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Critical Wavelength and Broad-Spectrum UV Protection
The UV absorbance of a sunscreen product can be determined in vitro over the entire UV spectrum (290nm – 400nm) using substrate spectrophotometry. The graph below is an example that approximates the absorption curves of a UV-B sunscreen (2-EHMC, 5%), a UV-A sunscreen (TDSA, 5%), a broad spectrum sunscreen such as Zinc Oxide (20%).
The shape of the resultant absorption curve represents the efficiency at which a sunscreen product blocks a given UV wavelength with respect to another. This UV absorbance spectra demonstrates the amplitude and breadth of protection provided (from 290nm - 400nm) across the UV spectrum. The amplitude of the absorption curve reflects the degree of protection. The higher the amplitude of the curve, the more protection provided at that wavelength. Within the UV-B portion of the spectrum this amplitude correlates with the SPF.
The greater the breadth of the curve, the more protection against long-wave UV radiation – in other words, the broader the spectrum of protection provided.
The wavelength below which 90% of the area under the whole absorption spectra curve falls is termed the “Critical Wavelength”. The critical wavelength value is based upon the inherent shape and breadth of the absorbance curve, not its amplitude, and therefore is independent of application thickness. Although SPF and critical wavelength are largely independent of each other, UV-A absorbance must increase commensurate with SPF to maintain the same critical wavelength value.
The critical wavelength determination does not promote the false idea that UV-B and UV-A are separate entities, but rather are part of the continuous electromagnetic spectrum. The critical wavelength for a UV-B sunscreen is less than that for a broad-spectrum (UV-B and UV-A) sunscreen. A higher critical wavelength ensures more UV protection, especially protection from longer wavelength UV-A rays. Calculation of a critical wavelength provides a convenient and reproducible method of evaluating the breadth of UV protection. When combined with in vivo SPF testing, the critical wavelength provides a simple and explicit means of communicating broad-spectrum photoprotection to the consumer.
A critical wavelength > 370nm appears to be a rigorous minimum that sunscreen products should achieve to be labeled truly “broad-spectrum”. A combination of in vivo SPF and critical wavelength provides a complete description of a product’s inherent photoprotective characteristics; SPF describes the amplitude of protection (at a given application rate), and critical wavelength provides a reliable measurement of a product’s absorption capability over the entire UV spectrum. No other efficacy measurements are needed.
In a study of 59 commercially available sunscreen products, only 10 % achieved a critical wavelength of at least 370nm. These products all contained at least one UV-AI filter: zinc oxide, titanium dioxide, or Avobenzone (Parsol 1789). However, not all products with these UV-AI sunscreen ingredients achieved a critical wavelength of 370nm. This proves that effective concentration and appropriate formulation are essential to a sunscreen product’s effectiveness. Simply including UV-AI filters in a formulation does not ensure a true broad-spectrum product.
FDA and Sunscreen Labeling
The sun protection factor (SPF) test is and should remain the singular in vivo method for evaluating sunscreens, and should be communicated to consumers as an SPF no greater than 50. We now know that it is the long ultraviolet rays (UV-AII and UV-AI) that are responsible for photoaging and some skin cancers. Unfortunately, many currently available sunscreens don’t protect against long-wavelength UV-A (UV-AI) insults. The American Academy of Dermatology has endorsed a critical wavelength of 370nm as a measure of effective UV-A protection, and recommended that the FDA use this as a requirement for those sunscreens labeled as “broad spectrum” coverage. This would ensure credibility to the term “broad spectrum”, and ensure that these sunscreens provide adequate protection against both UV-B and UV-A radiation.
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Sunlight & UV rays
SPF Rating
Sunscreen Testing
Critical Wavelength
Sunscreen Ingredients
Photostability
Skin Sensitivity
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