How much oxygen do we need to breathe?

TonyLoco · 2007-07-20 11:51:56

I understand that on Earth we have an atmosphere that is just 21% Oxygen.

And that terraforming Mars would probably begin by introducing algae and later plants to build up oxygen levels in the atmosphere.

But at what point will this atmosphere become breathable? Is just 5% oxygen sufficient, or does it need to be similar to Earth (21%)?

I read somewhere that deep divers sometimes use a gas mix that is just 16% oxygen, but that may be due to different pressures at depth making the oxygen % rise???? Not sure.......

Is there a simple tipping point, or would a low level, say 8% allow people to at least survive in the atmosphere for a certain amount of time?

RobertDyck · 2007-07-20 13:02:44

It isn't a matter of pressure or percentage, it's a matter of partial pressure. The Earth has 14.69 psi or 1013.25 millibar or 101.325 kiloPascal pressure on average at sea level. Barometric pressure varies with weather. Oxygen is 20.946% so that percentage of 14.69 psi is 3.0769674 psi. Or 212.235 mbar or 21.2236 kPa. You can reduce the amount of nitrogen and other gasses, but have to retain oxygen partial pressure to breathe. NASA used 3.2 or 3.3 psi pure oxygen in Apollo spacesuits, that gave 10% margin in case of leaks. However, Air Force studies show you don't need that much oxygen. If you spend weeks going through high altitude training, you can breathe 2.0 psi oxygen in standard or slightly reduced pressure just fine. High altitude training stimulates the body to grow extra red blood cells. However, you can't tollerate that little oxygen if pressure is extremely low. A strong test pilot after acclimation can breathe 2.5 psi pure oxygen indefinately, or 2.0 psi pure oxygen for up to 30 minutes before passing out. That 2.5 psi is 172.37 mbar, but it's only accurate to 2 significant figures so I round off to 170 mbar.

By the way, deep divers use 3.0 psi partial pressure oxygen, but mix in other gasses to balance the pressure at depth. If you breathe too much partial pressure of nitrogen you can get nitrogen narcosis; that's why they use either argon or helium. For the deepest dives they mix all 4 gasses: O2, N2, Ar, He.

The debate is what other gasses do we need to mix in for a breathable atmosphere. Too much CO2 is toxic; tollerable partial pressure is well documented. Can we do with little nitrogen? How much other gasses and where do we get them from?

More pressure is better, a very low pressure causes lung tissue to dry out. At 170 mbar you will require quite humid breathing air to prevent damage to lung tissue from dehydration. You can do that easily with a breathing mask, but it's more difficult in a cabin or the surface of a planet. Apollo and Skylab used 5.0 psi total pressure with 60% oxygen / 40% nitrogen. That pressure was fine, they could live there indefinately without danger of lungs drying. Notice 60% of 5.0 psi is 3.0% psi, the partial pressure of oxygen on Earth at Cape Canaveral. They didn't require any high altitude training. High altitude training takes 6 weeks to 3 months, the trip from Earth to Mars takes 6 months so they have plenty of time.

Terraforming does come back to the question of where we get nitrogen. There appears to be plenty of CO2, and if we convert that CO2 into oxygen we will have more than enough oxygen to breathe. Recent results from Mars Express showed there's plenty of water frozen as underground ice. The remaining question is whether there are substantial nitrate deposits in the soil anywhere. We haven't found any yet.

The good news is for a partially terraformed Mars. An atmosphere above 170 mbar CO2 will permit settlers to go outside without a pressure suit. They will need a breathing mask and tank of oxygen, that means warm winter clothing and scuba gear or a rebreather. On a warm summer day near the equator you could go outside just with shirt sleves and an oxygen mask with a little bottle of oxygen.

SpaceNut · 2007-07-20 13:21:12

While these are not humans they are a step in the understanding. Martian Air Breathing Mice

TonyLoco · 2007-07-20 14:05:45

Interesting, so you are saying that we need nitrogen in the air aswell as Oxygen to survive? Or that we need to have much lower levels of Carbon Dioxide in the atmosphere? Or both? If, lets say we could transform the Martian atmosphere to a 50/50 oxygen/Carbon Dioxide mix could we survive?

RobertDyck · 2007-07-20 16:54:42

Uh, no. We could survive in a pure oxygen atmosphere at 170 mbar pressure, but you would have to be careful to breathe humid air or use a wet cloth over your face or something.

Actually gasses have other purposes. Ultraviolet light on Mars is dangerous. UV will convert O2 into oxygen, but it also breaks ozone into O2. That is 3 O2 -> 2 O3, and back again 2 O3 -> 3 O2. As long as you don't do something stupid to destroy ozone, like releasing CFCs, ozone will spontaneously form from oxygen. If you make the partial pressure of oxygen the same as on Earth, I expect the O2:O3 equilibrium will cause UV intensity at the surface to be the same as Earth.

The other factor is X-rays. Nitrogen absorbs medium X-rays, but I don't know how much you need. There might be enough nitrogen on Mars right now to block X-rays; I just don't know. But you do need some.

Thick atmosphere will block heavy ion radiation. In fact, the atmosphere of Mars is so thick that less than 1% of heavy ion radiation will reach the surface below the datum. That's the nastiest kind of radiation because shielding like sand bags filled with Mars regolith will cause radiation particles to split, creating smaller/slower particles but more impacts with living cells. The radiation that does get through the atmosphere can easily be blocked by sand bags. A thick atmosphere would extend this effect further, only proton radiation would get through.

But as others have pointed out, nitrogen fixing microorganisms require nitrogen gas to work with. If you want to grow protein, plants need nitrogen. That means either nitrogen fixing microorganisms such as the rhyzobium bacteria or cyanobacteria, or you need nitrate fertilizer.

CO2 on Earth, according to Wikipedia:
"A person's exhaled breath is approximately 4.5% carbon dioxide. It is dangerous when inhaled in high concentrations (greater than 5% by volume, or 50,000 ppm). The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an eight-hour work day is 0.5% (5,000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less."
That is for 1013.25 mbar pressure; using partial pressure as the model expect those numbers to double if you half the pressure. Or quadruple if pressure is one quarter, etc. So let's see, converting to PP (Partial Pressure):
0.5% × 1013.25mbar = 5.06625mbar
round to 1 significant figure you get 5mbar PP CO2.
Add that to 170mbar O2 and you get 175mbar total pressure.

According to the book "Terraforming: Engineering Planetary Environments" by Martyn J. Fogg, page 271, table 6.1 the surface pressure of CO2 that will be released by warming Mars has been calculated. He quotes sublimating polar caps as <a few mbar, current atmosphere as ~7 mbar, and adsorbed in regolith as either <280 or <190 mbar. His references are Fanale et al for the 280 mbar figure, or Zent et al. for the 190 mbar figure. Either way the surface pressure on Mars will be between 200 and 290 mbar. That's good news for walking around without a pressure suit. That's bad news for creating oxygen. Remember the ~7 mbar of current atmosphere is 95.32% CO2, so this thick atmosphere will be almost entirely CO2. How do you get the PP CO2 down to 5mbar? Coverting to O2 would produce a breathable atmosphere, but that's a lot of CO2. It'll take time for plants to do that.

RobS · 2007-07-20 23:08:02

I suspect lower partial pressure of oxygen is possible. The Bolivian altiplano is a high plateau at 12,000 to 14,000 feet. LaPaz, a city of 1.25 million people, is at 12,000. According to Stephen Dole, Habitable Planets for Man, p. 15, the town of Aucanquilcha, Chile, is at 17,500 feet above sea level and the men ascend to 19,000 feet every day to work in a mine there (no doubt hard physical labor, too; this book first appeared in 1964) and at that altitude they're getting an oxygen partial pressure of 68 millimeters of Mercury. That's 68/760 =9% of normal atmospheric pressure, or 1.3 pounds per square inch. He quotes a mountain climber who says people can adjust to and live indefinitely at 23,000 feet, which has 53 millimeters of partial Oxygen pressure, equal to a bit more than 1 pound per square inch.

NASA would never be willing to go that low, but Martian settlers might decide to reduce their operating pressure as low as practical because it results in thinner domes and lighter pressurized structures.

It might be reasonable eventually to build Martian settlements with 1.5 psi of oxygen and another 0.5 to 1.5 psi of nitrogen. You'd want some extra pressure because (1) humidity would add about 0.5 psi, and (2) if someone needs an oxygen mask, you have to replace some other gas with oxygen.

-- RobS

RobertDyck · 2007-07-21 00:29:40

According to this calculator, 19,000 feet has 48.546 kPa pressure. That's 47.9% of the pressure at sea level.
http://hurri.kean.edu/~yoh/calculations … tdAtm.html

23,000 feet works out to 40.999 kPa pressure; which is 40.5% of sea level.

My understanding is base camp on Mount Everest has 1/2 an atmosphere pressure. It takes climbers several weeks to acclimate. They have to walk up carrying a heavy pack, then walk down. Then walk up and trudge around for a while, and walk down. Each time stay a little longer until they can stay at base camp. They sleep, too dead tired to do anything the first day. Then they walk around, then they climb just for exercise, then they start making climbs higher up the mountain carrying a heavy pack. By the time they're done, living at base camp feels like normal. That's called high altitude training.

Once acclimated, they make a dash up to the summit, then dash back down. The summit has only 1/3 an atmosphere pressure. You can't acclimate to that, in fact it's considered safety gear to carry a bottle of oxygen. If you get too tired coming down, use the oxygen; but only briefly. Make sure you dash down to base camp quickly.

These stories of Mount Everest were what I quoted before another member found the Air Force study. It had a very professional science paper. Nothing like a clinical study to prove numbers. Living at 19,000 feet is documented; living indefinitely at 23,000 feet is supposition. But I do agree with the conclusion that Mars settlers may be able to drop O2 PP to 1.5 psi and N2 to another 1.5 psi. I don't know about dropping N2 to 0.5 psi though, the total pressure is rather low. Remember the Air Force study finding that in extremely low pressure humans can't tolerate O2 PP as low.

X · 2007-07-24 18:04:41

Large amounts of atmospheric nitrogen also acts to prevent fires, but there are other gases that could be substituted for that purpose I think.

noosfractal · 2007-07-25 00:36:40

As a follow-on to the post I made about maintaining a Lunar atmosphere, I did the same calculations for Mars. But quite interestingly, I learned that having a high proportion of CO2 is a great way to cool your exobase. Both Venus and Mars have exobase temperatures ~200K, whereas Earth's averages ~1500K because the high proportion of oxygen is heated by XUV radiation. However, even Earth's exobase temperature has been cooling (~40K per century) because of the increased atmospheric concentration of CO2.

I don't know how to work out the exobase temperature for Mars given 170 mbars of oxygen, so I'll just give the loss rates and power requirements for several temperatures ...

exobase temp. (K) --- oxygen loss rate (tonnes/yr) --- Replace Power

1500 --- 4100 --- 23 MW
1300 --- 430 --- 2.4 MW
1000 --- 3.5 --- 14 kW
900 --- 0.2 --- 1.2 kW
700 --- ~1E-4 --- ~1 W
500 --- ~1E-6 --- ~1E-6 W

I'm using 5 kWh/kg to calculate the power required to replace the lost oxygen (electrolysis of H2O). The interesting thing is that the 500 K exobase temperature isn't entirely unreasonable for a Martian atmosphere that is, say, 170 mbars oxygen and 170 mbars CO2. Even if the exobase temperature rises to 1000 K, 2.4 MW is entirely reasonable as a power expenditure to maintain your hard won atmosphere.

The high CO2 proportion exobase cooling effect may also work for Luna. Maintaining 170 mbars at an exobase temperature of 300 K requires just 9.4 MW.

*** EDIT

Updated the figures. The previous figures were way too conservative.

Midoshi · 2007-08-01 19:04:06

The ability of human beings to breathe and operate effectively in a given atmosphere depends primarily on how much oxygen reaches the alveola in the lungs. When you breathe in air from the atmosphere it is warmed and becomes saturated with water vapor from your internal tissues. Since the pressure in your lungs is (normally) the same as the pressure outside your body, water vapor saturation displaces some of the oxygen in the air you breathed in. The effect of displacement by carbon dioxide diffusing from the bloodstream must also be considered. Quantitatively, we have the Alveolar Gas Equation:

pAO2 = fO2*(pAtm - pH2O) - paCO2/R

pAO2 is the alveolar oxygen pressure
fO2 is the fraction of oxygen in the atmosphere
pAtm is the atmospheric pressure
pH20 is the water vapor pressure which at 37°C (body temperature) is 47 mm Hg (63 mbar)
paCO2 is the arterial pressure of carbon dioxide, which is usually 40 mm Hg (53 mbar) at sea level (it decreases somewhat with altitude)
R is the "respiratory quotient" which is 0.8 in normal air at sea level, but is 1.0 when on 100% O2 and can be over 1.0 under unusual circumstances. It is a metabolic parameter and equal to (CO2 released)/(O2 absorbed).

The effects of low pressure on human respiration have been studied most in the context of high altitudes. Human beings have lived at 5,100 m (16,730 ft) in a permanent settlement, and they have great difficulty acclimating above 5,500 m (18,040 ft). Above this, the human body begins to experience degradation rather than acclimation. There is documented evidence of people living at 5,950 m (19,520 ft) for 2 years (West, 2002). Above 7,000-8,000 m (25,000-26,000 ft) the human body cannot sleep or carry out digestion and many bodily functions shut down. This is the Death Zone, where acclimation is impossible.

The next paragraph is a bit technical, and for those who want to play with data and numbers themselves.
As noted above, paCO2 can decrease as one acclimatizes to higher altitudes. Some numbers can be found here in Table 3. Although the 5th column says "alveolar CO2", which would be the CO2 pressure in the lungs (paCO2/R), the numbers don't quite work for low altitudes. All the other columns seem to be correct. I am inclined to suspect that the 5th column is actually reporting the "arterial CO2" (paCO2), because then the discrepancy can be taken into account by the metabolic parameter R, which would then be ~0.86 at sea level and increase towards unity as you ascended. R would then exceed unity after entering the Death Zone, which makes some sense since normal metabolic processes are disrupted there. I only mention this if anyone wants to play with the data themselves. All that really matters for our purposes are the pAO2 values in the table, which are correct.

Here are measured and calculated pAO2 for some representative situations:

0 ft (sea level):
Measured pAO2 = 137 mbar

10,000 ft (about the height of Leadville, Colorado):
Measured pAO2 = 81 mbar

18,000 ft (the highest one can fully acclimate, and about Everest Base Camp):
fO2 = 0.21, pAtm = 506 mbar (0.50 atm). For R~1 and pACO2 ~ 40 mbar, we get pAO2 ~ 53 mbar (40 mm Hg), which agrees with interpolation from measured values

20,000 ft (the highest people have lived for extended periods):
measured pAO2 = 45 mbar (34 mm Hg)

25,000 ft (the Death Zone; acclimation impossible and steady degradation of body functions experienced):
measured pAO2 = 40 mbar (30 mm Hg)

RobertDyck's fighter pilots:
fO2 = 1.0, pAtm = 170 mbar (2.5 psi), R = 1.0
for unacclimated pilots (paCO2 = 53 mbar) we get pAO2 ~ 57 mbar
for acclimated pilots (paCO2 = 40 mbar) we get pAO2 ~ 70 mbar
fO2 = 1.0, pAtm = 138 mbar (2.0 psi), R= 1.0
even for extremely acclimated pilots (pACO2 ~ 36 mbar) we get pAO2 ~ 35 mbar

From this we can clearly see why pilots last only a short time at 2.0 psi pure oxygen. Even for extremely acclimated individuals it is as if they are well into the Death Zone. One would need to acclimate to low pressue for many days like Everest climbers to have any hope of significant activity. Anything less would allow one to only maintain consciousness for a short time.

Conversely, we can see why 2.5 psi of pure oxygen is acceptable for pilots. Under these conditions (depending on their level of acclimation) a pilot would feel as if they were between 10,000 and 18,000 ft, which is the range in which people can still operate fairly normally, but tire somewhat easily and aren't as capable as they are at sea level.

Now, to apply this to terraforming Mars. It's a bit cumbersome to keep playing with numbers to shift from a 21% oxygen atmosphere to a 100% one, so I made the graph below. I just took the Alveolar Gas Equation and solved for the oxygen fraction in terms of the total atmospheric pressure, then plotted out curves using the parameters for various altitudes:

Any combination of fO2 and pAtm that falls on an altitude curve will have the respiratory effects of being at that altitude on Earth. For example, if you simply drew a horizontal line at fO2 = 21% the intersection point of each altitude curve with it would indicate the atmospheric pressure at that altitude on Earth. Thus, at 21% O2 the 0 ft curve is about 1010 mbar (1 atm) and the 18,000 ft curve is just above 500 mbar (0.5 atm).

But on Mars it would be difficult (and unnecessary) to obtain a 79% backfill of nitrogen, argon, and other gases. So how much oxygen do we need to use to get the same effects as, say, 18,000 ft on Earth? Well, let's say we want a pure oxygen atmosphere, since that'd be the thinnest possible. Just follow the 18,000 ft curve up to the cut-off at the top where fO2 = 1.0 (you obviously can't have an atmosphere more than 100% oxygen). Looking down at the pAtm axis, we see that we would need a bit under 160 mbar total atmosphere. If you actually plug in the numbers, you get 156 mbar. If you plug in for pure oxygen at 20,000 ft, you get 147 mbar. Given these numbers, it's reasonable to deduce a minimum pure oxygen atmosphere of 150-160 mbar (2.2-2.3 psi) for permanant human habitation.

But we probably won't have a pure oxygen atmosphere. It'd be more realistic to have, say, a 50% oxygen atmosphere with the rest made up by nitrogen, carbon dioxide, etc. It cuts down on flammability (a complex issue by itself) and some living things need nitrogen and/or carbon dioxide in the air anyway. So, we find where the 18,000 ft curve has an fO2 value of 50% and we get a total atmospheric pressure of 250 mbar (3.6 psi), 125 mbar (1.8 psi) of which would be oxygen. If you follow the 20,000 ft curve you get a total pressure of 230 mbar (3.3 psi), 115 mbar (1.7 psi) of which is oxygen. Exactly what percentage of the atmosphere you want to be oxygen will depend on how much flammability you're willing to tolerate, as well as what other gases are present, e.g. a given partial pressure of CO2 is more flame retardant than an equal pressure of N2. And of course in real life it'll depend on what's availible.

RobertDyck · 2007-08-03 05:26:36

My reason for calculating minimum pressure was not to advocate a pure oxygen atmosphere for a terraformed Mars, but at what point can we replace a pressure suit with an oxygen mask.

It also helps calculate a reasonable atmosphere mix. Many people expected pressure would have to exactly duplicate Earth, one atmosphere pressure with 78% N2, 21% O2, 1% other stuff. Reality is we can live with reduced pressure. Mars doesn't have as much gas as Earth, Mars will never have 1000mbar pressure, but Mars can have an atmosphere sufficient to live.

Antius · 2007-08-03 06:15:30

The issue of nitrogen crops up again and again in Mars terraforming discussions.

The question is, how much nitrogen is actually needed for nitrogen fixing bacteria to work within soil? Without those, we would be forced to either import masses of N2 into the Martian atmosphere or artificially produce and propogate nitrates across the biosphere.

It may turn out that a 200mb atmosphere which is 85% oxygen, 14% water vapour and 1% nitrogen is perfectly sufficient to sustain an active biosphere.

TonyLoco · 2007-08-03 07:12:50

I dont really undestand the role nitrogen plays.

Where does nitrogen come from on Earth?

What do we and other plants/animals need nitrogen for?

RobertDyck · 2007-08-03 22:56:54

Carbohydrates such as sugar and starch are made from carbon, hydrogen, and oxygen. Starch is a long chain of sugar molecules. Cellulose is also carbohydrate, and lignin is modified carbohydrate. There's starch in foods like potatoes, cellulose in grass, and cellulose and lignin are the primary constituents of wood.

However, protein is made of carbon, hydrogen, oxygen, and nitrogen. Protein is a chain of amino acids. DNA is a chain of nucleotides, each of which has 3 parts: phosphate, ribose sugar, and either a pyrimidine or purine. Those last two are the bases of DNA, and are very similar to amino acids. Ribose is a 5-carbon sugar, most sugar we eat is a 6-carbon sugar.

Life requires proteins for enzymes, organic catalysts to promote and control chemical reactions. Mammals including humans use a protein called collagen for structure. Skin is soft but doesn't tear; it gets its strength from collagen. Most people think of calcium when they think of bones, but bone is a combination of things. It is mostly hydroxylapatite, a mineral of calcium, phosphate, and OH from water. Like concrete it has good compressile strength but poor tensile strength. Collagen fibres through bone add tensile strength, fulfilling the same function as steel bars in concrete. So no nitrogen, no life.

On Earth nitrogen starts in the atmosphere. Single cell organisms like rhyzobium bacteria and cyanobacteria take nitrogen and water to make nitrate. Plants soak up nitrate with their roots, using it to make protein.

noosfractal · 2007-08-04 02:16:51

Thank you for making that graph Midoshi. I'd read the top and bottom figures before, but I've never seen the whole graph.

The question is, how much nitrogen is actually needed for nitrogen fixing bacteria to work within soil? Without those, we would be forced to either import masses of N2 into the Martian atmosphere or artificially produce and propogate nitrates across the biosphere.

McKay et al, Making Mars habitable, Nature 352, Aug 1991
http://www.nature.com/nature/journal/v3 … 489a0.html

... is a good general reference for terraforming. They say 10 mbar of N2 is required for nitrogen fixation. They reference ...

Klingler et al., Biological nitrogen fixation under primordial Martian partial pressures of dinitrogen, Adv Space Res, 1989

... which lowered N2 levels in a 1000 mbar atmosphere and found that microbial activity started decreasing at pN2 = 400 mbar, and that there was still some activity at 5 mbar, but none at 1 mbar.

For human habitation, McKay et al. concludes N2 is the only suitable buffer gas. They reference ...

Carr, Mars: A water-rich planet?, Icarus 68, 1986

... as saying there may be 100-300 mbar of N2 currently locked up in mineral form.

RickSmith · 2007-08-04 18:28:34

The question is, how much nitrogen is actually needed for nitrogen fixing bacteria to work within soil?

Hi Antius,
I've found that a partial pressure of 0.5 kPa (5 mBar) is sufficient for plants to fix nitrogen (actually the bacteria in their roots of course).

I am working on the UV problem right now...from some preliminary models it looks like an early 100 mbar CO2 with only slightly enhanced oxygen levels would produce a significant ozone layer. I can't say much quantitative at the moment since I'm still working on the model, but I'll post data if/when anything comes of it.

Hi Midoshi,
I don't like to clutter up the threads with 'nice post' posts, but again I thank you for your research in finding hard to find data. (In this case finding the O2 levels needed by mosses.)

On ozone levels, Fogg in his Terraforming text says:

Joel Levine and colleagues and a group heading by James Kasting have modeled the problem and their calculations suggest that a functional ozone layer could exist at 10% the present atmospheric level of oxygen...
... Thus we might assume that the complete protection from ultraviolet light requires a column mass of O2 equivalent to ~20 mBaron the Earth (0.1 PAL); satisfactory protection from most wavelengths is possible down to a terrestrial column mass of O2 equivalent to ~2 mBar (0.01 PAL). ...

PAL = present atmospheric level (PAL) <Thanks to Midoshi for the correction.>

A bit earlier in the discussion of ozone and UV light he points out that at minimum levels of O2 we would have O3 forming near the ground which is very harmful to life. Likely we will have to depend on aquatic plants to push the partial pressure of O2 high enough that the O3 will form high enough in the air to avoid oxidizing ground level plants. Mars' higher scale height will help us here. (Ozone that forms high in the atmosphere is further away from the ground than the ozone layer is on Earth.)

See also:
Levine, J.S., Boughner, R.E. and Smith, K.A. "Ozone, Ultraviolet Flux and Temperature o fthe Paleoatmosphere," Origins of Life, vol 10, 199 to 213, 1980.

Kasting, J.F., Holland, H.D. and Pinto, J.P., "Oxidant Abundances in Rainwater and the Evolution of Atmospheric Oxygen," J. Geophys. Res., vol 90, 10,497 to 10,510, 1985.

Warm regards, Rick.

New Mars Forums

Announcement

#1 2007-07-20 11:51:56

Re: How much oxygen do we need to breathe?

#2 2007-07-20 13:02:44

Re: How much oxygen do we need to breathe?

#3 2007-07-20 13:21:12

Re: How much oxygen do we need to breathe?

#4 2007-07-20 14:05:45

Re: How much oxygen do we need to breathe?

#5 2007-07-20 16:54:42

Re: How much oxygen do we need to breathe?

#6 2007-07-20 23:08:02

Re: How much oxygen do we need to breathe?

#7 2007-07-21 00:29:40

Re: How much oxygen do we need to breathe?

#8 2007-07-24 18:04:41

Re: How much oxygen do we need to breathe?

#9 2007-07-25 00:36:40

Re: How much oxygen do we need to breathe?

#10 2007-08-01 19:04:06

Re: How much oxygen do we need to breathe?

#11 2007-08-03 05:26:36

Re: How much oxygen do we need to breathe?

#12 2007-08-03 06:15:30

Re: How much oxygen do we need to breathe?

#13 2007-08-03 07:12:50

Re: How much oxygen do we need to breathe?

#14 2007-08-03 22:56:54

Re: How much oxygen do we need to breathe?

#15 2007-08-04 02:16:51

Re: How much oxygen do we need to breathe?

#16 2007-08-04 18:28:34

Re: How much oxygen do we need to breathe?

Board footer