Final answer:
A hardened sebum that completely clogs an oil duct without surface follicle openings creates a closed comedo, also known as a whitehead. Proper skin cleansing can prevent comedones, while infected comedones may lead to severe acne.
Step-by-step explanation:
When sebum hardens and completely clogs an oil duct so that there are no surface follicle openings, it creates a closed comedo. Closed comedones, also known as whiteheads, occur when the follicle is completely blocked, preventing sebum from reaching the surface of the skin. The overproduction of sebum and the accumulation of dead skin cells can lead to the development of both whiteheads and blackheads (open comedones). Blackheads have a dark appearance due to the oxidation of lipids in sebum when exposed to air.
It's important to maintain proper hygiene by cleaning the skin daily with mild soap to prevent the excessive buildup of oil and dirt and thus reduce the risk of acne. If the comedones become infected by acne-causing bacteria such as Propionibacterium acnes, it can lead to inflammation and potentially more severe acne lesions including papules, pustules, and cysts.