A Thin Atmosphere is a atmosphere with a standard oxygen/nitrogen mix, which is breathable without assistance.
An atmosphere rated as Thin has a mean surface pressure ranging from 0.43 atm to 0.70 atm.
Surface Pressure & Water Properties
History & Background (Dossier)
At altitudes above 2,500 meters on Terra, the air pressure drops to around 0.70 that of sea level. This is equivalent to the IISS Scout Code atmosphere type of Thin. Sophonts who are not adapted to such conditions can suffer from altitude sickness, with headaches, nausea, extreme tiredness, breathlessness and body pain. Associated high blood pressure can cause complications for pregnant women. 
References & Contributors (Sources)
- Marc Miller. Worlds and Adventures (Game Designers Workshop, 1977), 5.Marc Miller, Robert Eaglestone, Don McKinney. Worlds and Adventures (Far Future Enterprises, 2019), 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. Core 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