Whaddya know, I’ve invented a working time machine. I’d like to go back to the Carboniferous era and see the cool plants.
The thing is that at times during that era, the oxygen content of the atmosphere reached 35%, quite a bit higher than today’s 20%.
I’m smart enough not to light a fire in such an enthusiastically combustible place. But would I be okay breathing that air or do I need a space suit? Could I just take shallow breaths?
Pure oxygen is breathed in medicine and aerospace.
Anything over 23.5 percent will make your body do weird things. More info here.
Oxygen toxicity is a concern for divers. When they are at depth, the gas they are breathing is at a pressure above atmospheric. Recreational scuba divers can breathe regular air at depths of ~100 feet (local pressure = 4X sea level) for limited time without toxic effects from the O2 (decompression, which is related to nitrogen dissolution in the blood, is a concern). To dive much deeper or for much longer, divers need different mixes of gases with different amounts of nitrogen and oxygen to limit the partial pressure of those gases (while at depth) to safe levels.
According to the Wikipedia page on O2 toxicity, “toxicity does not occur below 0.3 bar” partial pressure of O2. This implies that there *are[/i ]toxic effects when the O2 PP is above 0.3 bar. You describe an atmosphere with an O2 PP of 0.35 bar, so presumably you might expect to develop problems if you were exposed for a long time.
Partial pressure is what’s important. Astronauts did breathe pure oxygen for a couple of weeks at a time in the early days of the space program, but the atmosphere in the capsules were at ~5 psi, so the partial pressure was about the same as at sea level.
(If you do go back to the Carboniferous, catch a sea scorpion for me. I always wondered if they are edible and, if so, what they tasted like.)
Depending on your gut flora, your farts (and possibly burps) will contain flammable gases like methane and hydrogen. Cite : Investigation of normal flatus production in healthy volunteers. | Gut
Methane itself is in the 10% range which is not good when you look at Lower flammability Level (LEL) in a 35% oxygen environment. Cite : Flammability diagram - Wikipedia
You may end up being a flame exhaling or flame farting being.
they will taste like chicken when chicken evolves
Was air pressure greater during the Carboniferous?
No, but PP is the airpressure multiplied by the O2 content. Since air is now about 0% O2, the PP of o2 is 0.2 bar. If you doubled the 02 content at sea level the PP rises to 0.4 bar even though the air pressure is the same.
Reasonably sure RickJay meant air is 20% O2 in the above post.
ISTM that if you double the total mass of O2 in the atmosphere (from its present quantity) while keeping the total mass of other gases constant, you will increase the PP of O2 by two means:
#1: you will increase the total atmospheric pressure to about 120% of its present pressure.
#2: you will increase the fraction of total atmospheric pressure that is due to O2.
So instead of an O2 PP of 0.4 bar, you’d end up with an O2 PP of 0.4*1.2 = 0.48 bar.
This makes me wonder whether total atmospheric pressure was in fact higher during the carboniferous period referred to by the OP.
Except that if you doubled the oxygen, it wouldn’t be 40%.
Are you saying we may have lowered the bar? 
Indeed so, doubling the mass of oxygen while holding the other gases constant would result in 0.4/1.2 or 33%.
RickJay has the partial pressure correct though. They’re a bit different from % of mass. We’re doubling the mass of oxygen while holding atmospheric volume the same. From the ideal gas law, the measured pressure of oxygen doubles since we’re holding the volume the same, which means the partial pressure of oxygen also doubles.
So doubling the O2 mass results in O2 PP of 0.4 while % O2 is 33%.
The example of the Apollo was given above. At launch, the cabin was held at 5 psi with a 60/40 O2/N2 mix. 60% of 5 psi is 3psi, which is the same as 20% of 15psi (atmospheric). It did go up to 100% O2 after they hit orbit, and this was a cause of some concern with too much O2, but the missions were of limited duration, and the partial pressure was nowhere close to pure O2 at sea level pressure.
None of this is an issue for the ISS, which is held at 14.7 psi (sea level) with appropriate O2 PP.
Missed the edit windows, but relating this back to the OP, assuming the total atmospheric pressure was the same as today but with 35% oxygen content, that’s an O2 PP of 0.35, which should still be ok for some time.
You can dive with (relative) safety at 1 bar of O2 PP for some time, so 0.35 bar shouldn’t pose a major issue for several hours of sightseeing. NOAA has a published document on standards including a table with 35% oxygen (at various pressures/O2 PP), so that’s a starting point. Note that the partial pressures all start about 0.5, so clearly humans do this with some regularity.
But turning this around, if overall atmospheric pressure was higher and still 35% oxygen, the O2 PP can be higher than 0.35 atm. How much higher the pressure will matter. But even with a doubling of atmospheric pressure, it should be relatively safe to walk around a bit.
Oxygen therapy side effects: oxygen therapy side effects at DuckDuckGo
Oxygen is not very nice. It tends to oxidize things.
Do we know how atmospheric pressure has varied throughout geological history? Is there a way to even figure that out?
According to Larry Niven, if you’re adapted to an atmosphere full of modern toxins, you’ll pass out when you leave your time-warp portal. Take along a tank of smog for safety.
According to White Wolf’s Blood Dimmed Tides, OT whilst diving occurs in three stages
#1 cramping and pain
#2 more pain and lungs filling with fluid
#3 alvioli in the lungs bursting and folks basically drowning in their own blood.
While most of the book is fiction, some of it is fact. I wonder where the above lies.
Here are facts on O2 toxicity for divers at 90 feet:
So there may well have been fire-breathing dragons!