Vacuum

A vacuum was a volume that was virtually empty of matter. The most commonly known was that of outer space. Most lifeforms could not survive in a vacuum for very long.

According to the Twelfth Doctor, there was no pressure in the vacuum of space, so one's lungs would explode if they tried holding their breath. Blood vessels would also rupture, and exposed areas of the body would swell up. The boiling point of water was much lower in a vacuum, so one's sweat, saliva and the fluid around their eyes would also boil.

Within 15 seconds, a humanoid exposed to the vacuum of space would lose consciousness, as oxygen bubbles would form in their blood. Within 90 seconds, they would be dead. (TV: Oxygen)

Vacuum survival
There were, however, those who were able to survive in the vacuum of space for extended periods of time or even indefinitely.

Time Lords could survive in a vacuum for limited periods, (TV: Four to Doomsday, The Doctor, the Widow and the Wardrobe, GAME: TARDIS) but extended exposure to the vacuum of space would have adverse physical effects on Time Lords, such as the Doctor. The Twelfth Doctor survived such an exposure, but went blind as a result. (TV: Oxygen)

Daleks, (TV: The Parting of the Ways) Cybermen, (TV: The Moonbase) Olympians, (COMIC: The Life Bringer!) Sensorites, (TV: The Sensorites) and star whales (TV: The Beast Below) were all capable of indefinite survival in a vacuum.

Wirrn could live within a vacuum for a period of years. (TV: The Ark in Space)

Toby Zed was able to survive in a vacuum whilst possessed by the Beast. (TV: The Impossible Planet)

Rutans could survive in a vacuum indefinitely. (PROSE: Shakedown)

Vacuum deaths
Katarina sacrificed herself to help the First Doctor and was blown out into space along with Kirksen. (TV: The Daleks' Master Plan)

Varan was killed in the vacuum of space after the Marshal of Solos destroyed a wall of his Skybase. (TV: The Mutants)

Lynda Moss was killed when Daleks broke the barrier between her and space. (TV: The Parting of the Ways)

Scooti Manista was killed in the vacuum when Toby Zed, possessed by the Beast, broke open one of Sanctuary Base 6's outer walls. (TV: The Impossible Planet)

The Chief Steward on the Titanic spaceship was blown out into space after accidentally opening a door to the outside following a meteorite strike on the spaceship. (TV: Voyage of the Damned)

Ahab was killed when teleported to the vacuum of space in the year 50,000 by Professor Alistair Gryffen's Space-Time Manipulator. (TV: The Bounty Hunter)