1. What are the atmospheric conditions (by percentages) that human's can breath on say an alien planet? I know that Earth's atmosphere contains 21% oxygen and 78% nitrogen and 1% other gaseous molecules. I would like to know if we could breath on a planet that has say 64% nitrogen 28% oxygen, 6% carbon dioxide and 2% water vapor and other gaseous molecules. And if so what would an increased amount of oxygen do to us as humans? (Science fiction question really).
What would the increased amount of carbon dioxide do to us as well?

2.

3. I'll try to get you started, but biology is not my best subject...

I have heard, if Oxygen falls below 16%, there would not be enough for humans to get whats needed in a breath. Above our current 21-22% with a quick increase with cause some problems, like hyper-ventilation.
Nitrogen, basically enhances life support systems, allowing growth etc...
CO2, can be handled, according to EPA, in enclosed buildings for instance, up to about 5% 50k parts per million w/o medical problems and water vapor in any fog is well above 2 %.

I would think under your scenario, higher oxygen, a human would be ok, certainly for short periods. The long range problems, could possibly be reduced by a slow or gradual change, via a space suit, much like deep sea diving is re-adjusted by gradually going back to sea level.

Probably not a sound post, but wanted to welcome you here as an occasional poster myself...

4. The biggest factor isn't percentage, but partial pressure. You can breathe 100% oxygen at 3 lbs pressure safely, but 50% oxygen at 1 atm is toxic and will kill you. Conversely, 14% oxygen at one atm will lead to anoxia, but you could get by on 12% oxygen at 1.5 atm .

EDIT: note that oxygen limits actually go from 7% to 53% at 1 atm, as per below)

Some more accurate numbers for breathability of atmospheric gases:

Oxygen would need to fall between 400mm and 50mm of mercury partial pressure. Above 400 and oxygen becomes toxic, below 50 and anoxia sets in.

Carbon dioxide content must be below 5mm partial pressure, after which it becomes toxic.

Partial pressure is what the pressure would be if all the other gases were removed from the mixture. Sea level pressure is 760mm of mercury. Earth normal partial pressure for oxygen is 160mm and for carbon dioxide 0.21mm

So for example, at standard sea-level air pressure oxygen could make up from 7% to 53% oxygen and still be breathable over long periods. (though at 7% I wouldn't be doing any wind-sprints)

At the same pressure carbon dioxide would have to be below .06 percent.

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