The Diaper's Outer Cover Is Full of Tiny Holes?! The Secret of the Breathable Backsheet That Blocks Water but Lets Air Through

The outer surface of a diaper is actually full of tiny holes. And yet no water leaks out. How can that be?

A diaper's outermost layer, the backsheet, is a PE film — vinyl. It's punched with millions of microscopic holes invisible to the eye.

The holes are smaller than water droplets

The secret is the size of the holes. They're smaller than liquid water droplets but larger than water vapor molecules. So urine can't pass through, but vapor escapes. Waterproof, yet breathable — the same principle as a raincoat.

Why diaper rash decreases

Thanks to this film, moisture escapes from inside the diaper, and diaper rash (diaper dermatitis) decreases too. That's why companies wage a hidden war over breathability, competing on pore size and density. Everyone remembers heat rash after diapering in summer, right?

3-line recap

A diaper's outer backsheet is a PE film punched with microscopic holes.
The holes are smaller than water droplets but larger than vapor — blocking liquid while letting vapor through, just like a raincoat.
Escaping moisture means less rash, and companies compete on pore size for breathability.

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