Photostability in Sunscreens: Importance and Testing Methodology

Photostability is a critical determinant of whether a sunscreen continues to deliver its labeled UV protection under real-world sunlight exposure. While Sun Protection Factor (SPF) and broad-spectrum claims are established at the time of testing, photostability addresses whether those claims remain valid during prolonged UV exposure over the product’s intended use period.

For brands operating in the United States, where sunscreens are regulated as over-the-counter (OTC) drugs, photostability sits at the intersection of product efficacy, safety substantiation, and regulatory risk management. Although not an explicit FDA labeling requirement, photostability and related photobiological safety testing play an important role in supporting durable SPF and UVA protection, as well as demonstrating due diligence in product development.

This article provides a US regulatory-aligned overview of photostability in sunscreen products, why it matters, and how photostability and photobiological safety testing are typically performed. It concludes with guidance on how independent testing can support compliant, defensible sunscreen claims.

What is photostability in sunscreen products

Photostability refers to a sunscreen formulation’s ability to retain its UV absorption performance when exposed to ultraviolet (UV) radiation over time, without significant degradation of its active UV filters.

Importance of Photostability

• Sustained UV protection
  
  Photostable formulations help ensure that labeled SPF and UVA protection are maintained during typical daylight exposure.
  
  
• Risk mitigation and safety
  
  Degradation of UV filters can reduce protection and may generate breakdown products associated with phototoxic or photoallergic responses, influencing the overall safety profile.
  
  
• Formulation integrity
  
  Photoinstability can affect both SPF and broad-spectrum performance, particularly where UVA filters such as avobenzone are insufficiently stabilized.

Regulatory context in the United States

FDA framework for sunscreens

In the US, sunscreens are regulated as OTC drug products by the Food and Drug Administration. Before marketing, manufacturers must substantiate SPF, broad-spectrum, and other performance claims using established testing methods.

Key regulatory elements include:

• SPF testing
  
  Measures protection against UVB-induced erythema compared with unprotected skin under controlled exposure conditions.
  
  
• Broad-spectrum testing
  
  Demonstrates adequate UVA and UVB protection based on critical wavelength criteria.
  
  
• Photostability and photosafety considerations
  
  While photostability testing is not a defined labeling requirement under the current FDA sunscreen monograph, the FDA recommends phototoxicity and photoallergenicity evaluations for active ingredients that absorb light.

Compliance considerations for US market entry

• Performance testing data should reflect expected in-use conditions, including potential UV-induced degradation over time.
  
  
• Photobiological safety data should be documented for UV filters that absorb strongly in the UVA and UVB regions.
  
  
• Photostability data, although not mandatory, can supplement regulatory files and internal quality standards, particularly for broad-spectrum products.

How photostability testing is performed

What photostability testing evaluates

Photostability testing assesses whether UV filters and finished formulations maintain their UV absorption characteristics following exposure to simulated solar radiation. Typical outputs include:

• Changes in UV absorbance spectra
  
  
• Retention of SPF and UVA protection
  
  
• Chemical integrity of individual UV filters

Common testing approaches

In vitro spectrophotometric methods

• Measurement of UVA/UVB transmission through irradiated samples applied to defined substrates
  
  
• Comparison of pre- and post-exposure absorbance curves to quantify degradation

Analytical chemistry techniques

• High-Performance Liquid Chromatography (HPLC) and related methods to quantify chemical degradation of specific UV filters
  
  
• Particularly relevant for filters known to be photolabile without stabilization

In vivo or optics-based assessments

• Techniques such as diffuse reflectance spectroscopy to assess SPF and UVA protection after controlled UV exposure
  
  
• Used selectively to correlate in vitro results with functional performance
  
  

Photobiological safety testing

Photobiological safety testing evaluates whether UV exposure in the presence of a sunscreen leads to adverse skin responses.

Key assessments include:

• Phototoxicity testing – evaluates acute skin reactions following combined product and UV exposure
  
  
• Photoallergy testing – assesses delayed hypersensitivity responses

For products containing light-absorbing actives, these studies form part of a comprehensive safety substantiation strategy aligned with FDA expectations.

Formulation strategy and commercial implications

Formulation optimization

• Photostabilizers
  
  The inclusion of stabilizing agents or complementary UV filters can improve the stability of labile actives.
  
  
• Filter combinations
  
  Certain UV filter systems demonstrate synergistic photostability, while others may reduce overall stability if poorly balanced.

Product positioning and claim support

Although photostability claims are not required by the FDA, brands targeting scientific credibility or risk-aware positioning often invest in third-party photostability data. Demonstrating minimal degradation under simulated exposure can support confidence in broad-spectrum performance over typical daily use durations and reduce regulatory uncertainty when expanded claims are considered.

Frequently asked questions

1. Is photostability testing required by the FDA for sunscreen approval?
   
   Photostability testing itself is not explicitly required for labeling under the current FDA sunscreen monograph. However, the FDA recommends photosafety evaluations for light-absorbing actives, and photostability data can support the scientific rationale for sustained SPF and UVA performance.
   
   
2. How does photostability affect product performance claims?
   
   Photostability influences whether SPF and UVA protection are retained during real-world exposure. UV filter degradation can reduce efficacy and affect safety profiles over time.
   
   
3. What testing strategy provides the most defensible photostability data?
   
   A combined approach using in vitro spectrophotometry, analytical chemistry, and selective in vivo or optics-based assessments provides comprehensive, correlated evidence.
   
   
4. Can photostability data support regulatory submissions?
   
   While not mandatory, well-documented photostability data can strengthen regulatory dossiers and reduce the likelihood of challenges related to broad-spectrum or durability claims.

How independent testing can support sunscreen development

Independent laboratories play an important role in generating objective, regulator-ready data for sunscreen products. The Eurofins network of companies includes laboratories with experience in sunscreen performance testing, photostability assessment, and photobiological safety studies aligned with US regulatory expectations.

Eurofins Consumer Product Testing laboratories support sunscreen brands by providing:

• Photostability testing using in vitro, analytical, and optics-based methods
  
  
• Phototoxicity and photoallergy studies as part of photosafety substantiation
  
  
• Data packages designed to support FDA-aligned performance and safety documentation

Conclusion

For sunscreen brands targeting the US market, photostability is a key scientific consideration linking formulation design, safety substantiation, and regulatory compliance. Integrating photostability and photobiological safety testing into development programs supports credible performance claims and helps demonstrate that labeled UV protection is maintained under real-world conditions.

By working with independent testing partners within the Eurofins network of companies, manufacturers can generate high-quality data to support compliant, defensible sunscreen products throughout their lifecycle.