Poison fog

Volcano Smog is an obstacle first found in New Super Mario Bros. Wii, normally seen as a enormous red-and-black cloud. It normally moves to the right very slowly. It is possibly based on a pyroclastic flow.

New Super Mario Bros. Wii
Volcano Smog first appeared in New Super Mario Bros. Wii, where it is solely found in World 8-1. If Mario touches it, it would make him lose a life regardless of powerup. However, neither the Raining Debris nor the Buzzy Beetles were affected by it.

New Super Mario Bros. U/New Super Luigi U
Volcano Smog reappeared in New Super Mario Bros. U in Magma-River Cruise, where it behaved the same way. No enemies are affected by it, still. It also appeared in the challenge Mole to Goal Returns and in New Super Luigi U's Magmaw River Cruise.

Super Mario 3D World
Volcano Smog is set to return in Super Mario 3D World, where instead of moving right, it will move down.

Trivia

 * Volcano Smog may possibly be based off a pyroclastic flow. However, it is slower than a real-life pyroclastic flow.