Elsevier

Inorganica Chimica Acta

Volume 360, Issue 3, 15 February 2007, Pages 794-802
Inorganica Chimica Acta

Inorganic and organic UV filters: Their role and efficacy in sunscreens and suncare products

Dedicated to Professor Vincenzo Balzani
https://doi.org/10.1016/j.ica.2005.12.057Get rights and content

Abstract

Minerals such as titanium dioxide, TiO2, and zinc oxide, ZnO, are well known active semiconductor photocatalysts used extensively in heterogeneous photocatalysis to destroy environmental pollutants that are organic in nature. They are also extensively used in sunscreen lotions as active broadband sunscreens that screen both UVB (290–320 nm) and UVA (320–400 nm) sunlight radiation and as high SPF makers. When so photoactivated by UV light, however, these two particular metal oxides are known to generate highly oxidizing radicals (radical dotOH and O2-) and other reactive oxygen species (ROS) such as H2O2 and singlet oxygen, 1O2, which are known to be cytotoxic and/or genotoxic. Hydroxyl (radical dotOH) radicals photogenerated from photoactive TiO2 specimens extracted from commercial sunscreen lotions [R. Dunford, A. Salinaro, L. Cai, N. Serpone, S. Horikoshi, H. Hidaka, J. Knowland, FEBS Lett. 418 (1997) 87] induce damage to DNA plasmids in vitro and to whole human skin cells in cultures. Accordingly, the titanium dioxide particle surface was modified to produce TiO2 specimens of considerably reduced photoactivity. Deactivation of TiO2 diminishes considerably, in some cases completely suppresses damage caused to DNA plasmids, to human cells, and to yeast cells compared to non-modified specimens exposed to UVB/UVA simulated solar radiation. The photostabilities of sunscreen organic active agents in neat polar and apolar solvents and in actual commercial formulations have been examined [N. Serpone, A. Salinaro, A.V. Emeline, S. Horikoshi, H. Hidaka, J. Zhao, Photochem. Photobiol. Sci. 1 (2002) 970]. With rare exceptions, the active ingredients undergo photochemical changes (in some cases form free radicals) and the sunscreen lotions lose considerable Sun protection efficacy only after a relatively short time when exposed to simulated sunlight UVB/UVA radiation, confirming the recent findings by Sayre et al. [R.M. Sayre, J.C. Dowdy, A.J. Gerwig, W.J. Shields, R.V. Lloyd, Photochem. Photobiol. 81 (2005) 452].

Graphical abstract

Minerals such as TiO2 and ZnO are extensively used in sunscreen lotions as active broadband sunscreens that screen both UVB (290–320 nm) and UVA (320–400 nm) sunlight radiation and as high SPF makers, as well as being well-known active semiconductor photocatalysts used extensively to destroy environmental organic pollutants. They have also been shown to induce damage to DNA plasmids in vitro and to whole human skin cells in cultures, thus requiring that they be made photo-inert for use in sunscreens and suncare products that also contain chemical (organic) UV filters (e.g., Padimate-O; see figure), whose photostabilities under simulated sunlight UV radiation in polar and apolar solvents and in actual commercial formulations have been questioned.

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Introduction

A variety of marketing strategies have led consumers to believe and rely on the notion that sunscreen lotions are meant to prevent skin damage (e.g., sunburns, skin cancers) while permitting gradual tanning, both of which are achieved when sunscreens absorb UVB {290–320 nm} and UVA {320–400 nm} sunlight radiation. To preclude sunburns and protect people from serious skin damage, sunscreens must possess several attributes. They must be photostable (ideally 100%) and must dissipate the absorbed energy efficiently through photophysical and photochemical pathways that rule out the formation of singlet oxygen, other reactive oxygen species, and other harmful reactive intermediates. They should not penetrate the skin, and should not be transported into the human cells where they can cause deleterious damage to DNA. Sunscreens should also minimize the extent of UVB and UVA radiation that might reach DNA in cell nuclei.

With few exceptions, sunscreens contain chemical filters (organic; absorb mostly UVB radiation) and physical filters (e.g., TiO2 and ZnO). The latter have been said to block UVB/UVA sunlight through reflection and scattering [1]. If this were so, since reflection and scattering are physical phenomena, the term physical UV filters was coined. However, these inorganic UV filters absorb considerable UV radiation [2].

Mineral compounds such as TiO2 and ZnO are used extensively in such cosmetics as foundations, powders, eye shadows and pencils. Indeed, titanium dioxide (TiO2) was reported as a sunscreen agent as long ago as 1952 [3]. The required feature of inorganic sunscreen filters is to screen/block UV light over the whole UVA/UVB range (290–400 nm) through absorption, scattering and reflection properties that in turn are determined by the intrinsic refractive index, the size of the particles, dispersion in the emulsion base, and by the film thickness. The ability of some minerals to act as the so-called physical filters in sunscreen lotions is determined by two major characteristics: the absorption/scattering property of the inorganic filter and its cosmetic acceptability.

An inorganic mineral sunscreen which functions well at reflecting light, however, tends to be opaque and white on the skin and consequently is unacceptable for cosmetic use. Cosmetic acceptability of metal-oxide sunscreens has required that the particle size of TiO2 and ZnO, among others (e.g., alumina, ceria and zirconia) be around 20–50 nanometers (nm). Occasionally, iron oxide (Fe2O3) pigments are added to give the cosmetic a brown tinge to improve the appearance of the suncare product. The so-called micronized metal-oxide particles are used in cosmetic products [4], as they are easily incorporated into emulsions, are transparent to visible radiation, reflection from the particle surface is minimal, and in some cases (TiO2 and ZnO) absorption is maximized. Partial light attenuation by these smaller particles is due mostly to Raleigh scattering with the intensity of scattered light following the power law Is  λ−4 (where λ is the wavelength). Thus, such small particles scatter UVB and UVA wavelengths more than the longer visible light wavelengths.

In 1978, an OTC Panel [5] reported that “… as a physical sunscreen agent, titanium dioxide (TiO2) is a safe, opaque and effective product that provides a barrier to sun-sensitive individuals against sunburns, because it reflects and scatters UVA and UVB radiation 290–400 nm) rather than absorbing the rays …”. However, TiO2 is a much stronger absorber of UV light than is a scatterer of UVB and UVA radiation [2], [6]. Particle sizes of TiO2 that are tenfold greater (range 200–500 nm) are best at reflecting visible light, which on application on the skin has the disadvantage of being opaque, albeit acting as a true sunblock.

Titanium dioxide is available in three different crystalline forms: viz., rutile, anatase and brookite, in addition to an amorphous phase. Only the rutile (bandgap energy, 3.0 eV; onset of absorption at 400 nm) and anatase (bandgap energy, 3.2 eV; absorption onset at 387 nm) polymorphs have relevance in sunscreen lotions and in heterogeneous photocatalysis, a technology that aims to remediate polluted soil, as well as aqueous and atmospheric ecosystems. The principal difference between rutile and anatase is the divergence in photoactivity with anatase being the more photoactive form of titanium dioxide; however, rutile is the more common form of titania even though anatase is the more stable form by about 8–12 kJ mol−1 [7].

Chemical organic filters are classified into either UVA (benzophenones, anthranilates and dibenzoylmethanes) or UVB filters (PABA derivatives, salicylates, cinnamates and camphor derivatives). Sunscreen lotions containing these active ingredients and/or inorganic UV filters are typically commercialized as cosmetic products in most countries [8], except in some as for example in the United States where they are treated as over-the-counter (OTC) drugs and thus regulated by the US Food and Drug Administration (FDA). Chemical UV filters are almost always used in combination because no single active agent, used at levels currently allowed by the FDA [1], provides high enough SPF (sun protection factor) protection or broad-spectrum absorption. Because of the photoinstability and possible unfavorable synergistic interactions between these agents, recent restrictions by the FDA’s Federal Register Administration Regulatory Affairs [1] have limited the choice of suitable combinations of UVB/UVA chemical organic UV filters.

A trend in sun protection is the increased use of inorganic UV filters, especially in suncare products for children and people with sensitive skin. This increased use of inorganic UV sunscreens is due partly to their low potential for producing irritant reactions, and partly to their cosmetic efficacy. Cosmetic chemists have formulated products with high SPF using only titanium dioxide. In combination with organic sunscreen agents, TiO2 gives impressive SPF numbers as well as displaying broad absorption in the UVB and UVA region [9]. Formulators have also incorporated micro-fine titanium dioxide into sunscreen preparations to avoid the decrease of SPF that can occur from the photoinstability of organic UV filters [10].

Our involvement in this field of sunscreen research was our, albeit late, recognition in the mid-1990s2 that sunscreen lotions inherently contained TiO2 in combination with various chemical organic filters. Table 1 summarizes three commercial sunscreen formulations available in Europe in the late 1990s. Note that today’s compositions are not very different from those that were available nearly a decade ago. We were rather concerned since TiO2 had been used for over a decade [11] as a very active photocatalyst to destroy, indeed mineralize pollutants organic in nature (e.g., chlorinated phenols, polychlorinated biphenyls, dioxins and the like) in aqueous ecosystems using the photogenerated highly oxidizing radical dotOH radicals (see below), to remove and dispose off toxic metals (Hg, CH3Hg+, and Pb) from the environment, and to recover precious metals with high selectivity from jewelry and photographic wastes (e.g., Au, Ag, Pt, and Pd).

Our studies on the photochemistry and photophysics of metal oxides have continued uninterruptedly during the past decade to further our collective level of understanding and knowledge of heterogeneous photocatalysis [11], [12] as part of our objective to place this technology with metal-oxide photocatalysts on a more fundamental scientific base [13]. Knowledge thus acquired has proven crucial in our efforts to inactivate titanium dioxide for possible use in sunscreen lotions.

Section snippets

Photochemistry, photophysics and photobiology of metal-oxide semiconductors

Fig. 1 illustrates, in a simplistic fashion, some of the more relevant events of interest to the photobiology of metal oxides. On absorption of UV light, titanium dioxide particles produce conduction band electrons (e) and valence band holes (h+). The electron is scavenged by the ubiquitous pre-adsorbed molecular oxygen to yield the superoxide radical anion (O2-), whereas the holes oxidize the surface hydroxyl groups (and chemisorbed water) to generate hydroxyl radicals (radical dotOH) that initiate the

Titanium dioxide and DNA damage

To cause damage to DNA in vivo, titanium dioxide must penetrate the skin, and most importantly must penetrate the cell nucleus. Inferences that TiO2 does penetrate the skin have been reported [18], [19], [20], [21], although some of the data were somewhat inconclusive [22]. For instance, X-ray microanalyses and scanning electron microscopy failed to reveal the presence of TiO2 in deep layers of the skin [23]. In a much earlier study, Dussert et al. [24] similarly found no intercellular or

Photostability of sunscreen lotions

Personal observations on several beaches in Italy and southern France indicate that beach lovers and suntan lovers apply sunscreen lotions perhaps once or twice daily and go to the beach at a time of day when the UV index is at its greatest, namely between 10:00 and 16:00 h. This is precisely the time period that the American Cancer Society recommends that beach goers limit their Sun exposure and to use a sunscreen with an SPF index greater than 15 so as to minimize the potential for skin

Concluding remarks

The role of sunlight UV radiation as an initiator of cancer and as a promoter of cancer is no longer in question [53]. Perhaps the most interesting and relevant new findings are from the recent work of Chiang and coworkers [54] who noted that “… sunscreens may prevent skin redness partly by UV absorption and partly by inhibiting the skin’s inflammatory response. As such, sunscreens might promote instead of protecting against melanoma”.

If this were not disconcerting enough, a study done at the

Acknowledgement

We are grateful to the Ministero dell’Istruzione, Universita e Ricerca (MIUR, Roma to N.S.) for support of our studies in Pavia.

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