Each human has two of these, unless removed or destroyed.
Each lung accounts for 50% of a person's 'breathing'. While it is possible to survive with only one, if the remaining one is destroyed or removed the colonist (or raider etc.) will die.
Lungs can be removed from colonists, prisoners or guests, to be sold, or transplanted into somebody else. Warning! Removing a lung from a guest or prisoner will decrease faction goodwill by 20 points down to the cap of -100. It always, even if it's from your own colonist, gives the victim -30 mood for ? time and the other colonists -5 mood for ? time. unless the organ was damaged? also is there a damage breaking point, or will just any damage not give mood penalties anymore?