Poison Gas is an obstacle that Link encounters in Spirit Tracks. The Dark Purple gas appears throughout the Forest Temple and if Link comes into contact with the gas, he will lose a 1/4 heart of damage. After acquiring the dungeon item, the Whirlwind, Link can blow the poison gas away, allowing him to walk by areas that were previously blocked off.
The enemy Vengas that appear throughout the Forest Temple will emit poison gas after they have been defeated. Furthermore, Poison Gas can be found during the dungeon boss battle with Stagnox. The poison gas protects Stagnox' backside and Link will need to blow away the gas so that he can reveal its weak spot.
Poison Gas on the backside of Stagnox.