The Deadliest Gas

One of the essentials of most - but not all - life as we know it is oxygen. To us, the end result of a long evolutionary line of oxygen-breathers, this appears obvious. However, as science fiction has pointed out, the properties of this highly reactive, corrosive gas might make its association with life far from obvious. From some perspectives, oxygen is probably the deadliest of gases.

Closed Environments

When space travel was first conceptualised in science fiction, the challenge of enclosed environments had to be faced in a way rarely true here on Earth. Human beings modify their environments constantly, consuming oxygen, breathing out carbon dioxide, and generating methane and other trace gases. Manufactured materials also outgas over time, while many materials gradually absorb trace gases from the atmosphere. Maintaining an atmosphere within the narrow range comfortable and healthy for most humans is far from simple.

Understandably, the majority of narratives where enclosed atmospheres cause an issue focussed on a shortage of oxygen (hypoxia) or an excess of other gases (e.g. carbon dioxide) which could harm any occupants, since these were primary concerns for even short journeys into space. There are nonetheless examples of authors who considered the reverse problem - the toxic properties of an excess of oxygen.

An early proponent of scientific plausibility, if not necessarily accuracy, in science fiction was John W Campbell, initially as a writer, and later as the highly influential editor of Astounding Science Fiction magazine between 1937 and 1971. As early as 1930, he wrote a letter to a different magazine, Amazing, noting the dangers of excess oxygen amongst other potential problems connected with spaceflight. 

“But the most serious effect is due to the fact that the man tends to "burn up"’ under the greater oxygen concentration. It is well known that men die in an excess of pure oxygen.”

Campbell points out the experience of aviators and others and stresses the importance of partial pressure of oxygen rather than its concentration, i.e. that the toxicity of oxygen depends not only on how much of it is present but also on the pressure and the contributions of other gases.

“If the pressure is increased on the other hand, the oxygen concentration is increased with it, until, when the pressure reaches that due to a depth of 175 feet of water, the men might as well be breathing pure oxygen at normal pressure. It is this effect of increased oxidation rate, faster living, that produces some of the dangerous effects in the caisson.”

While his argument in the letter is challenging for those without technical knowledge to follow, Campbell was amongst many authors to consider the problem of excess oxygen in his own fiction. While the highest oxygen levels are deadly, a moderate excess has the same intoxicating effect as a moderate excess of alcohol. In The Tenth World (novelette, Thrilling Wonder Stories, Dec 1937), Campbell described two of his recurring scientist-explorers, Blake and Penton, visiting a frigid world beyond Pluto and finding it teeming with unexpected (and deeply implausible) life. Trapped, and with their oxygen running low, they first experiment with trying to thaw some of the world’s frozen oxygen sand in one of their suit supplies, but this leads to technical difficulties:

“Stupid,” he muttered. Shtupid beassh. Stupid hydrushen, stupid oxyshen. Won’ burn. Here, shtupid, water. Make thish shtuff.” Blake was gloriously drunk; his oxygen control was stuck again, wide open, and he was thoroughly intoxicated by the excess oxygen.

Fortunately, Blake’s companion is able to stabilise his suit controls, while his impulse actions under oxygen narcosis lead to the breakthrough that allows the explorers to escape.

A similar problem, although in a different context, was visited by Ross Rocklynne in his novella Slave Ship to Andrigo which appeared in Planet Stories in July 1951. This story story focuses on the titular slave ship, the S S Selwyn, which has set out to capture a group of plant people to sell at a slave market on a nearby planet. When the Selwyn’s oxygen reprocessor is damaged during a mutiny, two men are imprisoned alongside the slaves and the oxygen supply to the prison deck cut by the crew, who are desperate to conserve the remaining supply. However the two prisoners soon find themselves becoming euphoric:

Hawk’s vision was blurred, but he focused long enough to see that the pupils of Single’s eyes were big and out of control. “Shstand still,” Hawk shouted thickly. “Schumping’s wrong — but it’s blasted good ! We’re blind drunk, oxshgen-drunk ! It’s the Greenies. That utter ignoramus Corpin!”

Single grinned laxly. “Have ’nother shigarette, Hawk of the Spashways. Burn up more oxygen. Too much oxygen, Hawk of the Spashways. Remember how they put us through oxygen tesh in a Spashship at Deshmal Point? Drunk then, too. Problem was to releash enough Shee-O-Two into the air to bring air back to normal. How’d you come out on that tesh, Hawk of Spashways?”

In fact, the large photosynthetic slave population themselves are soon struggling:

“But his scheme backfired. The Greeniesh naturally breathed in all the Shee-O-Two and breathed out oxshgen. That makesh ij bad for ush, worsh for them. They’re shuffocating, we’re burning up. Thing is, bring air back to normal like I did at Deshimal Point when nasty professhors wanted to kick me out.”

Fortunately the excess oxygen and a cigarette lighter make for a blow-torch like flame that breaks the old Decimal Point academy classmates out of the brig, and permits them to start a fire of broken furniture that replenishes carbon dioxide in the prison deck and helps rebalance the atmosphere. When the ventilation throughout the ship is reestablished, the Selwyn’s crew and the population of (now released) Andrigans even out and allow the ship to survive to planetfall.

A slightly different scenario of excess oxygen in the context of human space travel can be found in The Flames of Life by Theodore Thomas (short story, Amazing, Dec 1960). Here a spaceship in need of emergency repairs makes a glider landing on a swamp-dominated world in which the atmosphere comprises 76% oxygen, 17% nitrogen and 7% methane. The air is human-breathable (presumably at an acceptable partial pressure of oxygen, though this is not explained) but it is so infested by insect life that the two crew are unable to crack open their helmets for more than a brief instant and the intoxicating effects are not explored. More importantly, they realise, the high oxygen concentration renders the atmosphere potentially explosive - such that they may never be able to make a rocket-powered take off, and may be trapped amongst the insects for life.

In fact though, under the conditions on the planet (slow rotation, 1.5 times Earth mass and twice its radius), they calculate that a flame front will propagate at just one foot per second, allowing the heat of combustion to escape as the surface of the burning bubble moves outwards. In other words, the “explosion” of the atmosphere is very slow, taking weeks to spread across the planet, releasing heat gradually rather than in a single instant, and has the benefit of generating water, seeding clouds and killing most of the insect life at the same time, while leaving enough oxygen in the atmosphere to remain breathable.

Tragically, science fiction involving high concentrations of oxygen took on an uncomfortable resonance in 1967 with the test of Apollo 1. The capsule was designed to support astronauts in a pure oxygen atmosphere, which was delivered at low pressure in their spacesuits but kept in high pressure in the capsule. During a test run in January 1967, a spark ignited the air in the capsule, resulting in a catastrophic fire and the death of three astronauts. This increased awareness of the danger of oxygen combustion, and stories about the more comic aspects of oxygen narcosis became rarer.

Earth made Alien

While enclosed environments are the most vulnerable to gas imbalances, science fiction has also examined the effects of excess oxygen on a global scale. 

One of the most urgent and eagerly sought technologies of the industrial revolution was a method to improve crop yields in order to support the growing urban population. After modern chemical analysis developed, a key target was the extraction of nitrogen (a vital component of fertilisers) from the abundant nitrogen gas component of Earth’s atmosphere. At the end of the nineteenth century, the nitrogen necessary for use in fertilisers was derived from mineral sources, notably South American salpeter deposits. Despite intensive study during the late 1800s, a commercially viable and efficient process for fixing nitrogen as ammonia was not developed until the Fritz Haber’s work between 1905 and 1909 created the Haber process, now used to extract virtually all of the nitrogen used by industrial and agricultural processes. Haber won the Nobel Prize for Chemistry in 1918 for this work.

The fact that such ideas were being discussed outside of purely scientific circles is evidenced by the novella Within An Ace of the End of the World by Robert Barr, published in Windsor magazine in 1900 [1]. It describes initial work in 1898 and 1899 which culminated in a gourmet “Millionaire’s Dinner” in December 1899 at the end of which inventor, Herbert Bonsel, announces that:

“All the viands you have tasted and all the liquours you have consumed were prepared by me in my laboratory. You have been dining simply on various forms of nitrogen, or on articles of which nitrogen is a constituent. The free nitrogen of the air has been changed to fixed nitrogen by means of electricity, and the other components of the food placed on the board have been extracted from various soils by the same means.”

When this artificial food catches on around the world, agriculture collapses, and at first all seems well. But a rising death rate over the course of several years, and then increasingly erratic behaviour amongst public figures, begins to sound alarm bells. By 1904, a small group of students at Oxford build an enclosed environment, adding nitrogen to replace that extracted from the atmosphere, and are able to point out in a letter to The Times that the atmospheric concentrations have been altered from around 23% oxygen to 77% nitrogen to almost the reverse (73%:27%) as a result of human industry. As they note, most of the world is now intoxicated by the excess oxygen. One of the students has a fiancée at Vassar College in New York to whom they send instructions on building an enclosed house. Soon steam trains burst into fire, great conflagrations break out in every city, and humanity succumbs to oxygen poisoning. Ultimately the eight English men and eight American women are the only survivors of the entire human race, and a transatlantic journey is necessary to reunite them and refound humanity.

The story is implausible in a number of respects, including but not limited to the idea that six untrained scholars could sail a yacht across the Atlantic, that no one but them would notice the problem, and that eight men and women will fall in love and repopulate the world. Perhaps the biggest implausibility is that even the replacement of all agriculture would have such a vast impact on atmospheric composition in four years - or that such an inversion of composition fractions would be necessary to cause disaster. As we now know from recent developments regarding carbon dioxide, much tinier changes in composition can have a devastating effect on habitability, while the total volume of human industry has taken centuries to make those changes (albeit at an ever-increasing rate), while sequestration of unwanted atmospheric components to restore balance would take just as long.

The idea that the Earth’s atmosphere might become super-oxygenated was revived in James Follett’s 1978 radio drama for the BBC, The Destruction Factor. This six-part drama began with a series of intense fires loosely associated with an agricultural research centre which aims to combat world hunger. Dairy products such as cream begin going off within hours. Soon it becomes clear that the fires are associated with a peculiar plant, as Nobel Prize winning scientist Max Flinders explains:

“You’ve heard of photosynthesis? Well, in simple terms it’s the process by which plant leaves produce their nutrients from solar energy. They also produce surplus oxygen which is released into the atmosphere through the leaf. All plants do this, of course, but this… this strange plant does it faster. So fast that it can enrich the oxygen content of the atmosphere in a closed room within a matter of minutes.” (episode 2)

This plant, initially rare, rapidly appears everywhere in Great Britain, and grows very fast. It has, in fact, escaped from the research centre and is named the Exon strain after the scientist who developed it. The world faces disastrous fires, intense corrosion, abandonment of technology and eventual extinction from oxygen poisoning, unless every individual plant across the planet must be found and wiped out. The alternative is an atmosphere so oxygen-rich that devastating fires will ignite globally and humans, even if not killed by the toxicity, will be reduced to a pre-industrial state. As Flinders points out: “our whole civilisation depends on our control of fire.”

While the change is more gradual than in Within an Ace of the End of the World, with a few tenths of a percent difference in oxygen concentration observed in the atmosphere over the UK, the problem is growing exponentially, and the annual bird migration threatens to make it worldwide. The solution involves repurposing a scientific earth observation satellite in the desperate hope of locating and exterminating each plant - before the United Nations opts to totally defoliate the UK in order to stop it.

As in Within an Ace the intentions here were noble: Exon had spent a lifetime working to alleviate famine and was desperate to find a food plant that would grow in equatorial Africa. But as in Within an Ace, the text also carries a warning of the dangers of meddling with nature. The plant was designed to be so robust that its escape was likely inevitable as soon as it was engineered, and the work to alleviate world hunger must be sacrificed to prevent a greater disaster.

While these two examples deal with disaster, a similarly world changing, but subtler and more insidious, effect of oxygen is found in the short story Success Story by Julian Chain (appearing in Astounding, May 1951). This describes a biochemist whose breakthroughs allow a house, together with its internal fixtures, plumbing and air conditioning, to be grown inside a living tree, at a fraction of the cost and industrial burden of a traditional dwelling. A friend tries to convince him that the return to agrarian, rural living that he envisages would result in the abandonment of space travel and its associated physical sciences. However a confluence of circumstances lead him to retreat to his tree and ignore this warning. The reader is left to wonder whether the bucolic, contented return to nature of Earth’s population or an expansion into space with colonisation, trade and alien encounters would be the better future for humanity.

An interesting feature of the tree house, and a possible explanation for its eventual overwhelming popularity (besides cost and convenience), is mentioned in the context of discussing the building’s air conditioning:

“This Tree, however, has been provided with a circulation system, so that the oxygen, cooled in the leaf by evaporation in a sort of heat-exchange process, is led into the interior of the tree. This results in a perceptibly higher oxygen concentration inside the Tree. We have noticed a curious consequence of this: The ideal temperature and humidity that prevails inside the Tree, together with the feeling of well-being due to the higher oxygen content, causes a psychological reluctance to leave after a period of time.”

In the context of the story, this is presented as a selling point for investors in the new technology, and it could certainly encourage the uptake of tree houses, but it does suggest something indulgent and even sybaritic about the choice to turn towards comfort, safety and a collapse in a previously outward-looking society. Here a change in the oxygen content of even the domestic environment has a global impact.

The Alien Perspective

The narratives discussed above all consider the effects on humans an oxygen concentration that deviates from Earth-normal. But is the oxygen-dependance of Earth life itself an anomaly? 

The most promising venue for life outside of Earth in early science fiction was the planet Mars. However, spectroscopic analysis of Mars in the late nineteenth and early twentieth century made it very clear that the planet’s atmosphere contains very little, if any, oxygen. Indeed, it was obvious that the Martian atmosphere, whatever its composition, is very thin compared to that of Earth. Perhaps naturally, the Martian perspective of Earth in mid-twentieth century science fiction is one of confusion and horror when they become aware of our atmospheric oxygen concentration.

Ray Bradbury’s The Martian Chronicles incorporated the short story Ylla (??, which was recently reprinted in the 2018 anthology Lost Mars, edited by Mike Ashley). This describes a Martian woman who dreams telepathically of the human astronaut who is about to arrive on the planet. However her husband reacts with scorn to her suggestion that the third planet Earth could be inhabited - as he notes: “Our scientists have said there’s far too much oxygen in their atmosphere.”

Arthur C Clarke returned to this concept in his fictionalised essay Report on Planet Three (first published in 1959). This was presented as a report by a Martian astronomer on Earth, but - through the comparisons made - acts as a report on Mars for the reading audience. It later appeared as the opening feature in a collection of non-fiction essays Report on Planet Three and Other Speculations published in 1972. In the report, the Martian expert is uncompromising in his opinions:

“The first great objection to terrestrial life - which many experts consider conclusive - is the intensely poisonous atmosphere. The presence of such large quantities of gaseous oxygen poses a major scientific problem, which we are still far from solving. Oxygen is so reactive that it cannot normally exist in the free state; on our own planet, for example, it is combined with iron to form the beautiful red deserts that cover so much of the world.”

Clarke’s Martian also comments on the horrifying phenomenon of fire (unknown on Mars) and (more subtly) on the fact that oxygen high in Earth’s atmosphere becomes ozone, blocking the Sun’s ultraviolet light. From the perspective of the alien observers, this precludes life, which (on Clarke’s Mars, a world that lacks photosynthesis) directly absorbs ultraviolet photons as a form of food.

By contrast, in Seamus (short story, Science Fantasy, June 1955), Martin Jordan imagined a future in which humanity (but not other Earth life) is wiped out by a catastrophe, and the sole survivor is found decades later by exploring Martians (themselves the descendants of centuries-past human settle ancestors):

“Not for the first time he began to think about earth-air. Its composition was certainly different from the Martian atmosphere which the filter-plants duplicated inside the city. Yet it was not fatal to a Martian — once he had breathed it himself for an experimental ten minutes. The effect? It was a guilty exercise to recall the effect, even to himself. Earth-air was poisonous, yet poisonously desirable . . .”

In fact, Earth’s atmosphere proves to contain both a strong intoxicant (oxygen) and an aphrodisiac (aphrophon, described as a product of nuclear fall out) to Martians. This atmosphere causes the “advanced” Martians, who reproduce artificially and have a social status tied to their IQ, to revert to the more primitive reproductive urges, encouraged by the manly man, Seamus.

Similar alien reactions to Earth’s atmosphere [2] were a recurrent feature of science fiction of the time, but fell out of favour towards the end of the twentieth century as humans began to be represented as more typical of the interstellar community, or even as inferior to most aliens, rather than as significantly different or exceptional. It’s interesting to note that human exceptionalism has recently resurged in the form of a subgenre of SF which flourishes on social platforms such as Reddit and YouTube. HFY [3] stories (mostly self-published through such platforms) focus on humanity as unusual, strong and often leaders in deep space contexts. As a result, several involve aliens reacting with horror, and/or awe, on discovering that humans are tough enough to not only survive but flourish on a world with a high oxygen content.

Oxygen and Habitability

Narratives of oxygen toxicity serve two key roles in mediating the popular understanding of planetary habitability.

The first, as seen in examples such as The Destruction Factor or The Tenth World, serves to highlight just how precariously narrow the range of environments habitable to humans really are. Changes in atmospheric composition of just a few percent are enough to upset the precarious equilibrium on which our respiratory needs for oxygen and carbon dioxide, both at an appropriate partial pressure, depends. While this is at its most sensitive in closed environments, the planetary sensitivity to subtle changes resonates too with contemporary concerns regarding climate change. 

The second important aspect of oxygen toxicity narratives, as seen in Report on Planet Three and others, is a reminder that our perception of habitability itself can become too narrow. The presence of free oxygen in an atmosphere is likely an adequate signature of life (since we know of no other way it can be produced). That doesn’t mean that any environment rich in oxygen is suitable for humans - a few percentage points change upwards in the partial pressure could be toxic for complex life such as ourselves, or could lead to catastrophic fires, while a similar change downwards could restrict the potential for oxygen-demanding activities such as organ differentiation and brain growth.

And while oxygen may be adequate to signify life, it is not a unique or necessary requirement. Life does exist for which oxygen, or at least oxygen in the concentrations seen in Earth’s current atmosphere, is deeply toxic. 

It is not widely recognised that the most dramatic mass extinction in Earth’s history was not associated with an asteroid impact, supernova, volcanism or other physical catastrophe. Instead, around 2.5 billion years ago a small group of microscopic organisms known as cyanobacteria began to produce a toxic byproduct of their metabolism, poisoning first other microbes around them and ultimately the entire planet. The anaerobic bacteria which had formed the dominant life on Earth for a billion years died en masse. Most of the cyanobacteria were themselves killed by their own effluent. This toxic effluent was oxygen and the mass extinction is known as the Great Oxygenation Event, the effects of which can be seen in fossil records, geological records and climate records.

By contrast, oxygen levels only rose to their current ~20 percent within the last half billion years.

In other words, for the vast majority of the history of life on Earth, and even of complex multicellular life, Earth’s atmosphere would be unlikely to satisfy our currently applied criteria for planetary habitability. Even now, some single cellular life can be found on Earth which are either indifferent to free oxygen or actively damaged by it. There is every prospect that environments such as deep ocean vents on the gas giants’ ice moons will reveal more such biologies.

Despite its toxicity, or - in many ways - because of it - the energy released by oxygen’s violent chemical reactions has been essential to the development of humanity as we know it. Science fiction reminds us though just how delicately that reliance is poised and opens our eyes to the prospect of other forms of habitability entirely.