Protective Pressure

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.

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

Examples:

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    Anime & Manga 
  • Rin experiences the Dramatic Wind invocation in Aoyama Pharmaceutical lab in the first episode of Mnemosyne.

    Film 

    Literature 
  • 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.

    Real Life 
  • 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.