What percentages of oxygen to carbon dioxide ratio can a human being inhale without losing conciousness?

Question

I know that before the industrial revolution began the Earth's atmosphere held close to 23% oxygen, and that humans start to get dizzy in rooms holding only 15 to 17 % oxygen, but what is the mandatory % of oxygen for humans to breath and remain concious? Also, what are the current oxygen ratios to the Earth's atmosphere at the present date of August 1st, 2007?

Answer 1

The air we inhale is roughly 78% nitrogen, 21% oxygen, 0.96% argon and 0.04% carbon dioxide, helium, water, and other gases. (% by volume)

The permanent gases in air we exhale are roughly 78% nitrogen, 15% to 18% oxygen, 4% to 5% carbon dioxide and 0.96% argon (% by volume). Additionaly vapours and trace gases are present: 5% water vapour, several parts per million (ppm) of hydrogen and carbon monoxide, 1 part per million (ppm) of ammonia and less than 1 ppm of acetone, methanol, ethanol and other volatile organic compounds.

Not all of the oxygen breathed in is replaced by carbon dioxide; around 16% of what we breathe out is still oxygen. The exact amount of exhaled oxygen and carbon dioxide varies according to the fitness, energy expenditure and diet of that particular person. Also our reliance on this relatively small amount of oxygen can cause overactivity or euphoria in pure or oxygen rich environments.

% Of oxygen and Breathing :
21.5 % Breathing easiest
19.5 % Minimum required by law
17 % Breathing faster & deeper
15 % Dizziness, buzzing noise, rapid pulse, headache, blurred vision
9 % Unconsciousness
6% Arrest...

Answer 2

You don't "know" of what you speak; the O2 level has been at 21 percent for a long,long time. You are correct that 15 to 17% oxygen levels can cause distress, but this is equivalent to the oxygen available in air at a 1 mile elevation (21% of 0.75 the mole/L of sea-level air). Using the elevation to percent oxygen analog, unless people are acclaimated, levels of 18,000 feet and above not only cause distress, but can cause adverse health effects (about 0.4 x sea-level pressure), while people with very good conditioning can function for a few hours as high up as the top of Mount Everest (about 0.2 x sea-level pressure).

While your initial query mentioned CO2, the rest of your presentation seems to have ignored it.
The CO2 levels in "clean" air are about 360 parts per million.