Environmental engineering personnel only sign
Environmental engineering

Kirk and Scalosians in the Enterprise's environmental engineering section.

Environmental engineering (or life support center) was a section aboard Constitution-class starships where all life support activities on the ship were controlled and monitored. This section was located in the primary hull.

In 2266,Leonard McCoy stood in front of a sign that said "environmental engineering personnel only". (TOS: "The Conscience of the King")

In 2268, when the Scalosians took over the Starship Enterprise, they installed a deep-freeze device in environmental engineering with the intention of putting the entire crew into suspended animation. (TOS: "Wink of an Eye")

In the alternate reality, this center was located (in the) aft of the warp nacelles. In 2259, during negotiations to return his crew to him, Khan Noonien Singh threatened to destroy this center if his request was denied. (Star Trek Into Darkness)

In the final draft script of ENT: "Two Days and Two Nights", it was established that Keyla Terrana's husband had been "an environmental engineer."