Glossary

Photostability

Written by Reviewed by the SYNC editorial team Published Last reviewed Editorial standards

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.

Encyclopedia

Further reading

This entry was written and checked against the sources below. They are published by clinical and scientific bodies, they are listed most readable first, and each one opens in a new tab. They are background for the whole entry rather than footnotes to individual sentences.

  1. Introduction to Sunscreens and Their UV Filters - Review of Fate, Exposure, and Effects of Sunscreens in Aquatic Environments and Implications for Sunscreen… StatPearls (NCBI Bookshelf) Reference text ncbi.nlm.nih.gov
  2. Questions and Answers: FDA posts deemed final order and proposed order for over-the-counter sunscreen U.S. FDA Regulatory and safety fda.gov
  3. Sunscreen: How to Help Protect Your Skin from the Sun U.S. FDA Regulatory and safety fda.gov
  4. Ultraviolet Filters: Dissecting Current Facts and Myths PubMed Central Peer-reviewed, open access pmc.ncbi.nlm.nih.gov
  5. A short review of alternative ingredients and technologies of inorganic UV filters PubMed Peer-reviewed (abstract) pubmed.ncbi.nlm.nih.gov
  6. Synergy effects between organic and inorganic UV filters in sunscreens PubMed Peer-reviewed (abstract) pubmed.ncbi.nlm.nih.gov

SYNC does not publish medical advice. Nothing here replaces a consultation with a doctor or a pharmacist about your own skin.