Squalene
In plain terms
A lightweight oil your skin already makes, used as an emollient. The catch is stability: unlike its hydrogenated cousin squalane, raw squalene oxidizes easily, and the oxidized form is comedogenic and can aggravate acne.
What the evidence shows
Squalene is a light, fast-absorbing oil that your own skin produces as part of sebum, so as an emollient it is well matched to skin. The consideration that sets it apart from its more common cousin, squalane, is oxidation. Squalene carries several double bonds that grab oxygen from air and light, and when it oxidizes it forms squalene peroxides. Those peroxides are the problem: applied to skin they are strongly comedogenic, more so than many ingredients known for clogging pores, and they drive inflammation in acne-prone skin. Squalane is squalene with those double bonds removed by hydrogenation, which is why squalane stays stable and rates non-comedogenic while raw squalene does not. Squalene itself is not toxic, hormone-active, or a cancer concern, and a fresh, well-stabilized product is fine. But because a buyer cannot tell how oxidized the oil in a jar has become, and because the stable squalane version exists, whether plain squalene suits you depends on the formula and how acne-prone your skin is.
On the label
Listed as squalene, and easy to confuse with squalane, the stabilized version that ends in -ane. If your skin is acne-prone, the squalane spelling is the more reliable choice, since it does not oxidize into pore-clogging peroxides the way squalene can.