Photostability
Photostability — the ability of an ingredient or formula to remain chemically intact and effective when exposed to light, particularly ultraviolet radiation. In sunscreens it is especially important, because filters that break down in sunlight can lose their protective ability over time.
Key points
- A photostable sunscreen filter maintains its UV-absorbing capacity during sun exposure, while a photounstable one degrades and offers reduced protection.
- Avobenzone is a widely used UVA filter that is prone to degradation but can be stabilized when combined with other filters such as octocrylene.
- Photostability affects not only sunscreens but also active ingredients like vitamin C, which can lose potency when exposed to light.
- Formulators improve photostability through ingredient combinations, stabilizing agents, and opaque or air-tight packaging.
Frequently asked
Why does photostability matter in sunscreen?
Because a filter that degrades in sunlight protects less as the day goes on, photostable formulas help maintain more consistent protection between reapplications.
How can I tell if a product is photostable?
It is not always obvious from the label, but stabilizing combinations and opaque or airless packaging are good signs, and reapplying sunscreen as directed helps regardless.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
