Traditional Chinese medicine describes the causative factors as external climatic factors, volatile emotions and improper diet that lead to internal heat in the blood, or xue.
EoR thinks that any large scale study of acne vis-a-vis these factors would quickly establish the truth of these claims, particularly since acne is most common in teenagers. He's convinced there will also be "volatile emotions", "improper diet", and there will definitely also be "external climatic factors" present. But a causal link?
During puberty the warm yang of the body is said to be exuberant and will cause this heat in the blood to move to the exterior of the body. Local stagnation of qi and xue occurs along the large intestine and stomach channels on the face, upper back and chest. [...] Emotional excess or frustration that constrains the emotions will lead to the emotions being transformed into fire. Fire and heat are said to blaze upward, giving rise to small red eruptions.
Hence, since hot air rises, the pimples appear on the head. Presumably.