Why is there a tiny hole in airplane windows?

The hole in the bottom of your airplane window is actually a very important safety feature. It's all-too-easy to let your mind wander when you're confined to a tiny box of space while hurtling 40,000 feet in the air at hundreds of miles per hour, but rest assured: every single window on the airplane has the same hole. More officially, it's called a breather hole and it's used to regulate the amount of pressure that passes between the window's inner and outer panes. In short, the system ensures that the outer pane bears the most pressure so that if there were a situation that caused added strain on the window, it's the outside panel that gives out (meaning you can still breathe). As shown in the Boeing 737 maintenance manual (the most widely produced jet airliner in aviation history), the window structure consists of three layers of acrylic – a tough, transparent and flexible resin – although only two of them have an actual structural function.

As can be noted in the diagram shown above, the breather hole is located in the middle layer of the window. This little puncture acts as a bleed valve ensuring that the pressure between the last two layers and the cabin always remains the same. This is necessary as a way of preserving the middle layer (the extra safety one) so it is only exposed to severe pressure differences in cases of emergency – that is, if the last layer the window is fractured in some way.

Furthermore, any possible cracks in the outermost layer of the window is enough to justify an emergency landing – even if the middle layer is fully capable, in principle, to maintain the appropriate cabin pressure conditions

The breather hole also serves to prevent freezing and fogging between the outer layers of the window.