A time traveler's survival guide. The vertical green bars are the only times in Earth's history with enough oxygen to breathe (hypoxia) and low enough to avoid oxygen toxicity (hyperoxia).
This was a screenshot I took months ago while watching a Geology Hub upload on YT. It was a lightbulb moment for my understanding of mass extinction events, (the largest was 250ma). I've referenced this multiple times, so thought I might share. Perhaps you find it as interesting as I do.
You can absolutely breathe higher partial oxygen pressure unaided for a long time. Hyperoxia isn't all that lethal and definitely not quickly, if you're only visiting, there's no problem.
And if you want to live there, you should be much more worried about all the brand new diseases you don't have immunity to, or the bugs that are bigger than you.
The low oxygen is definitely a problem, especially if you need to run away from the stuff mentioned above.
Also someone like a proper Sherpa sherpa from the Himalayas can function with oxygen that's comparable to 7% sea level oxygen.
And there are towns with elevation so high the oxygen is equivalent to 15% sea level oxygen.
So this chart has pretty narrow limits. Sure, the legend does specify "breathe forever unaided", but someone like a well accustomed sherpa who regularly climbs Mt Everest would have a much wider range than 20-25%
We are merely the space ships of the microbes. Being caught between their war of the worlds would likely be unpleasant. However, I have a feeling we would be the true harbinger of death in that world of far more simple food webs than our present reality.
Our deaths would be miserable upon return, due to the evolutionary life of our passengers having many millions of years to plot our deaths when we return to find a completely different world and the little ones waiting to say hello.
I do not believe the table is intended for individual timescales, like how I tried to make the title more relatable. It is intended on geological timescales of species survival.
IIRC the person on YT also mentioned there were times when there was not enough oxygen to sustain fire, which is another fascinating thought and helps make sense of things like hydrocarbon deposits like oil and coal in some instances.