This type of fog is a poisonous, corrosive vapour released by the Gamemakers in the 75th Hunger Games as one of hourly horrors. It came between two and three, and spread through the jungle. It is mentioned to look like an "after-rain fog" that then grows and becomes thick and acrid. It kills muscles, makes its victims twitch, and kills them. The fog is made by the Gamemakers of the third Quarter Quell.
The effects of the fog include itchy scabs, convulsions, painful pustules, and rapidly atrophying muscular tissue and sometimes death. Katniss Everdeen accounts that when she was helping Peeta escape, the fog had reached one side of his face which caused the muscles on that side to collapse, shudder, and die.
Haymitch gives Katniss a thick, tar-like ointment to treat her scabs and scars and she says that it feels very soothing, even though it turns her skin a sickly green color while it is healing. Soaking in salt water relieves the effects of the fog, though this is a very painful process. Using this method requires that the affected enter the water bit by bit, or else risk intense agony.
Mags, the oldest tribute in the 75th Hunger Games, was the only known tribute to die from the fog. She realizes that Peeta will die unless Finnick carries him so she sacrifices herself. Up until now Finnick had carried her everywhere, but at this point, she knew Peeta would die. She got off Finnick's back, kisses him good-bye, and walked into the fog. Her death was described by Katniss as being like a dance, this shows the fog's power of muscle manipulation. Peeta, Finnick, and Katniss later suffer severe muscle damage by stray droplets of the fog. Mags was notable for this and respected by Katniss Everdeen.