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Part 4: Sulphur, Phosphorous, Halogens, and Other Compounds


Sulphur compounds mainly appear in atmospheres from volcanism and industrial activity. On some unusual worlds they may be substitutes for water or otherwise figure prominently in local geology e.g. Io.


Compound Melting Point Boiling Point
SO2 -73 -10
SO3 17/30* 45
H2S -83 -62
SO2F2 -120 -52
SOF2 -110 -30
SF6 -51** -63

* - Melting Points for alpha and beta forms respectively.

** - sublimes ; estimated melting point is at an ambient pressure of 10atm.

Sulphur oxides

These are strong irritant gases, forming sulphurous and sulphuric acid on combination with water.

SO2 levels (mcg/m3) Effect
80 to 365 Typical urban atmosphere
800 Mild hazard in susceptible persons
1600 Moderate hazard in susceptible persons
2100 Moderate hazard, everyone
2620 Severe hazard, everyone

Hydrogen Sulphide (H2S)

Hydrogen sulphide is a chemical asphyxiant, binding avidly to respiratory cytochromes. It is more potent than cyanide.

Concentration (ppm) Effect
0.002 Odour threshold: 'rotten-egg smell'
10-15 occupational exposure limit
100-150 nose/eye irritation
250-500 sore throat, cough, chest tightness, pulmonary oedema
500-1000 headache, confusion, cyanosis, convulsions
>1000 death

Treatment of poisoning is currently unsatisfactory. Antidotes for cyanide are ineffective.

The fluorine compounds of sulphur are simple asphyxiants. SF6, sulphur hexafluoride, is very poorly soluble in water and has been used in respiratory physiology research. The occupational limit for SF6 exposure is 1000ppm.



Compound Melting Point Boiling Point
PF3 -160 -95
PF5 -92 -85
PH3 -133 -88

Phosphine (PH3) is used as a fumigant and pesticide and as a dopant in the semiconductor industry. The adult lethal dose by ingestion is 1.5g (PH3 in salt form).

Occupational exposure level 0.3ppm
Maximum concentration tolerated 100-190ppm for one hour. Levels above 2000ppm are lethal within 5-10 minutes.

Toxicity acutely is to the liver and heart. Chronic low level exposure leads to liver disease (yellow degeneration of the liver) and gradual breakdown of the lower jaw bone ('phossy jaw').

The fluorophosphates are simple asphyxiants.


  1. Chlorine (Cl2)

    Molecular Weight 70.9, Melting Point -101C, Boiling Point -35C, critical temperature 144C

    This green-yellow gas is as reactive as oxygen and could be an important part of an alternate biochemistry. It is unlikely to be found in significant amounts in an atmosphere without biological regeneration.

    Industrially, it is a very important chemical, being used in the synthesis of a wide variety of substances. Hypochlorite (HClO-)solutions are commonly used as bleaches.

    Chlorine is a powerful irritant gas. Initially irritation of the eyes, nose and throat are the dominant symptoms. With further exposure, nausea and vomitting ensue, with the eventual onset of permanent eye damage, pulmonary oedema, laryngeal spasm and respiratory arrest.

    Statutory exposure limits: Occupational: 1 ppm; Ceiling: 3mg/m3 air

    Hydrogen chloride (HCl)

    Data: Melting Point -114C, Boiling Point -85C, critical temperature 51.4C

    Dissolved in water, this is hydrochloric acid. It is therefore a strong irritant to eyes and mucous membranes.

    Occupational limits to exposure: 5 ppm; Ceiling limit: 100ppm

  2. Fluorine (F2)


    Compound Melting Point Boiling Point Occupational exposure limit
    F2 -220 -188 1 ppm
    HF -83 -19 3 ppm
    OF2 -224 -145 0.05 ppm

    Fluorine is extremely reactive, and will not be encountered in its elemental state in nature under most conditions.

    HF is hydrofluoric acid or hydrogen fluoride; OF2 is oxygen difluoride or fluorine oxide. HF is much more stable than OF2 (the latter is almost as reactive as F2).

    From the exposure standpoint, fluorine and its compounds are very similar to chlorine. Fluoride salts are potent poisons to Earthly life ; the lethal dose of sodium fluoride (NaF) in man is 5-10mg/kg.


These two compounds might be present as unusual taints:

Compound Melting Point Boiling Point
Arsine (AsH3) -116 -57
Diborane (B2H6) -165.5 -92.5

Arsine is used in the semiconductor industry as a dopant. Like most arsenic compounds, it is highly toxic. Levels of 25-50ppm will be fatal within 30 minutes, 250 ppm within a minute.

Occupational levels for all arsenic compounds are about 0.01mg/m3 air (just under 0.01ppm).

Diborane is used in the manufacture of semiconductors and organoboron compounds. It is very reactive and will combine explosively with most oxidising agents (diborane is a good rocket fuel). Low level lung toxicity will ensue at the occupational ceiling levels of 0.1ppm, similar to that seen with ozone.

Arsine and diborane exposures have similar clinical effects to the halogens and their compounds; they are potent irritant gases.

Compounds of selenium, germanium and antimony behave like arsenic (as one might expect from the Periodic Table).

Given their extreme reactivity with water and oxygen, they are likely to be the products of some very unusual biological activity - or be encountered industrially.

Metal Fume Fever

A name applied to a syndrome produced by the inhalation of metals or metal salts.

Dusts of zinc, copper, magnesium, cadmium, nickel,aluminium, chromium, manganese, beryllium, silver and vanadium can lead to the following symptoms: fever, chills, drooling, headache, cough, shortness of breath.

This can progress to an overwhelming acute lung inflammation, which is rapidly fatal, especially in the cases of beryllium, cadmium and nickel.

Low level, long term exposure can lead to a variety of other pathology:

Dose levels are roughly similar to those described generally for particulates.

The mainstay of treatment (apart from prevention) is administration of chelating agents and usual supportive care (remember Airway, Breathing, Circulation).

Closing Remarks and Exotic, Corrosive and Insidious Atmospheres

Many of the compounds described in this series of posts represent distinct fire hazards in the presence of oxygen, or in their own right (strong oxidisers like chlorine, fluorine and fluorine oxide): H2S, AsH3, B2H6, N2O, N2H4, CO, and the hydrocarbons and alcohols.

However, it's certainly possible that they could be present in 'tainted' atmospheres, given the potency of some of the compounds.

Let's look at three more unusual atmosphere types:

  1. Exotic - defined as a gas mix where 'an oxygen supply is required but no protective suit is necessary'.

    So the basic requirements are a hypoxic gas mix (less than 0.01 atm oxygen) and a temperature and pressure range where sealed clothing isn't needed.

    There is no need for a separate 'low oxygen' taint : all such worlds have 'exotic' atmospheres by definition.

    Irritant or chemical asphyxiant gases can only be present at very low levels.

  2. Corrosive - temperature or gas mix necessitates protective clothing. This category combines temperature extremes with high levels of irritant or chemical asphyxiant gases.
  3. Insidious - extreme conditions will defeat portable protective measures within 2-12 hours. As per (2), but worse. Hydrogen is offered as the prototypical insidious atmosphere gas via its diffusion properties and subsequent explosion/fire risk. Other substances that react readily with oxygen are good candidates as well.

    Extremes of temperature and pressure are the most likely candidates (steam and oxygen at multi-atmosphere pressures and temperatures in the hundreds of degrees, or the atmosphere of Venus).


Inert : N2, noble gases, N2O, CO2, hydrocarbons, alcohols

Irritant : O3, NO and higher oxides, NH3, N2H4, Cl2, HCl, HF, F2, OF2, CS2, SO2, B2H6

Chemical asphyxiant : NO, CO, cyanides, H2S

Other toxic effects : radon, N2O, CO2, CS2, PH3, AsH3

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