When diving underwater, oxygen is typically considered toxic at a partial pressure of 1.4. However, hyperbaric chamber treatment tables regularly require taking patients up to 60fsw on 100% O2, which translates to a partial pressure of 2.81. Why is the same amount of oxygen more toxic underwater than in the chamber, if the pressure exerted on both the body and the gas are the same in both scenarios? What variable when we’re “dry” allows us to tolerate more than double the amount of oxygen than when we’re “wet”?
In: Biology
Oxygen under water is more toxic than in a very large room for several reasons. The water in the environment affects how the body absorbs and uses oxygen. In a hyperbaric chamber, air exchange is most effective under dry conditions. Increased downstream pressure can increase the production of reactive oxygen species (ROS), resulting in significant toxicity. In addition, divers are exposed to high levels of oxygen for extended periods of time, increasing the risk of toxicity.
I am not an expert and don’t have an answer, but would like to propose that this is not a result of the oxygen, but of the Co2 we produce. That likely determines how effective we are at processing it.
When diving, you are on your own. Well maybe with a buddy. If you start convulsing, you may well drown.
In a chamber, you are monitored by medical professionals. Also, if you are in a chamber, you are there for a reason. The risk of oxygen toxicity is lower than the risk of staying outside.
You will definitely run into oxygen toxicity/seizure territory in a chamber in the low-mid 2atm range. Safety guidelines basically disallow you being above 2.5atm on pure oxygen for longer than 30 minutes. Generally if you’re getting a treatment at those pressures, you will be given “oxygen breaks” where the gas is occasionally switched to normal air instead of oxygen. There’s a whole bunch of science, partial pressures, exchange rates under pressures with different gas mixtures, etc. that I’ve forgotten since taking a hyperbaric operator course, but in general the answer is it’s not really that different, you just have a lot more control of a chamber than under water.
There might also be some confusion on terminology, as 33′ of water is 1 ATM of water but it is 2 ATA because of the 1 atmosphere of air above it. I generally calculated a dive with atm, but the chamber I ran also showed in PSI which I felt was a bit easier to read and adjust for.
When you’re in the hyperbaric oxygen tank, it’s for a limited time and with purpose. Typically we use it to enable healing of some wounds or for any type of poisoning by a chemical that binds to your hemoglobin better than oxygen (CO, NO, CO2, actually, almost every gas in existence binds to your hemoglobin better than O2). O2 treats these poisonings by basically forcing its way in via overwhelming numbers and attrition.
Oxygen for extended time at high concentrations is toxic though, regardless of environment. Too much O2 forces the N2 out which we also want in our lungs. N2 kind of acts as a lubricant for the air sacs.
When you’re diving , you’re moving around which increases the demand for more oxygen but when you’re relaxing in a chamber the oxygen demands are less.
I think you have a units error here somewhere. 60fsw is about 1.8 atm. If it’s pure oxygen then the oxygen partial pressure is also 1.8 atm, not 2.8. Which is actually pretty close to the 1.4 used for the dive tables. The Navy allows 1.7 for four hours.
But also think about *why* you’re treating in the hyperbaric chamber…its not that you want 100% O2, it’s that you want 0% of whatever the thing it is we’re trying to get out of you (nitrogen if it’s a diver). And the only gas we can give you at 100% without killing you is oxygen, so that’s what we do.
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