In vitro sunscreen photostability testing

Detecting instability to correctly predict in vivo sunscreen performances

Photostability is essential to sunscreen effectiveness, as organic UV filters naturally degrade under sunlight. While some photodegradation is expected, instability due to filter-filter or filter-carrier interactions can accelerate SPF loss, disrupting protection. UV-B filters mainly determine SPF, but UV-A filters are critical for preventing long-term skin damage. Photoinstability may cause specific or broad-spectrum protection loss and is influenced by both the product’s composition and irradiation dose. ISO 24443:2021 now sets a maximum irradiation dose for in vitro testing, helping to benchmark photostability. Photoinstability manifests in distinct spectral patterns—flattened spectra, targeted wavelength drop, or overall absorbance loss—while stable formulations, especially with physical filters, show minimal degradation.