Current Concepts in Photoprotection

Key Points

Photoprotection includes the complementary strategies of seeking shade when outdoors, using protective clothing, wearing a wide-brimmed hat, application of sunscreen, and wearing sunglasses.
Protection from both UVB and UVA is important, and knowledge is also emerging regarding the cutaneous biological effects of visible light and infrared radiation.
Avobenzone is currently the only organic ultraviolet filter available in the United States with effective protection within the UVA1 range, and photostabilized formulations of this filter are an important component of broad-spectrum sunscreen products.
The photoprotective effect of clothing is quantified by the ultraviolet protection factor (UPF), which varies based on several factors.

Introduction

Photoprotection broadly encompasses the various techniques used to shield the body from the harmful effects of ultraviolet radiation (UVR) produced by the sun. The ultraviolet (UV) spectrum is subdivided based on wavelength. UVC (260–290 nm) is largely filtered by the Earth's atmosphere and does not reach the surface of our planet in appreciable amounts.

Historically, attenuating UVB (290–320 nm) was the primary goal of sunscreens, as UVB is the major contributor to sunburn while also provoking DNA damage implicated in the formation of non-melanoma skin cancers. UVA radiation (320–400 nm), the major spectrum responsible for tanning and photoaging, has also been shown to play a role in photoimmunosuppression and photocarcinogenesis. Prompted by this knowledge, UVA protective filters have been developed in recent years, and application of a broad-spectrum sunscreen covering both the UVB and UVA ranges is currently recommended. Yet, optimal photoprotection is obtained not only by the application of sunscreen, but also by seeking shade, wearing ultraviolet (UV) protective clothing, wearing a wide-brimmed hat, and the use of sunglasses. These measures, in addition to sunscreen application, are essential components of a complete photoprotective strategy.

History

The use of topical sunscreen application dates back to the ancient Egyptians, who attempted to use olive oil as a photoprotectant. The first modern use of sunscreen is credited to Veiel, who described tannin as a photoprotectant in 1887. By the 1920s tanned skin had become a popular fashion trend. As individuals spent more time outdoors in hopes of bronzing their skin, they also sought to minimize the risk of sunburn, resulting in an increased demand for sunscreens. In 1928, a product containing benzyl salicylate and benzyl cinnamate became the first commercial sunscreen.

However, sunscreen was not widely popularized until the military use of red veterinary petroleum during World War II. By the mid-1950s, several UVB filters had been developed and popularized, including para-aminobenzoic acid and salicylate derivatives. In the late 1970s, effective UVA filters were developed and combination products containing UVA filters and UVB filters allowed for broad-spectrum protection. Dibenzoylmethane derivatives were the first available long UVA filter.

Sun protection factor (SPF) was adopted for use by the United States Food and Drug Administration (FDA) in 1978. In the mid-1980s, public awareness campaigns against sun exposure were developed by the American Academy of Dermatology further serving to increase public awareness of sunscreen use. In 1989 and 1992, respectively, micronized forms of titanium dioxide and zinc oxide became available. The first FDA sunscreen monograph was published in 1978, with subsequent revisions in 1993 and 1999.

Sunscreens

Mechanisms

UV filters are classified into two categories: organic UV filters and inorganic UV filters. The primary difference between these categories is the mechanism of action each uses to attenuate UVR. Organic filters were previously known as chemical sunscreen filters and primarily function via a mechanism of absorption. When UVR contacts an organic filter, the molecules absorb the UVR photon, resulting in temporary excitation to a higher energy state. From this excited state, the energy may be dissipated via several reactions, including fluorescence, photoreactions, or redistribution of the energy within the molecule itself. In the latter, the energy is further attenuated by the release of heat or by collision with neighboring molecules.

Inorganic UV filters were previously known as physical sunscreens and primarily function via reflection and scattering; two of the widely used ones are titanium dioxide and zinc oxide. Scattering occurs when photons of UVR contact submicroscopic sunscreen particles and are dispersed in various directions, thereby attenuating the incident photon energy. Reflection of visible light photons from the skin surface is the reason that application of inorganic sunscreen products results in whitish discoloration of the skin. It should be noted, however, that as the particle size of inorganic filters reaches the range of nanoparticles (10–50 nm in diameter), absorption of photons occurs. Therefore, a lesser amount of incident photons reaching the skin is reflected, resulting in more cosmetically acceptable final products. The majority of commercially available sunscreen products contain both organic and inorganic filters.

UV filters in sunscreen products

The United States Food and Drug Administration (FDA) views and regulates sunscreens as over-the-counter (OTC) drugs. In 1999, the FDA published the latest version of its sunscreen monograph, describing regulations for sunscreen manufacturers. The FDA identified 16 sunscreen agents which can be incorporated into commercial sunscreen products. When incorporating multiple sunscreens into a commercial product, manufacturers are required to comply with the maximum concentration assigned to each individual sunscreen, while maintaining a sun protection factor (SPF) at or above a level of two. Commonly used sunscreen agents are summarized in Table 9.1 , including the 16 agents identified in the FDA monograph and others available in other parts of the world; they are further discussed below. Figure 9.1 schematizes the absorption spectra of these agents. A proposed amendment to the final monograph was released by the FDA in August 2007; it proposes to allow SPF claims to a maximum of 50, and recommended in-vitro and in-vivo testing methods for UVA protection of sunscreen products. At the time of this writing, the final version of this proposed amendment has not yet been released by the FDA.

Table 9.1

Available Sunscreen Agents

Sunscreen	λmax (nm)	US Availability	UV Spectrum	Summary
PABA	283	Yes	UVB	Photo/contact allergen. Stains clothes. Rarely used
Padimate O	311	Yes	UVB	Most commonly used PABA derivative
Octinoxate	311	Yes	UVB	Most common UVB filter. Photolabile. Contact allergen
Cinoxate	289	Yes	UVB	Rarely used cinnamate derivative
Octisalate	307	Yes	UVB	Weak UVB absorber
Homosalate	306	Yes	UVB	Weak UVB absorber
Trolamine salicylate	260–355	Yes	UVB	Used in hair cosmetics
Octocrylene	303	Yes	UVB	Photostabilizes avobenzone. Poor substantivity
Ensulizole	310	Yes	UVB	Used in cosmetic moisturizers
Parsol SLX	312	No	UVB	Weak UVB absorber. Large molecule (>6000 daltons)
Uvasorb HEB	312	No	UVB	Some consider as best UVB filter
Univil T 150	314	No	UVB	Some consider as the second best UVB filter
Oxybenzone	288,325	Yes	UVA/UVB	Photolabile. Most common sunscreen photoallergen
Sulisobenzone	366	Yes	UVA/UVB	Rarely used
Dioxybenzone	352	Yes	UVA/UVB	Rarely used
Meradimate	340	Yes	UVA	Rarely used
Avobenzone	357	Yes	UVA	UVA-1 filter. Combine with others for photostability
Ecamsule	345	Yes	UVA	Photostabile. UVA absorption > benzophenones
Univil A Plus	354	No	UVA	UVA-1 filter. Photostability > avobenzone
Neo Helioplan AT	334	No	UVA	Water-soluble. Synergy with oil-phase sunscreens
Silatriazole	303,344	No	UVA/UVB	First broad-spectrum UVA/UVB photostable filter
Bisoctriazole	303,344	No	UVA/UVB	Photostable. Has inorganic and organic properties
Bimotrizinol	305,360	No	UVA/UVB	Photostable UVA/UVB filter
Zinc oxide		Yes	UVA/UVB	Micronized form attenuates UVA-1 > titanium dioxide
Titanium dioxide		Yes	UVA/UVB	Micronized form attenuates UVB > zinc oxide

Figure 9.1, Absorption spectra of UV filters listed in the FDA sunscreen monograph. Avobenzone is the only agent listed with an effective absorption peak within the long wavelength UVA-1 range.

Organic UVB filters

Para-aminobenzoic acid

Para-aminobenzoic acid (PABA) is an organic UV filter with a maximal absorbance within the UVB spectrum at a wavelength of 283 nm. PABA is an effective UVB absorber and its high substantivity makes it durable in the face of friction and moisture. However, it is now rarely used for three main reasons. First, it has the unpleasant ability to stain clothing. Second, PABA has the potential to invoke allergic contact and photoallergic dermatitis. Last, there is in-vitro evidence implicating PABA as a carcinogen, although in-vivo confirmation of this finding is lacking.

Padimate O

Another organic UV filter within the UVB spectrum is padimate O, which absorbs maximally at a wavelength of 311 nm. This PABA derivative is less likely to stain clothing or result in contact hypersensitivity, but is less effective as a UV filter than its parent compound, PABA. Of the PABA derivatives, padimate O is currently the most commonly used, although overall it is rarely incorporated into commercial sunscreens.

Octinoxate

Octinoxate, a cinnamate, is the most commonly used UVB filter in the United States and has a maximum absorbance of 311 nm. The agent is photolabile. Moreover, as a weak UVB absorber, it must be combined with other UV filters in order to achieve an adequate SPF. As a cinnamate, there is the potential for cross-reactivity with cocoa leaves and cinnamon-containing compounds, resulting in allergic contact dermatitis in sensitized individuals. Octinoxate and other cinnamates may also contribute to a greasy sensation when applied to the skin, due to their composition of polar oils. Octinoxate has the advantage of being the most widely studied sunscreen in regards to protection against photocarcinogenesis, and in general has an excellent safety profile.

Cinoxate

Another cinnamate derivative, cinoxate, absorbs maximally within the UVB range at 289 nm. As a cinnamate, it shares many of the drawbacks of octinoxate. Currently, the agent is rarely used.

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