I love how Ai wears a magic 8 ball hat. He has 9 heads total, after all (2 of them ripped off), corresponding to the 9 bodies in the sludge-filled crates. But Ai himself (the melted face) is the core head, so he wears a hat symbolic of the other 8 - which surface when you shake him, like answers in a magic 8 ball. :P  
Another important symbol constantly associated with Ai is snakes. They are his totem of sorts. Like the vertebrae tentacles attached to his heads, they emerge out of corresponding skulls. They are shown slithering out of every pore in his body and they are emblazoned on his magical door.

Some would liken them to a hydra, who grows back two heads for every one cut off, to represent Ai's quasi-immortality. Yet his heads don't grow back. Once they're cut off, they're gone.
Heads & Snakes
Snake Door
Snakes are symbols of medicine and self-renewal. This is reflected in Ai's medical genius and incredible physical resilience. But Ai's snakes are also poisonous.

This theme of toxicity weighs heavy on Ai. The "gas" mask he wears is a constant reminder of it. The noxious sludge that pervades his body and warps his insides defies humanity.

Perhaps it is significant that Haze and Haru are also associated with snakes. Haze has snake tattoos, and is shown holding snake specimens in jars. Haru has a snake bracelet. Since their knowledge allowed Ai to become what he is today, they are his spiritual father and mother.
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DISCLAIMER: all story, art, characters are copyright Q Hayashida. This is a fansite. No copyright infringement is intended. Translations by Gantz Waiting Room, VIZ Media, and sometimes myself. Dorohedoro is published in Ikki Magazine.