The Toxicology Badge is a tier 2 trinket found in the Pond Depths, inside a skeleton located near one of the breakers that opens the Pond Lab, being the one under a large amount of wreckage. Specifically, the skeleton is buried halfway in mud at the end of the hall just before the large room that has the breaker in it.
When equipped, it gives a flat 90% immunity to all Gas Hazards, including the Haze, Stinkbugs, Green Shield Bugs, Large Rotten Food, Gas Arrows, and Super Gas Arrows when worn. It also gives immunity to the dust created by Dust Mites and Termite Soldiers. However, as a tradeoff, it increases the damage caused by all Poison sources by 100%, doubling all damage from poison.