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Part 3: Carbon Compounds

The enormous variety of possible carbon compounds offers some interesting taints for the creative world builder.

Carbon dioxide (CO2)

Data: Melting Point -57C (at 5.5 atm), Boiling Point -78C (turns from solid to gas at this temperature at 1 atm pressure), critical temperature 31C.

A 'simple asphyxiant'.

Given the abundance of carbon and oxygen in the universe, there is going to be a lot of CO2 around - or opportunities for its formation. On Earth, the carbon dioxide produced by living things is variously dissolved in the oceans (it is one of the most water soluble gases), converted by plants into sugars or locked up in the form of mineral carbonates.

The atmosphere now contains an average of 340ppm CO2, up from 280ppm in the 1850s. It is conjectured that atmospheric CO2 levels were much higher early in the Earth's history. Even as recently as the Cretaceous Era (to ~65 million years ago), CO2 levels were on the order of 600ppm.

The advent of photosynthesis and oxidative (aerobic) respiration changed all that. Carbon dioxide is a potent greenhouse gas, trapping solar infrared radiation within the atmosphere. In one climatological model (exploring the 'frozen pre-Cambrian Earth' theory), it was calculated that CO2 levels of 0.15 atm would have been sufficient to melt ice sheets covering the entire globe.

So worlds with atmospheres containing large amounts of CO2 will either be 'pre-' or 'post-aerobic' in the main. There may be intriguing worlds where the high levels of CO2 are enough to act as a powerful stimulus to plant growth - then the plants produce enough oxygen to incinerate themselves, and the cycle starts anew.

Other transient sources of CO2 may include warming or agitation of a body of water [or other solvent] - leading to a violent outgassing e.g.the lakes in Cameroon where several nearby villages had their populations asphyxiated.


Carbon dioxide is one of the waste products of the oxidation of sugars, proteins and fats. While the human body lacks oxygen stores, the solubility of CO2 and its rate of production enables the accumulation of the equivalent of three litres of gas (1 atm pressure, 25 C) per kilogram of tissue.

Carbon dioxide reacts with water to form carbonic acid ; this is mostly temporarily converted to bicarbonate to limit the adverse effects of increased acidity on cellular function. The only way to eliminate carbon dioxide is to breathe it out.

So an increase in the blood (tissue) levels of carbon dioxide is a powerful stimulus to ventilation. The minute volume doubles (product of respiratory rate and tidal volume) for each 0.03 atm increment in the partial pressure of CO2.

Breathing gas mixtures high in CO2 'flattens out' the tissue to lung pressure gradient, leading to retention of the gas. Eventually the ability of the body to limit the increase in acidity is lost ; this results initially in a rise in blood pressure and cardiac output (in a futile attempt to deliver more CO2 to the lung for elimination). The worsening acidosis depresses heart and brain function, leading ultimately to unconsciousness and death from cardiovascular collapse.

Initial symptoms include headache, dizziness, shortness of breath, muscular weakness, drowsiness or agitation, ringing in the ears and an acid taste in the mouth or a burning sensation in the nose, mouth and throat.

Gas mixtures containing more than 0.06 atm of CO2 cannot be breathed for more than five to ten minutes - collapse ensues from respiratory muscle fatigue. General anaesthesia ensues from the inhalation of 0.15 atm CO2. Death will ensue in a few hours with levels above 0.25 atm.

Carbon monoxide (CO)

Data: Melting Point -205, Boiling Point -190. A potent chemical asphyxiant.

The most obvious source for CO is the incomplete combustion of organic material e.g. cigarette smoke (1%), automotive exhausts (10% CO by volume) and 'coal gas' and house fires (up to 20% CO).

Carbon monoxide may also be produced by the hepatic metabolism of dichloromethane (methylene chloride, CH2Cl2), or the decomposition of phosgene (COCl2) in water (e.g. the moist air in one's lung).

Physiologically, CO is produced by the degradation of the porphyrin ring of haemoglobin, mediated by the enzyme haem oxidase. Perhaps some lifeforms could produce CO as a defensive or offensive mechanism.

Like nitric oxide, carbon monoxide has also been found to be a neurotransmitter substance. This may explain some of the neuropsychiatric sequelae seen in some survivors of carbon monoxide poisoning.


CO has great affinity for Fe2+ and binds avidly to haemoglobin (240x better than O2) rendering it useless for oxygen carriage. Reduction in the oxygen carrying capacity of blood is profound:

0.01% 14
0.05% 44
0.10% 60

Firefighters not using face masks and engaged in strenuous activity can have COHb levels of 75% in less than 1 minute.

Interestingly, severity of poisoning doesn't correlate with the level of carboxyhaemoglobin. Tissue load is obviously very important, but cannot be measured in living organisms.

Mild intoxication is usual with COHb over 20% ; severe symptoms and signs are usually evident with COHb above 50%. The heart and brain, being the most metabolically active tissues, are first to suffer the effects of hypoxia. Headache, drowsiness, confusion are early symptoms, with the eventual onset of coma. Heart muscle commonly infarcts (dies) in the face of decreased oxygen supply and increased work (reflexes lead initially to hyperventilation and increased cardiac output in an attempt to get more oxygen to the tissues!).

The first line treatment is using high concentrations of oxygen to displace CO off haemoglobin (exploiting the law of Mass Action).

Roughly, the half life of COHb is:
3-5 hours in air
30-120 minutes with 1.0 atm oxygen
20-30 minutes with hyperbaric oxygen

Methane and the Hydrocarbons


Substance Melting Point Boiling Point Critical Temperature Flash Point
Methane (CH4) -183C -162C -82.25C -188C
Butane (C4H10) -138.4C -0.5C 153.2C -60C
Pentane (C5H12) -130C 36.1C ? -49C

Melting and boiling points increase with increasing chain length. Compounds less than 5 carbons long are gases at 15 C ; compounds less than 16 carbons long are liquids at this temperature.

Hydrocarbons are common compounds. Significant deposits may be found on worlds in the outer zone of a star system (e.g. Titan with its tholin deposits and ethane lakes) or in certain rock formations of Earthlike worlds.

In oxygen based ecospheres, methane is a product of the anaerobic (no oxygen) decomposition of organic material. It is the main compound found in natural gas and has a wide range of industrial uses.

Methane is also a potent greenhouse gas, roughly 20x as effective as CO2 weight for weight.

Apart from the fire hazard associated with these compounds, the smaller hydrocarbons are simple asphyxiants.

Occupational health and safety limits:

Compound Occupational exposure limit
Methane 1000ppm - 8 hour
Butane 800ppm - 8 hour
Pentane 610ppm for 15 minutes

At levels ranging from 5 to 10x the above limits, general anaesthesia ensues. The hydrocarbons as a group are irritant to the heart and readily provoke abnormal rhythmicity. They are also corrosive to lung and gut tissue.

Halogenated Hydrocarbons

Substitution of halides (chlorine, fluorine, bromine, iodine) for hydrogen leads to compounds with varying physical properties but similar toxicological problems.

Some Methyl Halides

Compound Formula Melting Point Boiling Point Exposure limits
chloromethane CH3Cl -97 -24.2 100ppm/8 hours
dichloromethane CH2Cl2 -95.1 39.8 250ppm/8 hours
chloroform CHCl3 -63.5 61.2 2ppm/hour
carbon tetrachloride CCl4 -23 76.7 2ppm/hour
bromomethane CH3Br -93 3.56 zero
iodomethane CH3I -66.5 42.4 2ppm/day (contact)

Exposure limits are those set by U.S. federal regulatory agencies. Methyl halides are produced by various bacteria and algae in order to limit competition. Toxicity is via the production of methyl and halogen radicals which indiscriminately bind to various intracellular sites.

Liver, kidney and central nervous system toxicity predominates. Symptoms include: nausea, vomitting, headache, sensory disturbances, abdominal pain, eventual anaesthesia, then death.

Dichloromethane has also been mentioned above due to its transformation to carbon monoxide.

Chloroform: The odour threshold in man is 400ppm. Concentrations of 0.47 atm cause general anaesthesia.

Carbon tetrachloride: Odor threshold (lower): 21.4 ppm Concentrations on the order of 1000 to 1500 ppm are sufficient to cause symptoms if exposure continues for several hours.

Bromomethane: Fatal poisoning has always resulted from exposure to relatively high concentrations of methyl bromide vapors (from 8,600 to 60,000ppm). Nonfatal poisoning has resulted from exposure to concentrations as low as 100-500 ppm.

These compounds are unlikely to be present in large amounts in the atmosphere of Earthlike worlds due their effects on stratospheric ozone. Biological sources are the most likely, as mentioned above.



Alcohol Melting Point Boiling Point
Methanol (CH3OH) -98 65
Ethanol (C2H5OH) -114 78
Hexanol(C6H13OH) -45 157
Octanol(C8H17OH) -15 195

Everyone is familar with the effects of ethanol (C2H5OH) in one way or another.

Methanol (CH3OH) is toxic, leading to blindness, liver and kidney toxicity. Alcohols with chain lengths > 6 are potent inhalational anaesthetic agents, just like the hydrocarbons, and exhibit similar toxicity.



Hydrogen cyanide (HCN) mp -14, bp 26
Cyanogen (C2N2) mp -27, bp -20

Cyanide salts are used widely in metallurgy and the electronics industry. They may also be encountered in stone fruits and some other plants e.g. cassava. Chronic low level exposure may lead to a contact dermatitis.

Hydrogen cyanide is readily liberated when acids (and water) react with cyanide salts. It is also produced in the incomplete combustion of organic substances e.g. house fires.

Occupational exposure limit: 5 mg(CN)/m3 air/10 minutes

Lethal ingested dose is typically 200mg cyanide for an adult human.

The cyanide ion has a great affinity for Fe3+. The reactive centres of the cytochrome enzymes that drive aerobic respiration contain Fe3+. Cyanide prevents cells from utilising oxygen. This leads to accumulation of lactic acid, as anaerobic catabolic pathways are used in an attempt to maintain cellular homeostasis. Acidosis leads to the problems mentioned above in the discussion of carbon dioxide toxicity.

Cardiovascular and central nervous system collapse ensue within minutes. Fitting is common initially. Treatment aims to displace cyanide from the cytochrome iron atoms by reducing haemoglobin's iron (Fe2+ -> Fe3+) to mop up cyanide and to enhance the activity of the liver rhodanese system which converts cyanide to the slightly less toxic thiocyanate.

Carbon disulphide (CS2)

Data: Melting Point -111.5C, Boiling Point 46.5C, critical temp 280C, flash point -30C

This substance is a potent pesticide and is used as a solvent and intermediate for the synthesis of various chemicals. It reacts explosively with most oxidants. It is a potent neurotoxin, causing depressed levels of consciousness and ultimately anaesthesia. In chronic low level poisoning, the effect on the nervous system is one of central and peripheral damage which may be permanent if the damage has been severe.

Recommended exposure limit to this compound in air: 1 ppm/10 hours ; Ceiling Limit 10 ppm/15 minutes every 10 hours

A biological origin for this compound is likely, in the absence of some rather unusual geology.

Biological materials

This is a big group made up of

Entry into the aerodigestive tract is a function of size (as per any particulate material). Detection of intruders by the immune system may lead to appropriate or inappropriate responses. In general, substances with a molecular weight of less than ~1000 daltons will not be detected unless they bind to some other molecule (e.g. a tissue protein) first.

  1. Appropriate
    • Identification - antibodies or complement proteins are bound to the foreign substance
    • Confinement - capillaries become leaky, causing localised oedema which slows egress from the tissue ; proteins are released into the circulation to limit the access of the intruder to various resources e.g. transferrin levels increase, leading to a decreased availability of iron.
    • Mobilisation - phagocytic cells multiply and move into the area ; fever is produced which enhances immune cell function.
    • Elimination - the intruder is either eaten or destroyed by the immune cells. Sometimes this is not achievable and the intruder is confined but at the cost of ongoing inflammation - e.g. tuberculosis and parasite infections.
  2. Inappropriate
    • Tolerance - the immune system is fooled into ignoring the intruder.
    • Immunosuppression e.g. HIV infection
    • Anaphylaxis - antibody release leads to inappropriate activation of signalling cells called mast cells. Mediators released from the mast cells cause blood vessels to dilate and become 'leaky' and contraction of the muscle lining the larger airways in the lung. Tissue swelling, bronchoconstriction and marked reduction in blood pressure ensue which can be fatal within minutes.
    • Allergy - similar mechanism to anaphylaxis but on a much less threatening scale.

Additional Notes on Carbon Compounds, by Mike Kanssen

Hydrocarbons are a much larger topic than Rob's comments above imply. As this is my field of expertise I offer some additional notes. Rob's section is in main actually referring to the alkanes (or paraffins) which are saturated carbon/hydrogen compounds (with the general formula CnH2n+2). As well as straight chain compounds (n-paraffins), to which Rob's data refers, there are various branched chain versions (isoparaffins). The number of branched compounds increases rapidly with number of carbons; there are literally thousands of possible C15H32 variants (called isomers).

The boiling points of branched chain compounds are lower than the corresponding straight chain paraffin. In the case of pentane (C5H12) the branched isomers boil at 9.5 & 27.8 C.

An additional class is the cycloparaffins (also called naphthenes) in which the paraffin chain loops around and rejoins itself. For each ring formed, the general formula for alkanes loses 2 hydrogens. Rings with less than 5 members are unstable so cyclopentane (C5H10, boiling point 49.3) is the first stable member.

Further complication is added by the ability of carbon to form double and triple bonds to itself, giving rise to the alkenes (double bonds) and alkynes (triple bonds), which are generally considerably more reactive than the alkanes. Again, the number of variants increases dramatically with chain length: As well as having branched variants, the position, type and number of the multiple bond(s) can change. Boiling points tend to be slightly higher than the corresponding alkanes (with 2-butyne being the exception to Robs guide on boiling points of <15 for less than 5 carbons - it boils at 27C). My data lists 12 of these unsaturated compounds with boiling points below room temperature. Generally I'd expect these compounds to be more harmful than the alkanes (1,3-butadiene is particularly harmful with a limit of 10ppm, however I can't find data for any of the other alkenes).

The final major class of hydrocarbons are the arenes (or commonly aromatics), which contain both a ring and multiple bonds (such that the electrons from the multiple bonds are shared all the way round the ring). Aromatics are more stable than the alkenes/alkynes, and also more harmful (quite a few are considered toxic or carcinogenic). The lowest boiling aromatic - benzene (C6H6, boiling pointt 80.1) is carcinogenic with no known safe limit (in Europe the current exposure limit is set at 1ppm). Rural air typically has benzene levels around 1ppb (1 in 109) with levels significantly higher near to roads.

The Aromatics also provide exceptions to another of Rob's guidance physical properties - Naphthalene (C10H8 - basically 2 fused benzene rings), and biphenyl (C12H10 - 2 linked benzene rings) are both solid at room temperature. Natural crude oil sources vary widely in aromatic content from about 10% to over 90%.

Many hydrocarbons are considered harmful by skin contact - bear this in mind if you're considering making do with air tanks in an exotic hydrocarbon atmosphere.

As a final comment, part of my job is determination of hydrocarbons in the air for occupational exposure (at a refinery). We generally see n-heptane (boiling point ~100C). So boiling point is not as simple a guide to what might be in the air as many people might think.

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