Smoke

A toxic cloud is a rare stage hazard, first appearing in Super Mario 64. When the character walks into a toxic cloud, they start to take damage at steady intervals.

Super Mario 64 / Super Mario 64 DS
In Super Mario 64 and Super Mario 64 DS, toxic clouds appear in the toxic maze in the Hazy Maze Cave and in the middle of the Goomboss Battle stage, exclusively in Super Mario 64 DS. The cloud covers most of the ground in the toxic maze. When Mario, Luigi, Wario, or Yoshi has his head under the toxic cloud, he loses one wedge of health on his Power Meter after several seconds. In Super Mario 64 DS, if Mario, Luigi, or Wario does not have his own cap on, he will lose one wedge of his health on the first hit, then lose two wedges of his health on the second hit, and the process will repeat. Metal Mario/Metal Wario and Vanish Luigi are not affected by the toxic cloud, however. In Super Mario 64, the gas is greenish-yellow. In Super Mario 64 DS, it is purple instead and actually stuns the character a bit, which can get really annoying.

In Super Mario 64, if Mario stays in the toxic cloud 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 is low on health underwater or in freezing water also plays when in the toxic cloud. The coughing sound effect is removed as well; however, characters still appear to cough (unless Luigi and Wario are using their Power Flower abilities).

Super Princess Peach
In Super Princess Peach, a similar substance, simply called "smoke" by a Message Block, appears in the Fury Volcano levels. Its effect on the player is similar to the toxic cloud in Super Mario 64 DS, dealing knockback when it damages the player, though it deals damage at a slower rate. The smoke can be cleared temporarily by using the Joy Vibe's cyclone to disperse it. The boss of Fury Volcano, Wiggler, can create smoke when its health gets low.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, a toxic cloud appears in the Subspace Emissary. It appears in some of the cave areas and the Great Maze. When the character enters this toxic cloud, they incur damage at a much faster rate than in Super Mario 64 and Super Mario 64 DS, at about 2% of damage per second. This toxic cloud cannot KO the player, since it does not do any knockback, but it does make the character more vulnerable to enemy attacks. The player can temporarily clear the toxic cloud by hitting a red switch often situated near the cloud.

Super Smash Bros. Ultimate
In Super Smash Bros. Ultimate, toxic clouds appear in certain spirit battles.

Names in other languages
Nube tossica