Phaser coolant was a substance used to prevent shipboard phasers from overheating. The gaseous material was toxic, and prolonged exposure could be fatal.
In 2266, the phaser coolant seal on the USS Enterprise was breached due to damage caused by an attacking Romulan Bird-of-Prey, causing coolant to fill the phaser control room. Both crewmembers manning the phasers, Lieutenants Tomlinson and Stiles, were rendered unconscious by the fumes. Although Stiles was saved by Spock, Tomlinson died as a result of his exposure. (TOS: "Balance of Terror")