Metal oxide sunscreens protect skin by absorption, not by reflection or scattering

• Published in: Photodermatology, photoimmunology & photomedicine Vol. 32; no. 1; pp. 5 - 10
• Main Authors: Cole, Curtis, Shyr, Thomas, Ou-Yang, Hao
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.01.2016
• Subjects: absorption; anatase; band gap; barium sulfate; Humans; Metals - chemistry; Oxides - chemistry; protection; reflection; rutile; scattering; Skin; sunscreen; Sunscreening Agents - chemistry; titanium dioxide; ultraviolet radiation; Ultraviolet Rays; zinc oxide; barium sulfate; ultraviolet radiation; sunscreen; reflection; scattering; absorption; titanium dioxide; protection; anatase; band gap; rutile; zinc oxide
• ISSN: 0905-4383; 1600-0781
• DOI: 10.1111/phpp.12214

Summary Background/Purpose The inorganic metal oxide sunscreens titanium dioxide and zinc oxide have been considered to protect against sunburning ultraviolet radiation by physically reflecting/scattering the incident photons and thus protecting the skin. Earlier publications suggested, however, that the primary action of UV protection by these sunscreen agents is through absorption and not by reflection. The purpose of this work was to quantitate the contributions of each of these modes of action to the protection provided by inorganic UV sunscreen filters. Methods An optical integrating sphere was used to measure the transmission and the reflectance of titanium dioxide and zinc oxide. Results The average range of reflection for zinc oxide and titanium dioxide throughout the UV range was only 4–5% (less than SPF 2), providing minimal UV protection via this mechanism. The remainder of the UV protection is provided by semiconductor band gap mediated absorbance of the UV photons. At wavelengths above the semiconductor band gap absorption energy levels (in the long UVA and visible wavelengths), they are predominantly reflectors of light (up to 60% reflection) and non‐absorbing. Conclusion Titanium dioxide and zinc oxide provide UV protection primarily via absorption of UV radiation and not through significant reflection or scattering.