Photobiological Safety Testing: Evaluating Photobiological Safety Beyond SPF Metrics

Sunscreens perform well during short periods of outdoor use. Under strong sunlight, users may observe minimal immediate erythema, potentially leading to the assumption of stable, long-term protection. However, visual assessment alone does not reflect the complete behavior of a formulation under extended UV exposure.

Controlled scientific studies present a more detailed picture. In one investigation, three out of six commercial sunscreens showed significant degradation of a commonly used UVA filter after UV exposure, despite all holding compliant SPF values. This highlights a well-recognized challenge in sunscreen development:

SPF does not indicate how a sunscreen performs under real sunlight, heat, humidity, or long-term exposure conditions.

As consumers increasingly expect protection that lasts throughout the day, manufacturers rely on photobiological safety testing to understand how formulations behave in real-world environments, support safety compliance, and substantiate long-lasting performance claims.

Why SPF Alone Cannot Indicate Long-Term Sunscreen Performance

SPF remains a critical measure of UVB protection, but several performance factors fall outside the SPF test method:

• UVA penetration, linked to long-term skin impacts
  
  
• Photodegradation of UV filters under sunlight
  
  
• High-energy visible (HEV) light and blue light hazard exposure
  
  
• Heat and humidity, which influence filter stability
  
  
• Interactions between filters and other formulation components
  
  
• Exposure time affects how protection changes during the day
  
  
• Artificial optical radiation from LED lighting or beauty devices

Because of these variables, two sunscreens with the same SPF value may behave differently once applied outdoors.

How Photobiological Safety Testing Helps Brands Build Longer-Lasting Sunscreens

1. Understanding UV Photostability

Photostability testing examines how a sunscreen responds when exposed to controlled UV radiation that simulates outdoor sunlight. Using calibrated light sources, laboratories measure:

• Changes in UV filter concentration
  
  
• Shifts in UVA/UVB protection ratios
  
  
• Performance under both natural and artificial light
  
  
• Effects of long-term exposure
  
  

These outcomes help brands understand whether their products maintain effectiveness throughout the day.

2. Identifying Potential Degradation Pathways

During sunlight exposure, certain UV filters may degrade faster than others. Photobiological testing helps reveal:

• Which ingredients lose stability
  
  
• Whether filter interactions reduce protection
  
  
• How long-term UV radiation affects overall performance
  
  
• Any structural changes within the formulation

This information guides formulation improvements and supports product optimization.

3. Supporting Claim Substantiation

Market standards increasingly require formulations to demonstrate:

• Broad-spectrum protection
  
  
• Photostability
  
  
• Long-lasting performance
  

To support these claims, brands require measurable evidence. Photobiological safety testing and optical radiation assessment provide data that helps verify statements about long-term protection and performance under sunlight.

These data can also contribute to internal risk assessments and safety documentation where required.

How Testing Is Conducted

Although methods vary, most UV testing involves three core elements:

1. Use of Calibrated Light Sources

Laboratories simulate natural sunlight using controlled light sources designed to replicate:

• UVA and UVB distribution
  
  
• Visible and HEV wavelengths
  
  
• Conditions similar to typical outdoor environments

Some assessments may incorporate xenon arc equipment during accelerated weathering testing to model prolonged exposure conditions.

2. Defined Exposure Times

To ensure results are traceable and repeatable, exposure time is precisely controlled. This allows consistent evaluation of how protection performance shifts during extended UV exposure.

3. Optical Radiation Analysis

Spectral analysis is used to measure:

• Irradiance
  
  
• Wavelength distribution
  
  
• Changes before and after exposure

These safety measurement methods help determine how effectively a formula maintains protection over time.

Designing Sunscreens for Real-World Conditions

Consumers now use sunscreens in environments that combine sunlight, heat, humidity and exposure to visible and blue light. To develop products that maintain protection across these conditions, beauty brands are increasingly relying on UV testing procedures and photobiological safety data.

Photostability and optical radiation assessments help evaluate how a formulation behaves when exposed to environmental stressors such as high temperatures, perspiration, water contact and shifts between shade and direct sunlight. These tests also provide insight into film formation and how the protective layer changes over time.

For brands developing multifunctional products—such as sunscreens with antioxidants, hydration benefits or visible light protection—these evaluations support evidence-based formulation choices. Integrating test results early in development helps teams refine filter combinations, improve long-term stability and generate clear, defensible claims about real-world performance.

Benefits for Manufacturers and Developers 

1. Enhanced Performance Data 

Testing helps brands:

• Understand real-world sunscreen behavior
  
  
• Identify vulnerabilities early in formulation
  
  
• Optimize ingredient combinations
  
  
• Improve long-term stability

This accelerates R&D and enhances product quality.

2. Substantiated Claim Support

Brands can use testing outcomes to support:

• “Long-lasting protection”
  
  
• “Photostable formulation”
  
  
• “Broad-spectrum coverage across UVA and UVB”
  
  
• Protection under varied natural and artificial light environments

Claims backed by measurable evidence help avoid overgeneralised or unverified statements.

3. Reduced Development Risk

By understanding how a formula behaves under sunlight and other relevant conditions, brands reduce potential issues later in development. Early testing helps streamline product validation, strengthen internal risk assessments, and support long-term quality assurance.

Conclusion

Photobiological safety testing provides brands with a clearer understanding of how sunscreen formulations behave under real-world conditions, where sunlight, heat, humidity and visible light exposure can influence long-term protection. While SPF measures UVB performance, UV testing procedures and optical radiation assessments reveal how protection levels shift during extended use, supporting stronger formulation decisions and evidence-based claims. Integrating these insights early helps develop sunscreens that remain stable and reliable throughout daily wear.

To explore these services, contact Eurofins Cosmetics & Personal Care companies

Frequently Asked Questions 

1. Why is SPF not enough to understand long-term protection?
   
   SPF measures UVB protection but does not reflect how a sunscreen responds to prolonged sunlight, heat, or UVA exposure.
   
   
2. Why do some sunscreens lose effectiveness during the day?
   
   Certain UV filters degrade after UV exposure, and environmental conditions such as heat and humidity can accelerate this.
   
   
3. What testing proves a sunscreen is photostable?
   
   UV photostability testing and optical radiation assessment show how protection changes after controlled exposure periods.
   
   
4. Do sunscreens need to protect against visible or blue light?
   
   Some brands assess protection against high-energy visible light depending on consumer expectations and product positioning.
5. How do brands support long-lasting protection claims?
   
   Testing results from UV photostability assessments, long-term exposure evaluations, and accelerated weathering testing help provide supporting evidence.