The representation of the fog in Chapter One, and the novel as a whole, is symbolic of vision and the way that patients such as Bromden are treated and how they experience life on the ward. Note how the fog is described when the fog machine is turned on when Bromden is caught to have his head shaved:
They start the fog machine again and it's snowing down cold and white all over me like skim milk, so thick I might even be able to hide in it if they didn't have a hold on me. I can't see six inches in front of me through the fog...
Fog is something that is linked to obscured vision, and is something that prevents the characters from seeing the world clearly. Bromden always sees fog when he begins to retreat from reality, either because of his medication or through fear, and he actually finds the fog comforting as he believes he can hide in it and ignore reality. The fog is also used to symbolically represent the impact of Nurse Ratched's dominance and routine on the ward, which keeps patients from seeing themselves and the world clearly. McMurphy is the character who seeks to take the patients out of the fog.