The life support belt was a device worn around the waist that emitted a personal force field which supplied the wearer with the appropriate atmosphere and protected them from the elements. Life support belts were used by Starfleet personnel in the late 2260s and the early 2270s aboard the USS Enterprise. (Star Trek: The Animated Series)
In 2269, Chuft-Captain asked the Kzinti Telepath whether Spock's life support belt was prohibiting usage of the Slaver weapon against him. The telepath replied in the negative, adding that the metal in the belt vibrated. (TAS: "The Slaver Weapon")
Although used exclusively in TAS, the life support belt was invented during the run of Star Trek: The Original Series. Judy Burns, co-writer of "The Tholian Web", thought of using battery-powered "force field belts" in that episode, but the series' producers decided to feature EV suits instead. "They felt strongly that if they started something like a force field belt," explained Burns, "it might have ramifications down the line on other stories. I was a novice in those days, but today I probably would have countered that it was a prototype model that had been given to us this one time. In 25 years, we would get it back again." (Captains' Logs: The Unauthorized Complete Trek Voyages, 3rd ed., p. 72)
In the script for "Beyond the Farthest Star", life support belts are introduced with the description, "The belts are wide, utility-type belts that glow with a pale, lime-yellow aura when turned on. This aura surrounds the wearer, like an envelope of air surrounds the Earth; it supplies air, and being a force-field, prevents harm to the person inside it, as it is almost totally impenetrable."
"Life support system belts" were additionally referred to in the first draft script of "In Thy Image", the story that gradually developed into Star Trek: The Motion Picture. In that script, Scott told Kirk – upon the captain's arrival in a transporter room with Xon and the Ilia probe – that they wouldn't require such belts for transport to Ve-jur, as a gravitational field and an oxygen atmosphere envelope were forming inside the craft. (Star Trek Phase II: The Lost Series, p. 204)
While the Starfleet life support belt was never seen again, there were a few other examples of personal force fields in later spin-offs.
In the novelization of TAS: "The Survivor" (as published in Star Trek Log 2), a Vendorian disguised as Carter Winston is said to have a life support belt that he deactivates moments after being beamed aboard the Enterprise.
In the novelization of TAS: "The Lorelei Signal" (also published in Star Trek Log 2), life support belts are considered for use when, just before beaming down to the unfamiliar surface of planet two of the Taurean system, Kirk asks Spock about them. Under the influence of an hallucination-evoking signal from the planet, however, Spock casually deems use of the belts unnecessary on that occasion.
The Star Trek: Department of Temporal Investigations novel Forgotten History explains that the usage of life support belts was discontinued due to the likelihood that any failures would result in the users' immediate deaths. Environmental suits, on the other hand, gave the wearer a window of opportunity to escape back into a safer environment.