The Atmosphere Code is the UWP code to represent the mixture of gases constituting an an atmosphere encountered on a world or moon. Some types of atmospheres require the use of protective clothing or masks.  The code shows the requirements for using protective gear on the world. The Atmospheric Code is a standard Metric.
Atmospheres have a mix of different gases, some in great amounts, while others are barely detectable. Atmospheric mixes include active and inert gases. On Earth, oxygen (02) is active, but forms less than 25% of the total atmosphere. Three times as much is nitrogen (N2), plus a few trace elements such as argon. These inert gases are not usually required by animal or plant life, and take no part in the chemical reactions essential for life (except for nitrogen, a special case). They are, nonetheless, part of the atmosphere. In this AAB article, when we refer to a specific gas such as methane making up an atmosphere, we are referring to the active element, which may make up only a fraction of the total atmospheric mix. 
World Atmosphere Classification Codes Table
As a generality, small worlds have insufficient gravity to maintain an atmosphere and tend to have thin or trace atmospheres while larger worlds grow increasingly dense. Corrosive and insidious worlds tend to be at extreme ends of the tolerable temperature spectrum.
|Atmospheric Code Descriptions|
|Symbol||Code||Specific Description||General Description||Pressure (Atm)||Remarks|
|0||Vacuum||Vacuum||< 0.001||Vacuum requires a vacc suit. The atmosphere has a pressure of less than 0.001 atmospheres, which requires the use of a vacc suit.|
|1||Trace||Vacuum||0.001-0.09||The atmosphere has a pressure of less than 0.1 atmospheres, which requires the use of a vacc suit.|
|2||Very Thin / Tainted||Vacuum||0.10-0.42||Very Thin tainted requires a filter respirator combination|
|3||Very Thin||Vacuum||0.10-0.42||Very Thin requires a respirator. The atmosphere has a pressure of 0.1 to 0.42 atmospheres, which requires the use of a respirator to ensure sufficient oxygen.|
|4||Thin / Tainted||Thin||0.43-0.70||Tainted requires a filter mask. The atmosphere contains an unusual taint such as such as disease, a hazardous gas mix, pollutants, or sulfur compounds which requires the use of a filter mask.|
|5||Thin||Thin||0.43-0.70||Shirtsleeve World: No survival gear required. The atmosphere has a pressure of 0 43 to 0.70 atmospheres. The atmosphere is a standard oxygen/nitrogen mix, which is breathable without assistance.|
|6||Standard||Standard||0.71-1.49||Shirtsleeve World: No survival gear required. The atmosphere has a pressure of 0.71 to 1.49 atmospheres. The atmosphere is a standard oxygen/nitrogen mix, which is breathable without assistance.|
|7||Standard / Tainted||Standard||0.71-1.49||Tainted requires a filter mask.|
|8||Dense||Dense||1.50-2.49||Shirtsleeve World: No survival gear required. The atmosphere has a pressure of 1.50 to 2.49 atmospheres The atmosphere is a standard oxygen/nitrogen mix, which is breathable without assistance.|
|9||Dense / Tainted||Dense||1.50-2.49||Tainted requires a filter mask.|
|A (10)||Exotic||Exotic , Conventional||varies||An unusual gas mix which requires the use of oxygen tanks, but protective suits are not needed.|
|B (11)||Corrosive||Exotic , Conventional||varies||A concentrated gas mix or unusual temperature creates a corrosive environment, which requires the use of a Hostile environment suit or vacc suit.|
|C (12)||Insidious||Exotic, Conventional||varies||The atmosphere is similar to a corrosive atmosphere, but extreme conditions cause the corrosive effects to defeat any protective measures in 2 to 12 hours.|
|D (13)||Dense, high||Exotic, Unusual||2.5 or greater ||Typically no survival gear required under many conditions. Pressure at or below sea level is too great to support life but is breathable at higher altitudes.|
|E (14) †||Ellipsoid †||Exotic, Unusual||0.5 or less ||Typically no survival gear required under many conditions. The world’s surface is ellipsoidal, not spherical. Because the atmosphere remains spherical, surface atmospheric pressure ranges from very high at the middle to very low at the ends. Breathable bands may exist at some point within the range of pressure.|
|F (15) †||Thin, low †||Exotic, Unusual||varies||Typically no survival gear required under some conditions. This world is large and massive, with a thin atmosphere which settles to the lowest levels of the terrain. The atmosphere is un-breathable at most altitudes except the very low ones (…as in depressions or deep valleys).|
† - NOTE: In MgT and T5, Atm type E is "Thin, Low", and Atm type F is "Unusual", which includes (but is not limited to) Ellipsoidal atmospheres.
Protective Measures Based on Atmosphere
The various atmosphere types require specific personal equipment for survival and protection. 
- Codes 0 and 1
- No atmosphere and trace atmosphere require use of a vacc suit and associated support equipment.
- Codes 2, 4, 7, and 9
- Tainted atmospheres require the use of specialized filter masks.
- Codes 2 and 3
- Very thin atmospheres require the use of compressors to insure sufficient oxygen to breathe. The tainted very thin atmosphere requires a combination respirator/filter mask for survival.
- Codes 5, 6, 8
- Thin, standard, and dense atmosphere are breathable without assistance.
- Codes A
- Exotic atmospheres require the use of oxygen tanks, but protective suits are not usually required unless corrosive or insidious.
- Code B
- Corrosive atmospheres require the use of protective suits or vacc suits.
- Code C
- Insidious atmospheres are similar to corrosive atmospheres, but will defeat any personal protective measures in two to twelve hours.
- Codes D, E, and F
- These represent "Unusual Atmospheres" with special conditions such as Ellipsoid Atmospheres. The worlds are partly covered with a breathable atmosphere. The survival gear required depends upon where you are located.
Basic Atmosphere Types
Combinations of the below atmospheres may exist and are not comprehensively listed:
- Corrosive Atmosphere
- Dense Atmosphere
- Ellipsoid Atmosphere
- Exotic Atmosphere
- High Pressure Atmosphere
- High Temperature Atmosphere
- Insidious Atmosphere
- Low Pressure Atmosphere
- Low Temperature Atmosphere
- Standard Atmosphere (Tolerable pressure, temperature, constituent gases, etc. for conventional lifeforms)
- Tainted Atmosphere
- Thin Atmosphere
- Trace Atmosphere
- Unusual Atmosphere
- Vacuum (Interstellar Medium)
- Very Thin Atmosphere
UWP & Atmospheres
The third digit in the UWP identifies the type of atmosphere that the world has. As a rule, only larger worlds have a gravitational field large enough to hold on to an atmosphere, but there certainly are exceptions. An atmosphere is essentially a layer of gas or gasses that cloak the surface of the planet and there are a number of reasons why a world may or may not be lucky enough to have one. As we have already mentioned, a big world has the muscle to keep its atmosphere and the bigger the world, in all likelihood the denser (...or thicker) the atmosphere will be. Conversely, moons and small planets with relatively low surface gravities will generally have only trace atmospheres, if they have anything at all. 
Other factors also come into play. A world sitting close to the main star might have its atmosphere stripped away by the destructive energy of the star's solar wind sweeping past. The only chance a planet has of retaining its layer of gasses in this situation, is if it also benefits from a molten core that is producing a protective magnetosphere. Compare the Earth, whose atmosphere is shielded by its magnetosphere, with Mars. The red planet has a trace or very thin atmosphere that was considerably thicker in ancient times, but as the planet cooled it first lost its magnetosphere and then its atmosphere. 
Yet there are exceptions; in the depths of the solar system, the moon Titan orbits the ringed planet Saturn. Titan is cold, far from the sun and seismically inactive. It has no magnetosphere and with a surface gravity of only 0.14 G should not be able to retain an atmosphere - yet it has a dense atmosphere that is 1.45 times thicker than that of Earth's Ganymede and Callisto, comparable-sized moons of Jupiter, are of a similar composition but do not have atmospheres. One theory suggests that Titan, being farther from the sun than the moons of Jupiter, was colder during its formation. Gasses were trapped in the ice at those low temperatures and later made their way into Titan's atmosphere. 
History & Background (Dossier)
All industrial societies gain an expanded understanding of atmospheres, air pressure, atmospheric constituents, gas behavior, industrial pollutants, and other factors in order to build a successful, modern interstellar starfaring society.
The typical sophont species needs to invent various kinds of protective equipment in order to colonize atmospheres to which it is not native.
Techno-Environmental Limits Atmosphere
Remarks 5, 6 or 8 TL-0 No survival equipment required. 4, 7 or 9 TL-3 Tainted atmospheres require filters. 2 or 3 TL-5 Very thin atmospheres require compressors. 13 (D) or 14 (E) TL-5 Unusual atmospheres require a variety of protective equipment. 0 or 1 TL-8 Trace or vacuum atmospheres require a vacc suit and oxygen tanks. 10 (A) TL-8 Unbreathable atmospheres require oxygen tanks. 15 (F) TL-8 Unusual atmospheres such as ellipsoids require survival equipment under selected conditions. 11 (B) TL-9 Corrosive atmospheres require a variety of capable protective equipment. 12 (C) TL-10 Insidious atmospheres require a variety of heavy-duty protective equipment.
- Advanced Base
- Atmosphere Tester
- Atmospheric Survival Suit
- Body Pressure Suit
- Combat Environment Suit
- Filter Mask
- Filter Suit
- Hazard Suit
- Hostile Environment Suit
- Oxygen Mask
- Oxygen Rebreather
- Oxygen Tank
- Pressure Tent
- Protective Suit
- Rescue Ball
- Tank Pressure Gauge
- Vacc Suit