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 potentially kills them. The fog is made by the Gamemakers of the third Quarter Quell.
The effects of the fog include itchy scabs, convulsions, 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, where the affected must enter the body of water bit by bit, else they risk killing themselves because of the huge amount pain brought by this method of healing.