The rules for suffocation read as follows (emphasis mine):
A character who has no air to breathe can hold her breath for 2 rounds per point of Constitution. [...] After this period of time, the character must make a DC 10 Constitution check in order to continue holding her breath. The check must be repeated each round, with the DC increasing by +1 for each previous success.
When the character fails one of these Constitution checks, she begins to suffocate. In the first round, she falls unconscious (0 hit points). In the following round, she drops to –1 hit points and is dying. In the third round, she suffocates.
These rules are clear for when a character simply has no air to breath, such as while being strangled or underwater. But what about a character choosing to hold their breath?
For example, a PC enters a room with an airborne poison and begins to hold his breath. After the number of rounds he can safely hold his breath expires, he begins to make the requisite Constitution checks.
What happens on the character's first failed save? Does he...
- ...fall unconscious and continue to suffocate, not breathing?
- ...fall unconscious and begin to breath, no longer suffocating but inhaling the poison?
- ...remain conscious but breath involuntarily, inhaling the poison?