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Protective Pressure

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Whenever the heroes enter a Mad Scientist Laboratory in an unconventional way, they'll experience slight air draft coming from inside (or the other way around). Same thing happens whenever a science-y MacGuffin is opened with a slight hiss.

In Real Life, keeping some 1.2 atmospheric pressures inside is part of airtight sealing of laboratories against dust and other dirt from the outside: even if dust manages to find a crack in the sealing, it'll be blown right out by the pressure difference. On the other hand, labs dealing with dangerous micro-organisms (bio-hazard levels 3 and 4) require the lab to maintain a negative pressure difference to suck whatever nastiness is developed there right back inside. This trope covers both cases.

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In fiction, it is mostly used to invoke some Dramatic Wind appropriate for the occasion.


Examples:

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    Anime & Manga 
  • AKIRA: the holding cell of the eponymous character is often sealed with the freezing negative version, across the adaptations.
  • BioMeat: An inexplicitly still functioning (solar panels) general stores has the positive version. The characters comedically startle when the air rushes out at them as they're used to living in a blighted hellscape at that point.
  • Rin experiences the Dramatic Wind invocation in Aoyama Pharmaceutical lab in the first episode of Mnemosyne.

    Film 
  • The telepods in The Fly do this by design, otherwise something horrible could have happened much earlier.
  • Used in Small Soldiers when the two company executives visit the company that made the titular toys' chips (Played for Laughs when the down-to-earth toy maker gets surprised and his Corrupt Corporate Executive friend has to explain "Don't worry, that's meant to happen.").
  • When the door to Nicodemus' study opens in The Secret of NIMH, Mrs. Brisby is met with a blast of wind and a blinding light. Not sure why that would happen, except that it looks cool.
  • Mentioned as a feature of the secret research facility in The Andromeda Strain.

    Literature 
  • Worm: Weaver noticed this when she gained her powers and is able to explain the phenomena when it's weaponized by a supervillain.
  • This is how the Big Bad Laboratory in Michael Crichton's Prey defended itself against Nanomachines. They failed. Mainly because the swarms took control of the humans. Is it any wonder it failed?
  • The X-Wing Series of books has the Imperial labs on Coruscant that were developing the Krytos Virus designed this way. In case of a containment leak and someone came in, the air would flow in instead of out, preventing the virus from leaving the building.
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    Live Action TV 
  • The "negative pressure" bit is used in NCIS when HQ suffers a Y. pestis bio-attack.

    Video Games 
  • In Virtue's Last Reward, a manual in the Pressure Exchange Chamber states that Rhizome-9 keeps its atmospheric pressure higher than its surroundings to keep out the Radical-6 virus. This is only partly true; the atmospheric pressure is higher inside the facility than outside because it's on the moon.

    Western Animation 
  • ThunderCats (2011): A tremendous amount of air pressure is released when Grune forces open a door to Mumm-Ra's suspended animation chamber.

    Real Life 
  • This is the idea behind Air supported structures the the internal atmosphere is enough to keep it upright.
  • Many stores with air-conditioning use a the positive variation of this trope to keep bugs and dust from getting in. Useful for cooling off on a hot day.
  • The smoking salon on the Hindenburg was kept at positive pressure to prevent any wayward hydrogen atoms from sneaking in. In the end it didn't help but it was a nice idea.

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