The qualifier 'tainted' simply means that the atmosphere in question contains some element that makes it unbreathable. Most likely it is a harmful gas, such as carbon dioxide, methane and/or hydrogen that most sophonts simply cannot breathe, which has serious side effects (such as nitrogen at relatively high partial pressures) or sulphur compounds, possibly resulting from global volcanic activity. Taints may also include bio-hazards such as pollens or alien pheromones. Taints require a filter mask for human survival. 
Basic Atmospheric Taints
Atmospheric taints are also known as irritants:
- Atmospheres with an imbalance of Ammonia (NH3)
- Atmospheres with an imbalance of Carbon Dioxide (C02)
- Atmospheres with an imbalance of Chlorine (Cl2)
- Atmospheres with an imbalance of Methane (CH4)
- Atmospheres with an imbalance of Organic-biological Taints
- Atmospheres with an imbalance of Oxygen (02)
- Atmospheres with an imbalance of Nitrogen (N2)
- Atmospheres with an imbalance of Sulfur Compounds
History & Background (Dossier)
Tainted: Tainted atmospheres contain some element that is harmful to mainstream sophonts, such as an unusually high proportion of carbon dioxide. A traveller who breathes a tainted atmosphere without a filter will suffer damage every few minutes or hours, depending on the level of taint. 
Selected Atmospheric Taints
- Ammonia Taint
- Carbon Dioxide Imbalance Taint
- Carbon Dioxide (CO2)
- Chlorine or Fluorine Taint
- Carbonyl Fluoride – (COF2)
- Chlorine – (Cl2)
- Chlorofluorocarbon – (CFC)
- Fluorine – (F2)
- Hydro Chlorofluorocarbon (HCFC)
- Hydrogen Taint
- Hydrogen (H)
- Inert Gas Taint
- Argon, Radon, Xenon, etc.
- Nitrous Oxide (N2O)
- Methane Taint
- Swamp Gas – CH4
- Nitrogen Compound Taint
- Nitrogen Trichloride (NCl3)
- Nitrogen Trifluoride (NF3)
- Nitrogen Dioxide (NO2)
- Organic-biological Toxin Taint
- Biologicals, bacteriological vectors, viruses, etc.
- Pollen, Spores, Disease-causing microorganisms, Complex airborne toxins, etc. Antihistamines may serve to reduce allergic reactions.
- VOC's (Volatile Organic Compounds)
- Oxygen Imbalance Taint
- Oxygen compounds
- Ozone (Trioxygen = O3) atmospheric holes
- Pollutant Taint
- Acid Rain, Particulates (Smog), Unburned Hydrocarbons, etc.
- Heavy-metals, Radioactive dust, Toxic smoke from volcanism or industrial pollution, etc.
- Carbon Monoxide, Nitrogen Oxides, etc.
- Lead, Heavy Metals, Radioactives, etc.
- Silicates (very fine dusts)
- Sulfur Compound Taint
- Disulfur dichloride (S2Cl2)
- Hydrogen sulfide
- Sulphur Dioxide (SO2)
- Sulphur Tetrachloride
- Sulfur Trioxide
References & contributors (Sources)
- 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.
- Author & Contributor: Lord (Marquis) and Master Scout Emeritus Adie Alegoric Stewart of the IISS
- Author & Contributor: Jonathan Sherlock
- Author & Contributor: Lord (Marquis) and Master of Sophontology Maksim-Smelchak of the Ministry of Science