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.
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