Hazmat zombies are zombies in zombies ate my friends. They wear yellow hazmat suits. Although they are of the generic zombie type, the game counts them as mini-bosses. They have more health than the average zombie and pack a bit of a punch but are more likely to drop items when killed. They are encountered from episode 2 onwards.
While the origin of their presence is unknown, they were most likely part of a big hazmat team sent into the city as part of a special cleanup operation. One thing to note is that half of the suit's visor has been torn away, implying that some,if not all of the infected hazmat members were infected when a zombie/zombies ripped their visors and bit/scratched their faces.