Atmospheric conditions

You might want to check your math there for a moment. Right now it's not making so much sense with your question.

(80 + 25 = 105)

The answer is ... yes we could still survive.

thothguard51 said:

The answer is ... yes we could still survive.

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Err... do you have any facts to check for that?

http://en.wikipedia.org/wiki/Oxygen - see 'toxicity' and 'Combustion and other hazards'

Also: http://en.wikipedia.org/wiki/Oxygen_toxicity

The real answer is 'it depends'

Also - http://tvtropes.org/pmwiki/pmwiki.php/Main/DidNotDoTheResearch

http://en.wikipedia.org/wiki/Oxygen

You can survive on a variety of mixes - those figures are overall numbers; they change with altitude for a start. Divers use weird mixes because the pressure affects how they absorb it, and then there's the obvious use of oxygen-enriched air in medicine and stuff like jet fighters. Really high concentrations can lead to stuff like oxygen toxicity, but a boost of a few percent should be fine. The body would tune the lung function and red blood cell count to get the right amount, so you'd feel weak for a bit if you switched back to normal, like altitude sickness.

You could survive just fine in terms of breathing. Infact you can survive on a higher percentage of oxygen than that with no ill effects. Although too high a percentage can have negative effects as mentioned above. And some people will have lower tolerance than others, which will all need to be taken into consideration.

Calliopenjo said:

What if, we lowered the nitrogen content to 75% and upped the oxygen content to 25%? Would we still be able to breathe without special equipment?

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Yes, this is perfectly fine. In fact, Earth's oxygen level used to be 30-35% about 300 million years ago. It was the higher oxygen levels which allowed insects like dragonflies to grow so large (so, if you're world-building a planet with higher oxygen, keep in mind that it will probably have really big bugs).

Wikipedia article that mentions this fact (scroll down to the "terrestrial invertebrates" section). I know it's Wiki, but I learned about this is school, so I'm pretty sure it's true.

Higher concentrations of O2 are used in medicine, mountain climbing, etc. all the time.

Calliopenjo said:

What if, we lowered the nitrogen content to 75% and upped the oxygen content to 25%? Would we still be able to breathe without special equipment?

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Absolutely. Oxygen toxicity is only really a factor with increased PRESSURE.

And even that isn't too much of a problem - for example even with 50% oxygen, 2 atmospheres of pressure isn't a huge problem.

Here's a guide:

http://en.wikipedia.org/wiki/Maximum_operating_depth

'10 meters' of depth is just '2 atmospheres'.

Good luck !

Mac

However, increased atmospheric oxygen might lead to a rise in long-term side effects.