelackington for the first time in the narrowest part of the inclined rift - no escape. I think he recognised me from my avatar - and no there isn't a prize if you meet me although if you are really unlucky you might be asked to...

ukcaving.com

 

[The Old Ruminator]

Mr O Doc rightly highlights the issue of CO2 particularly when digging in a cave. Work has stopped at one deep Mendip cave mainly due to the fact that CO2 levels usually exceed %2 though variable at differant depths. A series of pitches make the issue more probmatical due to the energy required in climbing them. Add a long digging session to that might cause a bigger problem though in truth we really dont know. The odd thing is that the cave was dug for nearly three years down to 100m plus before anyone was aware of the higher levels. Certainly there is a psychological issue at play here. If you are looking for issues you can well imagine that you have found them. In our case ignorance was bliss and any puffiness was put down to our extreme age. Certainly I never got a headache in the 100 plus visits I had there. Now that the issue is known the cave is descended in extreme trepidation and any residual problems blamed on the CO2 whether real or imagined. Our other member referred to above had severe headaches rending him incapable for doing much for a couple of days. These occurred mainly the day after digging rather than during the session so the causal effect cannot be certain. It seems that factors are variable. Energy expended with increased breathing rates. Depths within the cave. Possible age of the caver. Fitness levels perhaps. Most certainly the extent of the exposure in terms of time in the environment. Sometimes the O2 levels were also reduced though by no means hypoxic.

 

[ChrisB]

Smokers certainly are more immune to the effects.

A climber I knew used to say that smokers did really well in the Himalaya, if they stopped smoking when they arrived there, as their lungs were accustomed to low oxygen (and I assume CO2) and had extra capacity. He may have been right, but sadly the downside was his fatal heart attack at Everest Base Camp.

 

[ChrisB]

Is the CO2 produced by cavers breathing and then being heavier than air, with no atmospheric turbulence, settles to the bottom of the cave? If the cave is draughting, or has a good water flow, it would be removed, but on a dig there might be neither.

 

[pwhole]

I remember abseiling to the very bottom of Nettle Pot to see the Red River Series maybe ten years ago, but as soon as we got to the entrance of the passage we knew there was a problem, and decided to leave immediately. That prussik back up was one of the worst ever - I'm a much faster climber now, but the breathlessness was miserable, and knowing my mate was still sat down there in it until I got to the top made it even worse. A meter was sent down the following week and it was about 2.6% as I recall - not extreme, but it did feel a bit like walking through treacle.

I've had similar in Water Icicle, but that's a much easier cave, so not really a problem, and the climb out is straight into fresh air. I was in a very low-oxygen passage once and was fine (I was told it was because I smoke), but we didn't have a CO2 meter, so no idea what the O2 was replaced with, but it didn't feel especially bad, though again, we were just walking in an adit rather than anything strenuous.

 

[Flotsam]

One question which comes to my mind is oxygen levels. Does the oxygen percentage remain the same as normal atmospheric at 20 9%? I guess it will be slightly lower simply because the raised CO2 is part of the mix.

 

[pwhole]

No, it goes down. A few months ago a group of us found a lamb stuck down a hole near Eldon Quarry, so called in DCRO to see if they could extract it. The hole was a fresh and narrow collapse about 7m deep, but on lowering O2 meters down there they were getting readings of around 9% oxygen. As the hole was a fresh collapse, and as it was a very still warm day, I assumed the depleted oxygen must have been down to the lamb breathing it all, in the same way that Neil Moss suffocated in Peak Cavern. Anyway, it was far too dangerous to descend with breathing assistance, but thankfully they were able to extract it safely (and alive) with a noose on a telescopic pole.

Obviously large caves like Nettle with no cavers in it are producing the excess CO2 without assistance, and the O2 level will generally vary in proportion to the CO2. A few months later after our visit it was tested again, and the levels were far more reasonable, so it varies a lot.

 

[Bob Mehew]

The settling topic is a nonsense. If it were true, then the world's atmosphere would have a layer of argon at the bottom followed by a thin layer of CO2 and then oxygen and nitrogen; the order being based on how dense each gas is. If you go back to basics, air comprises of a vast number of molecules whizzing about, hitting each other and surfaces. That movement is a reflection of the temperature of the air. So provided the air is somewhere above absolute zero tempperature, there is a mixing process going on. That is in addition to drafts and thermal currents and so on.

The reason for layers of CO2 in caves (and other situations like half empty brewing vats) is that there is a source of the CO2. Starting with something simple to visualise, the dregs in the bottom of the brewing vat still evolve CO2 and the mixing process takes time. So you get a concentration gradient from very high just above the liquid surface to much lower at the top of the vat to almost background a distance away from the vat. In UK caves, direct decomposing vegetation in cave is uncommon. Rain water has a small amount of CO2 dissolved in it, depending upon the partial pressure of CO2 in air according to the equation:

CO2(gas) + H2O(liquid) = H2CO2(dissolved)​

(I am going to ignore the ionisation process which whilst relevant, is just an added complication.) It is an equilibrium process (so the use of an = sign is slightly misleading) which is set by the amount of CO2 in air. As water starts percolating from the surface down through soil, it picks up more CO2 from the decomposing vegetation in the soil which has a much higher level of CO2 associated with it. That CO2 laden water then percolates through the limestone where it can dissolve some calcium carbonate according to the equation:

CaCO3(solid) + CO2(dissolved) + H2O(liquid) = Ca(HCO3)2(dissolved)​

(again ignoring the ionisation process). This is also an equilibrium process which is set by the amount of dissolved CO2 which in turn is set by the level of CO2 in the air. So the more CO2 dissolved in the water, the more dissolution of CaCO3 takes place. When the water issues out of the limestone into a drafty cave passage, the level of CO2 in the air is such that the chemical equations above reverse and thus calcium carbonate is deposited as formations and some CO2 is released into the air from the water. The draft obviously helps mix the released CO2 into the air so that the slightly higher level of CO2 is not noticeable. In a less drafty or zero draft passage, the mixing takes more time so there can be a bigger concentration gradient. That is the underlying physics as to why a source of CO2 can give an impression of there being a layer.

Of course people breathing in confined non drafting spaces create a CO2 problem which can linger, just like bang fumes. Carbide lights were also a problem in the old days. And coal mines are different in that the seams of coal can release CO2 (as well as other gases like carbon monoxide and methane).

Oh it is often a fallacy to take an oxygen reading and assume the difference between it and 20.9% is CO2. That is based on the assumption that the missing oxygen is converted into CO2. That obviously may not be the case either because CO2 may have been added to the air or because other reactions have occurred to remove oxygen such as rusting.

 

[thehungrytroglobite]

Smokers certainly are more immune to the effects.

A data logger at the base of the p90 in that cave shows levels varying from at least 0.8 to 3.3, with higher results at this time of the year (CO2 dissolves in water, so expect lower levels when water high - although you probably don't want to go in then either!):
"CO2 study
It is well known that in the Ardèche the basements can have extraordinary CO2 levels.
Instead of the usual 0.04% found in the ambient air outdoors, in the Ardèche cavities it is common to find rates >1% but even going up to 3% or beyond at rates that the descent forbids us to mention."

Etude de CO2

Le CO2 dans l'Aven de Noël. Le Dioxyde de Carbone en Ardèche dans les grottes.

a-r-s-p-a-n-fr.blogspot.com

Oh that's really interesting info, thanks!

 

[The Old Ruminator]

The cave situation and geology play an interesting part. My post above refers to Vurley Swallet on Mendip which lies in a large karst basin. The cave was buried under seven metres of loess and until recently did not act as a true swallet. Though epigenic in nature the cave has formed by agressive drip . There are no formations as such as they either do not form or are dissolved away. Rock structures have a dissolved and fluted appearance and small vadose trenches are still forming at depth. I cant see anything paragenic. In fact there is hardly any sediment in the entire cave.

 

[Fulk]

Did you mean, Bob:

Rain water has a small amount of CO2 dissolved in it, depending upon the partial pressure of CO2 in air according to the equation:

CO2(gas) + H2O(liquid) = H2CO3(dissolved)?

My understanding is that the amount of H2CO3 is low, and that most of the CO2 is present as dissolved gas.

 

[Duncan Price]

Did you mean, Bob:

Rain water has a small amount of CO2 dissolved in it, depending upon the partial pressure of CO2 in air according to the equation:

CO2(gas) + H2O(liquid) = H2CO3(dissolved)?

My understanding is that the amount of H2CO3 is low, and that most of the CO2 is present as dissolved gas.

Correct - I frequently use sparkling water in STEM demos and the figure I quote is that 1 litre of water can hold 6 litres of gaseous CO2 (more under pressure). I add universal indicator to demonstrate that it is a weak acid and then degas it under vacuum and watch the pH rise.

Here's my go to table of the body's response to elevated CO2

Tolerance to increased atmospheric CO2 concentration
ppCO2 in inspired air	Expected tolerance for useful activity on continued exposure to elevated CO2
Duration	Major limitation
0.0004	lifetime	normal atmosphere
0.005	lifetime	no detectable limitations
0.01	lifetime
0.015	< 1 month	mild respiratory stimulation
0.02	< 1 month
0.025	< 1 month
0.03	< 1 month	moderate respiratory stimulation
0.035	< 1 week
0.04	< 1 week	moderate respiratory stimulation, exaggerated respiratory response to exercise
0.045	< 8 hours
0.05	< 4 hours	prominent respiratory stimulus, exaggerated respiratory response to exercise
0.055	< 1 hours
0.06	< 0.5 hours	prominent respiratory stimulus, exaggerated respiratory response to exercise, beginnings of mental confusion
0.065	< 0.25 hours
0.07	< 0.1 hours	limitation by dyspnea and mental confusion

Above 0.1 (10%) you are in danger of having a heart attack regardless of any hypoxia.

 

[Bob Mehew]

My understanding is that the amount of H2CO3 is low, and that most of the CO2 is present as dissolved gas.

I was being simplistic by trying to avoid detail which would complicate matters and avoiding getting into discussing equilibria.

Whilst you are correct in your understanding for a solution of CO2 in pure water, the key point is that where there are other compound like calcium carbonate (CaCO3) present, the dissolved CO2 will 'feed' the dissolving of solid CaCO3 to form calcium bicarbonate (Ca(HCO3)2). That dissolving process will continue until an equilibrium is reached. Thus the total amount of CO2 present in the system in one form or another can be much larger than just that due to dissolved CO2.