Gas toxicity

[Erwin Kodiat]

Hi Doctor,
I'm still in the middle of Divemaster course, and had some question regarding gas toxicity.

1. How CO (Carbon Monoxyde) can be a poison? Is it related to partial pressure?
2. How can be CO2 (Carbon Dioxyde) not forming bubble while Nitrogen does (especially in the hypercapnea condition)? How about the other gas, like Helium?
3. After I dove with EAN 32, I got my Suunto Stinger alert me about the high OLF I had, I unintentionally exceed the maximum depth for EAN 32 (exceed 1.6 a.t.a). Is it true that the OLF is high once you exceed the maximum depth even though only no longer than 1 minute at that depth?

Thanks for the answer.

 

[cpalerm]

I guess I can give you a quick explanation, and leave DrDeco to fill in whatever gaps there might be.

1. Carbon Monoxide is poisonous as it binds to hemoglobin (what transports oxygen and carbon dioxide in the blood), taking the spot where oxygen would normally bind. Bound CO means less oxygen can bind to hemoglobin.

2. The carbon dioxide is a biproduct of respiration, your body produces it; the gas you breath in has no carbon dioxide in it, so the general effect is for the body to get rid of it.
With nitrogen, the gas you breath has a good proportion of it; in the case of EAN 32 it is about 68% of the mix. Under pressure the number of molecules of the gas you breath in increases, meaning that more nitrogen will go into your body and into solution in your blood. As pressure is decreased the nitrogen exchange in the lungs might not be fast enough so that the gas starts to come out of solution, forming the bubbles.

3. Part of the factors in oxygen toxicity is the time you are exposed to high concentrations of oxygen at depth. You could be diving EAN 32 within MOD limits but exceed the recommended time limits and still be at risk for CNS toxicity (albeit this is hard to do). I guess this is one point DrDeco could probably provide a clearer explanation than I...

You might want to check the following book out:
Diving physiology in plain English by Jolie Bookspan (ISBN: 0-930406-13-3)

Hope this helped...

 

[donacheson]

Originally posted by cpalerm


2. The carbon dioxide is a biproduct of respiration, your body produces it; the gas you breath in has no carbon dioxide in it, so the general effect is for the body to get rid of it.

The atmosphere, and hence the compressed air breathed from a tank, has a bit of carbon dioxide (CO2) in it, about 320 parts per million as I recall. The global warming controversy is in large part about the increasing amounts of CO2 in the atmosphere produced by the combustion of fossil fuels (coal, petroleum, natural gas).

 

[DennisW]

Not only is CO poison because it binds to your hemoglobin, it does not release like oxygen and has an accumulative effect. It will not leave the blood until the hemoglobin is replaced through natural means or forced out via compression therapy. Most people that get CO poisoning have to take a chamber ride to rid themselves of the CO in their blood. CO poisoning is extremely dangerous because you cannot smell it and you can become incapacitated before you know anything is wrong.

 

[Erwin Kodiat]

Thanks Cesar, Greybeard and Dennis,
After reading your answer and further reference (PADI Encyclopedia for Recreational Diver), I have several further questions, as follows:

1. So the percentage of CO in the gas we breathe that will poison us. Let say if the gas we breathe contained 1% of CO, the more you inhale, the more the CO bind to the hemoglobin since CO would not easily unbound with hemoglobin. Does the partial pressure affect the posionous degree of CO? Let say we took the contaminated gas to 40 meters, hence the pp for CO became 5 times though the percentage is still the same, does it affect something?
2. I still don't get the answer why CO2 does not form bubble just like Nitrogen does when, let say, we ascend too fast (the pressure gradient too high)? I've heard that Helium will form bubbles as well as Nitrogen. Additionally, the only reason divers use trimix to keep clear thinking (not narcosed).
   [/list=1]
   
   Again thanks for helpful answer and some new knowledge, such as CO2 and global warming effect.
   
   Regards,

 

[cpalerm]

Hi Erwin,
Regarding CO, if the air you are breathing contains it, then under pressure you will inhale more, just as you do with oxygen and nitrogen. It is not so much that the pressure would increase the degree to which CO is poisonous, it just means that more will get into your bloodstream faster. I certainly would not like to have any CO in the air I breath under pressure...

Regarding CO2... The air you breath has a negligible amount of CO2; that is why you can assume it has none when compared to the amount of the other gases present. The exchange of gasses in your lungs depends on the gradients on each side of the tissue.
On the surface the amount of nitrogen dissolved in your blood is roughly the same as that in the air (this is, they are in balance); oxygen in consumed by the body, so the tendency is to have a slight imbalance that brings in oxygen into the blood; the same goes for CO2 in the other direction.
Under pressure, the amount of nitrogen and oxygen you breath is greater, so now the difference will push nitrogen into the blood; the deeper you go, the more will dissolve. The tendency is to get the amount of the gas on both sides of the tissue to balance. When you go up, the pressure diminishes and now you have a greater concentration of nitrogen in your blood than in the air you breath; still, it takes time to get this excess nitrogen to vent off. The greater the difference, the faster the off-venting. That is why decompression divers change the mixes they breath at different deco stops, including pure oxygen at shallow stops.
The gradiente for the exchange of CO2 remains in the same direction throughout your dive (assuming an open scuba circuit; rebreathers are a different story); every time you breath CO2 is more concentrated in your blood, so it vents-off all through your dive, so it never gets to the point that there is too much CO2 dissolved in the blood to generate bubbles.

Hope this helps...

 

[TwoBitTxn]

Regarding CO, if the air you are breathing contains it, then under pressure you will inhale more, just as you do with oxygen and nitrogen. It is not so much that the pressure would increase the degree to which CO is poisonous, it just means that more will get into your bloodstream faster. I certainly would not like to have any CO in the air I breath under pressure...

Umm... no.

Under pressure you don't inhale more. The issue is partial pressure. The same as with Nitrogen and Oxygen. CO is generally measured in ppm or parts per million. 10000 ppm is 1%. 10ppm is an accepted maximum level of CO in SCUBA air. Cigarette contain about 30-60 ppm CO. Since CO has an affinity (binds easier) of 200 times more than O2 then a level of 30-60ppm would bind about 5-10% of your hemeoglobin. So from a partial pressure point....

60ppm at the surface =10% bound Hb
60ppm at 30ft =120ppm at surface =20% bound Hb
60ppm at 60ft=180ppm at surface=30%bound Hb

and so on.

Source: Scuba Diving Explained by Lawrence Martin

TwoBit

 

[The Iceni]

May I jump in?

1) Forgive me TwoBit, but you appear be confusing the issue.

For any given gaseous fraction the greater the ambient pressure the more molecules of that gas are available in a given volume.

The effects of the gas are not simply determined by its fraction but by the number of molecules delivered to the tissues. Thus if the CO fraction is 1 ppM it exerts a partial pressure of 0.000001 bar at the surface (the same figure written differently). If, for the sake of this argument a litre of surface air contained a million molecules (It doesn't), at 10 metres this same mix would contain two million molecules of which two are CO and so on.

In my humble opinion cpalerm is right.

2) As for CO2 it is just as likely to cause bubbles and DCI if its concentration exceeds the tissues abilities to dissolve it. It does not present a risk of DCI simply because its partial pressure will never rise to a level at which sufficient is dissolved to even approach its surface saturation. Except when contaminated mix is breathed, the only source of CO2 is from the body's metabolism and it is immediately excreted very efficiently from the lungs.

For this reason, regardless of the depth the pp CO2 in the blood remains around or below 0.1 bar. (If it rises above this level it becomes toxic in its own right.)

If pollution does give us a fCo2 of 320 ppM it is still less than 1% or the equilvalent of 0.04 bar at 30 metres.

Compare this with a pp N2 in air of 0.79 bar on the surface, 1.58 bar at 10 mters, 2.37 bar at 20 metres and 3.16 bar at 30 metres.

Or a helium mix of 50% which gives a pp He at these depths of 1 bar, 1.5 bar or 2 bar at 30 metres.

What is OLF?

I hope this helps, Erwin

 

[Erwin Kodiat]

OK then, Dr Paul Thomas, your info really help me to understand some topics. OLF is the measurement of Oxygen Exposure in Suunto Stinger dive computer.

 

[madmole]

I would also ask, what you where doing on Nitrox on a dive when you obviously dont understand Oxygen toxicity or the very very real threat of dying from it if you exceed the OLF for even a short period

If you are not trained on it, dont use it. If you have trained, I suggest you do the course again.

You cannot treat Nitrox like AIR with a depth limit. You should be planning and recording your CNS toxicity and pulmonary Tox for every dive

Also you shouldn't use a dive computer unless you know how it works and what all the bits mean

Sorry to come down on you, but its your life your risking without having the full knowledge of that risk