Tridium

From Memory Alpha, the free Star Trek reference

Tridium is a gaseous element or compound that is highly toxic in concentrated doses. Though not immediately fatal, tridium poisoning is usually deadly within four years of exposure to the compound.

In 2369 Jadzia Dax suggested flooding the central core of Deep Space 9 with ionized tridium gas in order to detect the source of an unknown power drain. (DS9: "Q-Less")