Why The Most Overused Movie Trope Is So Aggravating

First of all, air ducts are filthy because, by design, they move around not only air, but also dust particles. In every single movie that uses this trope, however, the HVAC systems behind the air vents are spotless and shiny as if the building in question was just erected, or if the ducts were just cleaned out by a third-party contractor. But it’s worth noting that the National Air Duct Cleaners Association (NADCA) suggests that despite the lack of established industry regulations, it’s a good practice to clean out your system every three to five years. 

Given how most villains aren’t necessarily rule followers, I think it’s safe to say that they’re not maintaining a cleaning schedule while they’re trying to execute their world domination plots. Even if they stayed on top of their building maintenance, five years is a long enough time for dust, dirt, and grime to build up inside of an air vent.