Extreme Atmospheres: Some of the more difficult atmosphere types to explain and describe are the 'extremes', types A, B and C which equate to Exotic, Corrosive and Insidious.
- Whereas the Very Thin, Thin, Standard and Dense atmospheres are generally Earth-like in pressures with Earth-like gases (Nitrogen-Oxygen-Carbon Dioxide mixtures) in a variety of amounts, the 'extremes' are terribly toxic, horribly lethal and utterly alien.
- The three types are really grades of lethality, and an A type and C type atmosphere may actually contain the same poisonous gas, just in different amounts. 
Lethality of Extreme Atmospheres Atmo. Code Class Name Danger Remarks A Exotic, Conventional Exotic Atmosphere DANGEROUS A gaseous mixture, which is unbreathable by mainline conventional life sophonts. B Exotic, Conventional Corrosive Atmosphere HIGHLY DANGEROUS Not only a dangerous gaseous mixture, but may have dangerous pressure, an temperature qualities in addition to corrosive (acidic) properties. C Exotic, Conventional Insidious Atmosphere EXTREMELY DANGEROUS Even more dangerous than a Corrosive Atmosphere, with persistently lethal properties that will kill most mainstream sophonts within minutes and destroy most protective equipment within two to twelve hours. D, E, F Exotic, Unusual Unusual Atmosphere DANGEROUS Unusual Atmospheres have livable respiration zones, typically at the heights or depths of a topography. They are rare worlds with very unconventional atmospheres such as Ellipsoid Worlds.
Basic Exotic-Extreme Atmosphere Types
- Extreme Low Temperature
- Very Thin, Low Temperature
- Very Thin, Moderate Temperature
- Very Thin, High Temperature
- Thin/Standard/Dense, Low Temperature
- Thin/Standard/Dense, Moderate Temperature
- Thin/Standard/Dense, High Temperature
- Very Dense, Low Temperature
- Very Dense, Moderate Temperature
- Very Dense, High Temperature
- Extreme High Temperature
Common Corrosive Atmospheric Components
Gases which give an atmosphere an extreme label include carbon dioxide, methane, nitrogen, sulphur, oxygen, hydrogen, ammonia, chlorine and fluorine. These gases are often found in combination with others on the list potentially mixing together with additional dangerous effects into one nasty airborne cocktail! 
Exotic or Unusual Atmospheres are most noted for widely diverging from the mostly acceptable spectrum of atmospheric temperature, pressure, and gas compositions from the more conventional codes for atmospheres well suited to the major races and conventional life. They typically have atmospheres very divergent from the roughly 80% Nitrogen and 20% Oxygen mixtures of Terran-like life-supporting worlds.
- Marc Miller. Worlds and Adventures (Game Designers Workshop, 1977), 5.
- J. Andrew Keith. Exotic Atmospheres (Game Designers Workshop, 1983), 1-16. (Special Supplement 2 from the periodical Journal of the Travellers' Aid Society 17)
- Marc Miller. Referee's Manual (Game Designers Workshop, 1987), 22.
- Joe Fugate, J. Andrew Keith, Gary L. Thomas. World Builder's Handbook (Digest Group Publications, 1989), 64-68.
- Terrance McInnes, Dave Nilsen. World Tamer's Handbook (Game Designers Workshop, 1994), 11-17.
- Jon F. Zeigler. First In (Steve Jackson Games, 1999), 70,72.
- Paul Drye, Loren Wiseman, Jon F. Zeigler. Interstellar Wars (Steve Jackson Games, 2006), 125-6.
- Gareth Hanrahan. Mongoose Traveller Main Rulebook (Mongoose Publishing, 2008), TBD.
- Marc Miller. T5 Core Rules (Far Future Enterprises, 2013), 409.
- Paul Elliott. The Universal World Profile (Zozer Games, 2016), TBD.
- Traveller Wiki Editorial Team
- Author & Contributor: Lord (Marquis) and Master of Sophontology Maksim-Smelchak of the Ministry of Science