The “ash” is a harmless stand-in
The drifting particles in Stranger Things’ Upside Down are made from cellulose, the plant-based material that makes up paper—not from soot or toxic ash. Lightweight cellulose fibers can float and tumble through the air, giving the camera the look of a contaminated world while actors work among them.
Why use a paper-like material?
Real ash would be a poor special effect: it is dirty, difficult to control, and could irritate people’s eyes and lungs. Cellulose can be scattered in a controlled way to create the slow, eerie fall the scene needs, then cleaned up after filming. Its movement also helps sell the setting: particles seem to hang in the air, making the Upside Down feel as if it is constantly shedding debris.
The contrast is part of the effect. On screen, the flakes suggest a dangerous environment; on set, they are a practical visual material chosen to create that impression without exposing the cast to actual ash.
