@JamesShewey Increasing the atmospheric pressure is not the same thing as increasing concentration. Pressure changes do not affect the contents of a closed system, and Earth's atmosphere can be more or less viewed as a closed system.

From what I understand about biology, increased pressure, but constant concentrations, may be harmful long term, but may also not be if there are enough antioxidants in the body to detoxify the reactive Oxygen species.

I may be wrong, and I am certainly not a doctor, but I do have a personal interest in this and I am involved in the medicine industry, so have heard many lectures on this.

That's true, but again, the best reference for this is diving. On the page for Nitrogen Narcosis they state "The cause of narcosis is related to the increased solubility of gases in body tissues, as a result of the elevated pressures at depth (Henry's law)"

So my understanding of that is that the increased pressure changes the amount of gasses in the bloodstream - even when it does not change the concentration in the atmosphere.

I believe this is know as the Minimum Alveolar Concentration which is a function of Henry's Law.

They also note "Carbon dioxide has a high narcotic potential and also causes increased blood flow to the brain, increasing the effects of other gases."

In deep sea diving they actually use a special type of oxygen mix called trimix or heliox which does not contain carbon dioxide. I presume this is because it is toxic. C02 of course goes out through the breathing regulator in diving.

Which does bring up an interesting point: with the presence of carbon dioxide, it may increase the effect of Nitrogen Narcosis thereby lowing the pressure at which the effects are noticed. I have no idea how much that would affect an individual though.

Maybe we could set up an experiment if you will volunteer to be the test subject? ;)

@JamesShewey Yes, higher pressure will produce more dissolved gasses, assuming the solvent is not saturated. The bloodstream is a closed system, so the mechanism is not that simple, but I do believe increased pressure increases the total amount of gasses in the blood.

@JamesShewey I'm not sure diving is the best data for this hypothetical. First, it's short term. Dives only last an hour or two at the most.

Second, they use air that is not representative of earth's atmosphere.

Third, the pressure on their bodies varies as they go up an down in the sea, while the pressure of the air they are breathing remains constant, and likely not commensurate with the pressure of the sea level at which they are diving.

This hypothetical atmosphere is nothing like that. All three of those scenarios can and do cause complications that may not exist in a steady atmosphere.

@JamesShewey I think we'll start with rats. I'm sure there must be a study on the long term effects of plain air, increased pressure, long-term exposure via hyperbarics.

Just put the rats in a chamber with plenty of food, space, and water, then double the pressure and measure their vitals over the following weeks. If it's really done right, there will be a control at 1 atm, then study groups at three or four points between 1 and 2 atm, with a reasonable timeframe for increasing the pressure.

:)

I posted a question about this to the Health Stack Exchange:

I will be interested to see what responses we get.