As with variants in gravity, a change in atmospheric conditions can cause major problems for characters. Unfortunately, not every planet will have the same atmospheric density or chemical composition as Earth, meaning that worlds otherwise hospitable to human life may not be ideal for humans born and raised on Earth.
Various atmospheric conditions (and their effects) are presented below.
Some atmospheres (breathable or not) contain corrosive chemicals and gases. Corrosive atmospheres slowly eat away at foreign equipment and can cause significant equipment failure. The corrosion can be particularly troublesome in atmospheres that demand special survival gear, as any breach in a protective environmental suit renders it useless. Unprotected equipment exposed to a corrosive atmosphere takes 1d4 points of acid damage per hour of exposure. This damage ignores hardness and deals damage directly to the equipment, eating away at it slowly.
Creatures not wearing protective gear in a corrosive atmosphere take 1d4 points of acid damage per round of exposure.
Planets with thin atmospheres have less oxygen per breath than the standard Earth atmosphere. Many thin atmospheres are the equivalent of being at a high elevation on Earth, such as on top of a mountain or in the upper atmosphere. A creature exposed to a thin atmosphere must succeed on a Fortitude save (DC 20) every hour. On the first failed save, the creature is fatigued. A fatigued creature that fails a subsequent save becomes exhausted for as long as it remains in the thin atmosphere. After 1 hour of complete, uninterrupted rest in a normal atmosphere, an exhausted creature becomes fatigued. After 8 hours of complete, uninterrupted rest, a fatigued creature is no longer fatigued.
Thick atmospheres are those that contain a more dense concentration of certain elements, like nitrogen, oxygen, or even carbon dioxide, than the standard Earth atmosphere. These dense atmospheres sometimes contain a different balance of elements, while others simply contain a higher number of gas particles in each breath. The effects of exposure to a thick atmosphere are similar to those of a thin atmosphere (see Thin Atmosphere, above), except the Fortitude save DC is 15 instead of 20
Some atmospheres (breathable or not) contain toxic gases that are debilitating or lethal to some or all forms of life. The atmosphere is treated as always containing a type of inhaled poison.
Despite some popular myths, moving into a vacuum does not cause the body to explosively decompress, nor does it cause instant freezing as heat bleeds away from the body. Rather, the primary hazards of surviving in the vacuum of space are the lack of air and exposure to unfiltered ionizing radiation.
On the third round of exposure to vacuum, a creature must succeed on a Constitution check (DC 20) each round or suffer from aeroembolism ("the bends"). A creature that fails the save experiences excruciating pain as small air bubbles form in its bloodstream; such a creature is considered stunned and remains so until returned to normal atmospheric pressure. A creature that fails the Constitution check by 5 or more falls unconscious.
The real danger of vacuum comes from suffocation, though holding one's breath in vacuum damages the lungs. A character who attempts to hold his breath must make a Constitution check (DC 15) every round; the DC increases by 1 each round, and on a successful check the character takes 1 point of Constitution damage (from the pressure on the linings of his lungs). If the check fails, or when the character simply stops holding his breath, he begins to suffocate. In the next round, he falls unconscious with 0 hit points. The following round, he drops to -1 hit points. On the third round, he drops to -10 hit points and dies.
Unfiltered radiation bombards any character trapped in the vacuum of space without protective gear. A creature exposed to this ionizing radiation suffers from severe sunburn as well as the effects of radiation exposure; the degree of exposure depends on the nearest star's classification (see Star Systems below for more information).
The sudden decompression of a starship, vehicle, or other object can be dangerous to creatures inside. Whenever a sealed environment within a vacuum is breached, all of the air inside rushes out quickly to equalize the air pressure. Creatures within the decompressing environment must succeed on a Reflex save (DC 15) or be thrust toward the breach (and possibly beyond it) at a speed of 60 feet per round. Creatures that are three size categories larger than the breach's size category are big enough not to get dragged toward the breach (no Reflex save required). For example, a Fine breach pulls only Fine, Diminutive, and Tiny creatures toward it; creatures of Small size or larger are unaffected.
If the breach's size category is larger than the creature's size category, the creature passes through the opening and is blown out into the vacuum. If the breach's size category is the same as the creature's size category, the creature is blown out into the vacuum and takes 1d6 points of damage as it gets pushed through the breach. If the breach is one or two size categories smaller than the creature's size category, the creature isn't thrust into the vacuum but takes 2d6 points of damage as it slams against the area around the breach. It takes another 2d6 points of damage each round until the air completely evacuates from the decompressed compartment or until the creature pulls itself away from the breach with a successful Strength check (DC 20).
The time it takes for all of the air to evacuate from a compartment depends on the size of the breach and the volume of the decompressing compartment, as shown in Table: Decompression Times.
Once the air has completely rushed out through the breach, the pressure equalizes and the interior environment becomes a vacuum.
Table: Decompression Times
Breach Size Decompression Time
Fine (1-inch square) 3 rounds per 10-foot cube of air
Diminutive (3-inch square) 3 rounds per 10-foot cube of air
Tiny (6-inch square) 2 rounds per 10-foot cube of air
Small (1-foot square) 2 rounds per 10-foot cube of air
Medium (2 1/2-foot square) 1 round per 10-foot cube of air
Large (5-foot square) 1 round per 10-foot cube of air
Huge (10-foot square) 1 round per 20-foot cube of air
Gargantuan (15-foot square) 1 round per 30-foot cube of air
Colossal (20-foot square) 1 round per 40-foot cube of air
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Hazards Of Space Travel
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