Photostability

Photostability describes whether an ingredient survives light unchanged. A photostable molecule absorbs energy and releases it harmlessly; a photolabile one breaks down, and once it has broken down it is no longer doing whatever it was in the formula to do.

The concept matters most for sunscreens, where the ingredient's entire job happens under the light that degrades it. Avobenzone is the best-known example of a filter that needs help: paired with a stabilising partner such as octocrylene or a photostable filter, it holds up far better than it does alone. This is one reason sunscreen filter lists are long, and one reason reapplication is advised regardless of what the SPF number says.

• Antioxidants such as vitamin C and some plant extracts also degrade in light, which is why they arrive in opaque or airless packaging.
• Packaging is part of the answer, not an afterthought — a clear jar undoes some of what a careful formula achieved.
• A product's colour darkening over time is often the visible end of this process.

Nothing on a label states that a formula was photostability-tested, so this is background for interpreting advice rather than something to verify at the shelf. Storing products out of direct sun and reapplying sunscreen through the day are the parts within a reader's control.