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 of Protective Pressure include:

Anime & Manga

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

Film

• The telepods in The Fly.
• 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

• This is how the Big Bad Laboratory in Michael Crichton's Prey defended itself against Nanomachines. They failed.
  • This was in place to keep the swarms out. This was made by the humans. 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.