The Atmosphere: The Tolerances of our Survival

We have evolved to breathe the air of Earth's atmosphere, the composition of which was described in the last article. However, what are the tolerances of our survival? This article looks at what happens if the concentrations of the various components of our atmosphere are changed. Hopefully this will also give us some insights into how amenable the atmospheres of other planets might be for humans.

The Evolution of Earth's Atmosphere

The Earth's atmosphere has evolved over the billions of years since the Earth was created out of the same second generation (in terms of stellar evolution) dust cloud that spawned the Sun and most of the rest of the solar system. The majority of this gas cloud was made up of Hydrogen and Helium. Some of the Hydrogen has stayed with us; locked up in heavier molecules because of its volatility. However, a lot of it, and most of the Helium, has leaked away into space and been blown out to the farther reaches of the solar system through the action of the solar wind.

This first generation atmosphere was replaced by one dominated by Carbon Dioxide (a much heavier gas) released from the planet through vulcanism. Although inimical to animal life forms of today, the heavy carbon content of the atmosphere and the actions of volcanic heating, lightning discharges and the presence of tidally effected liquid water were all instrumental in the creation of more complex organic molecules, and might have even led to the creation of life on Earth (a debate for another time, perhaps).

No matter how it came about, the presence of life on Earth assisted in the evolution of the third generation atmosphere that we possess today by turning CO2 into diatomic Oxygen and biomass and by boosting the concentration of the relatively inert Nitrogen gas in the atmosphere as a by-product of organic decay. This evolution is still occurring — it is thought that the concentration of Oxygen during the time of the dinosaurs was more like 30 percent than today's 17 percent — but as we evolved along with the atmosphere we are well suited to its current mix.

Now let's see what happens if we change that mix.


Nitrogen gas is diatomic with strong bonds between its two atoms. This makes it chemically relatively inert, but it can still have dramatic effects on us if we change the conditions under which we breathe it.

Under conditions of high pressure, humans breathing normal air, can suffer from Nitrogen Narcosis. Also known as the 'Rapture of the Deep', this effect occurs when high pressure forces Nitrogen gas into the solution which we call our blood through the lungs. Although the process is not completely understood it seems that the Nitrogen gas molecules interfere with the correct working of our nervous system bringing about a state similar to intoxication. Although this effect is not normally dangerous itself, an intoxicated diver is generally considered to be in danger because of the danger of the environment. People have been known to forget about the limited nature of their air supply because of Nitrogen Narcosis.

The reverse situation, exposing a subject to low pressure, can be deadly in and of itself if the pressure drop occurs rapidly. In this case, the Nitrogen gas absorbed in the blood literally boils as the pressure drops forming pockets of the gas within the body and blood. This effect, commonly called the 'bends', but more formally known as Decompression Sickness can be induced by a fifty percent variation of pressure. This equates to coming up from a dive under as little as ten metres or water or ascending quickly to 5000 metres above sea level without suitable protection. Note, that this effect is avoided if the pressure change is slow as the excess Nitrogen in the blood can be exhaled normally through the lungs given the chance. However, skin divers who leave the water and immediately climb onto a plane to go home at the end of a diving holiday have been known to experience decompression sickness, so the changing of the concentration of Nitrogen in the blood does take some time.


We die quickly without Oxygen; it is a necessary part of the body's energy generating processes. Without it, all our biochemical systems quickly run down and cease to function. However, too much Oxygen is also bad as it is a volatile element and excessive oxidation within living cells can lead to the effective poisoning of the body.

This situation is further complicated by the fact that the rate of absorption of Oxygen into the blood in mammals is determined by its partial pressure in relation to the total pressure of the gases in the lung. This leads to the, perhaps surprising, fact that deep sea divers use a concentration of Oxygen in their breathing gas that would be insufficient to sustain them on the surface. The pressure under water brings the partial pressure of the Oxygen in their mix up to the levels required, whereas normal concentrations would be too rich.

So, what do we take away from this as speculative fiction writers?

It is insufficient to just have Oxygen in an atmosphere and assume it will sustain humans in the longer term. At sea level it is recommended that the concentration of Oxygen for bottled air lies between 17 and 23 percent; less or more than this causes problems. The other variable in this equation is the air pressure. Increase the air pressure and the Oxygen is absorbed at levels poisonous to the body. Decrease the air pressure and the body can't take up enough of the Oxygen to sustain life even if the concentration is in the right range. Therefore, for designers of their own worlds, staying close to Earth size (and similar air presssure) and having a similar evolution to Earth is the right way to go if you want to introduce humans from Earth to your planet and have them walk around without special breathing apparatus.

Aside: If, as some scientists suggest, the concentration of atmospheric Oxygen during the age of the dinosaurs was thirty percent, then it is interesting to note that modern day man could not have survived unassisted in that period. Similarly, dinosaurs would probably suffocate in today's air.


Like Nitrogen, Argon is not chemically reactive, and has similar effects on the body under extremes of pressure. However, as it is not a common element (about 1 percent of our atmosphere) its effect is normally swamped by the other more common gases.


Carbon Dioxide is also not in high concentrations in the atmosphere, but it is not by any means inert, so even in small amounts it can cause problems. A safe work place concentration of Carbon Dioxide at sea level is considered to be less than .5 percent. At a concentration of 3 percent the gas has a narcotic effect. At five percent those exposed experience dizziness, headaches and shortness of breath. At eight percent unconsciousness sets in after about five minutes exposure and ten percent is considered a deadly concentration.

Perhaps of most interest to writers of speculative fiction is the fact that Carbon Dioxide is a heavy gas. It tends to hug the ground and flow to the lowest point, like a fluid, concentrating itself and ten percent of Carbon Dioxide in the air will kill you even if you have twenty percent Oxygen. Also, Carbon Dioxide is odourless and colourless, but if inhaled leaves a sour taste in the mouth and a stinging sensation in the nose and throat as a result of the production of Carbonic acid through its reaction with the water in those sensitive areas.


Not much to say here that isn't obvious, but I would like to point out that it is very lucky for life on Earth that the Earth's orbital radius is what it is. Under our protective atmosphere most of the water on Earth is in liquid form. Closer to the Sun and water would exist only in gaseous form. Further away and it would be all locked up in the form of ice. In liquid form, it not only makes up most of the mass of living things on the planet, but also provides a protective medium in which the majority of the Earth's life forms exist (i.e. the oceans).

Despite the length of this article I haven't finished with the atmosphere yet. More next week ...


About the evolution of the Earth's atmosphere

More about the possible creation of life on Earth

The Wikipedia on Nitrogen Narcosis

More about Decompression Sickness

The Wikipedia on Oxygen Toxicity

The Wikipedia on Breathing Gases

More than you ever probably wanted to know about Carbon Dioxide

N.B. Please note that I although I use the Wikipedia (and WikiMedia Commons) a lot for references, this is for expediency and the familiarity of my readers. Anyone interested in further studies should make use of the references where available and understand that the Wikipedia is a co-operative project contributable to by anyone and must always be looked at in that light.

Phill Berrie, October, 2008.