Smoke

A toxic cloud is a rare stage hazard, first appearing in Super Mario 64. When the character walks into a toxic cloud, they will 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 only in the Toxic Maze in Hazy Maze Cave. The cloud covers most of the ground in the Toxic Maze. When Mario, Luigi, Wario, or Yoshi enter an area of the cloud, they will lose one wedge of health on their Health Meter after several seconds. If the player 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 the toxic cloud, however. In the Nintendo 64 version, the gas was greenish-yellow. In the remake, it is purple instead and would actually knock the character back a bit, and it also appears in the middle of the Goomboss Battle stage.

Super Princess Peach
In Super Princess Peach, a similar substance, called simply "smoke" by a Perry 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 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 cloud can not 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.

Trivia

 * In Super Mario 64, if Mario stays in 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 will also play when in the toxic cloud. The coughing sound effect is removed too.