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

Abstract

SUMMARY Background/PurposeThe 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.

A review of inorganic UV filters zinc oxide and titanium dioxide

“…These particles provide photoprotection mostly be absorbing UV radiation; a small degree of scattering may also occur. 7,8 In the United States, nanoparticles are regulated by the FDA with a broad definition that does not differentiate between variably sized particles smaller than 100 nm. 10 This inclusive definition does not discriminate amongst the different characteristics of nanoparticles of even the same molecule such as size, shape, and surface area.…”

“…7 These molecules absorb, reflect and refract UV photons but function in photoprotection primarily by absorbing UV radiation. 8 The initial formulations of mineral filter-containing sunscreens often left a white, chalky…”

Quantifying the visual appearance of sunscreens applied to the skin using indirect computer image colorimetry

“…This poses an additional challenge in product formulation. Inorganic sunscreens, which were previously felt to primarily scatter ultraviolet radiation, have been demonstrated to actually provide UV protection primarily via absorption . The challenge remains to formulate a sunscreen able to provide protection in spectral homeostasis with the sun with minimal visible light reflection.…”

“…Inorganic sunscreens, which were previously felt to primarily scatter ultraviolet radiation, have been demonstrated to actually provide UV protection primarily via absorption. 25 The challenge remains to formulate a sunscreen able to provide protection in spectral homeostasis with the sun with minimal visible light reflection. At 2.0 mg/ cm 2 , the inorganic SPF 50 sunscreen in our experiment had a ΔE c of 12.5, which approximates the ΔE between vitiligo and normal skin in Caucasians with phototype 3 or 4.…”

Photobiologische Wirkungen der Sonnenstrahlung auf die Haut und Schutz durch Sonnenschutzmittel

“…Obwohl es sich hier um partikuläre Systeme handelt, konnte gezeigt werden, dass die Wirkung zum weitaus überwiegenden Teil in Absorption der einfallenden Strahlung besteht und deren Streuung nur eine untergeordnete Rolle spielt [13]. Das Gleiche gilt übrigens auch für Titandioxid und Zinkoxid, die durch ihre Halbleiterabsorption überhaupt erst Effizienz in Sonnenschutzmitteln zeigen [14]. Der Sonnenschutzfaktor ist der Faktor, um den die erythemwirksame Bestrahlungsstärke durch ein Sonnenschutzmittel abgeschwächt wird.…”