Sebum

Sebum is composed of fats and other lipids and reaches the surface mainly through hair follicles. On the skin it blends with sweat and dead cells to form a thin protective film over the barrier. When sebum is abundant, skin can look shiny or feel greasy, particularly on the face and scalp. It plays no direct part in cooling, but it shapes how sweat spreads and how skin feels to the touch. Because it is oil rather than water, it does not evaporate and so lingers on the surface longer than sweat. Its production tends to follow hormonal rhythms rather than the temperature swings that govern sweating. The film it forms is part of what keeps skin from drying out and cracking. It can also carry a faint scent and mix with sweat, contributing to how skin smells. Distinguishing it from sweat matters because the two look similar on the surface but do very different jobs.

The slick feel on the nose in warm weather is often sebum rather than sweat, though the two can be hard to tell apart by touch. One clue is that sebum leaves a lasting oily film, while sweat that beads up will cool and eventually evaporate. Blotting the nose lifts sebum away, but it does not lower the skin's temperature the way evaporating sweat would.

Sebum is produced by sebaceous glands and is chemically distinct from the watery fluid made by eccrine glands. It travels through the same follicles used by apocrine glands.