David Shaw wrote:

> but what would be the physiological effects, if any,
> of the opposite, moving to a thicker atmosphere than
> the one you're adapted to?

Think about the problems divers face at depth.

One standard atmosphere = 760mmHg or 100kPa; as a rule of thumb, each 10m of depth on Earth adds an atmosphere of pressure.

- increased work of breathing: About 5% of metabolic rate is used to breathe; increased ventilation scales with increased metabolic rate
(e.g. exercise, critical illness) to ~10x basal. Denser gas mixtures contribute to flow resistance; doubling gas density doubles work of breathing.

- oxygen toxicity: lung inflammation with partial pressures greater than 400mmHg/52kPa, then seizures
become increasingly likely with levels above 1040mmHg/137kPa

- "inert gas" toxicity: nitrogen and noble gases have anaesthetic effects above certain pressures:

Gas: Pressure (mmHg/kPa)
nitrogen: 2,300/302
argon: 1,220/160
neon: 3,900/513
krypton: 350/46
xenon: 160/21
helium: >60,000/>~7,900
hydrogen: ??

- gas toxicity
Carbon dioxide has respiratory, cardiovascular and ultimately anaesthetic action as concentration rises.
Noticeable effects >7mmHg/0.9kPa

Other trace constituents: consult MSDS entries on the web for threshold exposure values and effects.

- high pressure nervous syndrome (HPNS): CNS irritability with tremor, muscle jerking (myoclonus), nausea, dizziness, visual disturbances,
impaired concentration. Threshold 16 atmospheres; effects in all >30 atmospheres.
Interestingly can be limited with nitrogen and hydrogen (nitrogen narcosis counteracts HPNS to a degree).

- heat loss: depends on the heat capacity of the breathing gas mixture. Helium and hydrogen are notorious in this regard.

Rob O'Connor