- “Danger ahead! Beware of the strange toxic cloud! Don't breathe it! If you feel faint, run for higher ground and fresh air!”
- —Sign outside Hazy Maze, Super Mario 64 DS
Toxic Gas is a rare stage hazard appearing in Super Mario 64, Super Mario 64 DS, and Super Smash Bros. Brawl. When the character walks into an area of toxic gas, they will start to take damage at steady intervals.
In Super Mario 64 and Super Mario 64 DS, Toxic Gas appeared only in the Toxic Maze in Hazy Maze Cave. The gas covered most of the ground in the Toxic Maze. When Mario, Luigi, Wario, or Yoshi entered an area of gas, they would lose one wedge of health on their health meter after several seconds. If one of the characters stays in too long, they will lose all of their health and lose a life. Metal Mario/Metal Wario and Vanish Luigi are not affected by toxic gas, however. In the Nintendo 64 version, the gas was yellow. In the remake, it was violet instead and would actually knock the character back a bit, and it also appears in Goomboss's stage.
In Super Smash Bros. Brawl, Toxic Gas appears in the Subspace Emissary. It appears in some of the cave areas and the Great Maze. When the character enters this toxic gas, they will incur damage at a much faster rate than in Super Mario 64/Super Mario 64 DS, at about 2% of damage per second. This toxic gas could not KO the player, since it does not do any knockback, but it does make the character more vulnerable to enemy attacks. The character could temporarily clear toxic gas by hitting a red switch often situated near the toxic gas.
- In Super Mario 64, if Mario stays in Toxic Gas for a period of time without moving, he will start to cough, even as Metal Mario.
- In Super Mario 64 DS, the same alarm sound that plays when a character's low on health underwater or in cold water will also play when in the toxic gas.