Life support belt

A Life support belt was a type of portable protective technology in use experimentally within the Federation Starfleet in the mid-23rd century.

Specifics
A gray, metallic belt worn at the waist generated a glowing personal force field. Within the field, pressure, heat and atmosphere remained at class M conditions, allowing the wearer to travel in space, underwater, or in other hostile climates without needing an environmental suit. A charged belt would last for four hours. They were disallowed in combat situations, as a phaser beam could potentially collapse the force field. Starfleet distributed belts on an experimental basis aboard various vessels. 

An internal belt radio transmitted speech through the force field or between users, allowing conversation without the need of a communicator even in space. However, the field blocked any sense of touch. There was a known upper limit to its pressure tolerance: it could not withstand crushing pressures in a deep ocean. 

Engineering and medical personnel trained in life support technology could repair a damaged belt. 

In practical use, the force fields not only withstood prolonged phaser fire for several seconds, but they also endured crushing pressure that would kill a Human. Significantly, they were unaffected by energy absorbers that disabled lights and deactivated equipment aboard spacecraft. They were also versatile enough to be worn over early 2270s security officer uniforms. 

The normally greenish-yellow glow would flare brighter to signal an impending failure of the force field. 

The belts were not available commercially. 

Service history
In the late 2250s, the USS Aeolus was attacked by Orion pirates. The ship was nearly lost, and afterwards second officer James T. Kirk discovered a life support belt hidden in the luggage of Cecilia Simons, a visitor he then suspected of being a saboteur. 

In 2269, life support belts were worn by a landing party to explore an insectoid pod ship exposed to open space. They were also worn by the crew after life support was partially shut down by the magnetic organism. It saved the life of Montgomery Scott, who was wearing one when the organism slammed the engineering core hatch on him. Both James T. Kirk and Spock wore belts during their final confrontation with the entity, which protected them from some of the effects of sustained phaser fire from the automated bridge defense system. 

A Vendorian impersonating beamed aboard the USS Enterprise (NCC-1701) from his disabled transport while wearing an activated belt. 

Before a landing party from the Enterprise beamed down to in the Taurean system, life support belts were discussed but deemed unneeded. 

Kirk felt that trying to swim underwater with a belt was too awkward. It was one reason Kirk preferred to use an aquashuttle while exploring the water world in 2269. 

When Kirk and Spock were mutated into water breathers on Argo, Scott wore a belt to talk with them underwater. To treat Kirk and Spock aboard the Enterprise within an aquarium tank, Christine Chapel and Leonard McCoy wore belts. 

In 2270, Nyota Uhura, Hikaru Sulu and Spock wore life support belts while searching for a Slaver weapon on a Beta Lyrae planetoid in the Beta Lyrae star system. Kzinti type-2 phasers were able to penetrate the force fields and stun the wearers. Although a Slaver energy absorber shut down a police web and lights aboard the Kzinti's Traitor's Claw, it failed to deactivate the belts. 

In 2273 of the  continuity, life support belts would have been used when stepping outside the Enterprise to explore V'Ger, but they became unnecessary after an oxygen atmosphere formed around both vessels. 

Eventually the belts were discontinued on the notion that any belt failure could kill its wearer, whereas a puncture to an environmental suit would give the wearer time to repair it or retreat to safety.