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Keywords:

  • antioxidants;
  • clothing;
  • photoprotection;
  • sunglasses;
  • sunscreen

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

Exposure to ultraviolet (UV) radiation is associated with a variety of harmful effects ranging from photoaging to skin cancer. UVB (290 to 320 nm) directly damages the cellular DNA leading to the formation of the 6-4 cyclobutane pyrimidine dimmers, and UVA (320 to 400 nm) indirectly damages the DNA via the production of oxygen radical species. In this review, we focused on the technological and scientific aspects of photoprotection using sunglasses and clothing while attempting to dispel some of the misconceptions. In addition to the basic knowledge relating to sunscreens, we reviewed the current guidelines for testing and labeling UVA protection around the world, controversies associated with nanoparticles, and future sunscreens actives waiting for the Food and Drug Administration approval. Lastly, we reviewed alternative agents, such as antioxidants, that can be used to supplement and augment photoprotection provided by sunscreens.


Introduction

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

Ultraviolet (UV) exposure from the sun or other sources is a known culprit for the development of skin cancer. Chronic UV exposure has been linked to the development of actinic keratosis, squamous cell cancer, and basal cell cancer, whereas intermittent and intense UV exposure is associated with the development of melanoma, the most dangerous type of skin cancer. UVB (290 to 320 nm) is the major wavelength that causes sunburn, and this portion of UV rays directly damages the cellular DNA leading to the formation of the 6-4 cyclobutane pyrimidine dimmers. By contrast, UVA (320 to 400 nm) penetrates deeper into the skin layer and indirectly damages the DNA via the production of radical oxygen species (ROS).

UV radiation also plays a major role in the acceleration of the photoaging. Clinically, these changes consist of wrinkling, dryness, telangiectasia, sagging, and pigmentation. Histologically, aging skin is reflected in the disorganization of collagen bundles, loss of elastin fibers, flattening of dermal-epidermal junction, and dilation of blood vessels. In addition, morphological and quantitative changes are observed in the keratinocytes, melanocytes, fibroblasts and Langerhans cells. On a cytokine and molecular level, UV radiation increases the production of matrix metalloproteinases (1), a group of protease enzymes responsible for the degradation of collagen and elastin fibers. Furthermore, UV exposure initiates a cascade of ROS and inflammatory cytokines via activation of the AP-1 and NF-κB pathways (2–5).

To reduce the potential harms associated with UV exposure, the medical community, sun care and cosmetic industries, government and non-profit agencies, and media have worked closely in the past decades to educate the public about the importance of photoprotection. Despite these efforts, large portions of the population fail to appreciate this risk and do not heed the warnings. Some segments of the population evenintentionally increase their exposure to achieve a tanned look. In view of the current state of photoprotection, continuous effort and education are needed. In this review, we will only focus on the technological and scientific aspects of photoprotection covering a range of existing and future modes of protection, although emphasis on improving compliance and modifying behaviors is equally important.

Environmental Factors

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

UV rays (200 to 400 nm) from the sun comprise a narrow window of the entire solar spectrum. Nearly 100% of the UVC (200 to 280 nm) and 90% of the UVB rays are blocked by the ozone layer in the stratosphere, which is situated approximately 40 km above the surface of the earth. Virtually all UVA rays pass through the ozone layer and reach the earth surface. Depletion of the ozone layer from substances such as chlorofluorocarbons has lead to a substantial increase in UV transmission to the Earth's surface.

Aside from the ozone factor, the amount of UV rays reaching the Earth's surface is also related to time, season, latitude, and altitude. For every 300 m of elevation, there is an increase of 4% of UV radiation that reaches the surface (6). For every degree of decrease in latitude, there is a 3% increase in the transmission of UVB rays. During the peak hours and summer seasons there is also an increase of UV transmission. When the sun is located at its zenith, the path of UV light through the absorptive ozone layer is significantly shorter. Additionally weather factors, such as fog and clouds, and even pollution, can dramatically reduce UV transmission by 10 to 90% (7). Terrestrial settings, such as water, snow, sand, and concrete, can reflect up to 90% of UV rays to the skin (7). Lastly, shade reduces UV radiation from the sun by 50 to 90%, with dense foliage providing superior UV reduction to a beach umbrella (8).

Clothing and hats

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

Wearing clothing and hats has been emphasized as an important mean for photoprotection (9). Compared with sunscreens, the most popular mode of photoprotection used by the public, clothing has a number of advantages. First, clothing and hats offer a balanced and uniformed protection for both UVA and UVB. By contrast, most of sunscreens available in the United States preferentially deliver more UVB protection. Second, clothing and hats offer more reliable protection as long as users remember to wear them. By contrast, a user needs to apply sunscreens 30 minutes prior to going outdoors and reapply them every 2 hours, requirements that are often ignored or forgotten. Furthermore, the degree of protection offered by sunscreens depends on application of the correct amount of the product. Many users apply only 1/4 to 1/2 of the amount used to assess the sunburn protection factor (SPF) for methodological reasons (i.e., 2 mg/cm2), and they do not apply the product evenly, leaving parts of the body unprotected (10–11). Lastly, clothing and hats are less costly than sunscreens (9), and they are devoid of any complications such as contact or photoallergic dermatitis.

As expected, the degree of UV protection offered by clothing and hats varies. Gambichler et al. showed that 1/3 of summer clothing provided poor protection from UV radiation, and only 75 % of fabrics were able to deliver a sufficient degree of protection (12). Other studies show that 25% of garments deliver at least a SPF 15 (9), which corresponds most likely to at least double the protection of a SPF 15 sunscreen for the reasons mentioned above. To accurately and quantitatively measure UV protection by garment, most regulatory agencies around the world have adopted the UV protection factor (UPF) as a measurement standard. To attain the UPF of any clothing, UV transmission through the clothing is measured with a spectrophotometer (13–14). UPF is then calculated based on the amount of UV transmission at each wavelength and the erythema response from each spectra of the UV (15). This standard was first developed and published in Australia in 1996 (14), and it was later adopted and refined by the European Committee for Standardization in 2003 (16). The Committee stipulated the clothing with UPF label must cover the area from the neck to the hip, across the shoulders, and three quarters of the upper arm. In addition, a cutoff of 40+ was set as the minimum UPF value allowed. It also created a requirement for UVA transmission, which had to be less than 5%. However, the standard did not factor in the effect of stretching and wetness in the UPF assessment; the Committee felt that clothing with a UPF > 40 would be adequate to compensate for these factors. In the United States, three documents addressing the issue of testing and labeling of the UV protective clothing have been published by the American Society for Testing and Materials and the American Association of Textile Chemists and Colorists (17). One US manufacture of UV protective clothing makes an SPF 30+ claim, whereas others follow the UPF testing and labeling standards outlined in the Australian, European, or US guideline.

A number of factors determine the UPF of clothing (Table 1). Fabric materials play a major role. Polyester and wool offer the highest UV absorbing capacity, and they have higher UPF values than cotton, linen, and rayon (18–19). However, polyester retains heat, and it is not a comfortable material. In much summer clothing, polyester can be combined with other materials to deliver both comfort and superior UV protection (18). The thickness of the fabric has a role as well; thicker fabrics provide better protection (19). Certain colors, such as blue and black, can enhance UV protection (12,20). Lastly, tightness of the weave or knit is a major determinant factor (18,21). Weaving (e.g., dress-shirt) or knitting (e.g., t-shirt) refers to the way the threads of the fabric are interlaced with each other. Gaps or holes between individual fibers permit direct access for UV transmission, but the UV radiation passing through the fiber (especially cotton) is normally more important.

Table 1.  Factors influencing the ultraviolet protection factor (UPF) of clothing
Increase UPFDecrease UPF
Fabric materialThinner fabric
(e.g. polyester, nylon)Lighter colors
Tightly woven fabricWetting
Thicker fabricStretching
Darker colorsBleaching
Washing 
Washing with optical whitening agents 
Washing with UV absorbing chemicals (e.g., Tinosorb FD) 

Washing and wearing also impact the UPF value of the clothing. After washing, certain fabrics shrink, thereby decreasing the gaps between the individual fibers and making the fabric thicker, thus increasing the UPF. This effect is most noticeable after the first wash (22) and is more pronounced with clothing made with cotton, rayon, and flax materials (9). By contrast, hydration or wetting decreases the UPF for most clothing (23), because the presence of water eliminates the scattering of UV light at the fabric/air interface (24). Therefore, average summer t-shirts may lose a large degree of UV protection when they are wet. This change applies mainly for lightly colored fabrics; clothing with dark colors are less so affected, because absorption rather than scattering becomes the dominating protection mechanism (24).

Chemical treatments with optical brightening agents, UV absorbers, and bleach can change UPF measurements. Optical brighteners are conjugated products that absorb UV energy and release the energy in the form of fluorescence in the visible wavelength range. These compounds absorb UV rays and increase UPF values, and they are found in many household laundry detergents (9,25). Similarly to the optical brighteners, there are laundry chemicals (e.g., Tinosorb FD) that serve as UV absorbers, which chemically bind to the individual fibers of clothing with each subsequent wash. Wang et al. (20) showed that UPFs of cotton fabric used for manufacturing summer T-shirts increased by more than 400% with the addition of UV absorber. Similar findings were confirmed by Osterwalder et al. (26). On the other hand, bleaching fabrics such as cotton and rayon reduces their UPF (19). Harsh treatment from the chemical damages the fabric materials and creates new or enlarges existing interstices in the clothing, resulting in additional UV transmission.

Sunglasses

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

As dermatologists, we tend to concentrate on the harmful effects of the UV exposure to the skin. However, acute and chronic damages from UV rays to the eyes are equally striking. Aside from the development of squamous and basal cell cancer on the eyelid and periorbital skin (27–28), UV exposure can also damage the cornea, conjunctiva, lens, and retina, resulting in a number of conditions collectively named “ophthalmohelioses” (see Table 2).

Table 2.  Non-ionizing radiation hazard to the eye
Eye damageClinical descriptionStructure involvedRisk factor
Acute damage
PhotokeratoconjunctivitisTransient mild discomfort to severe pain with tearing and photophobiaCorneal epitheliumExcessive UVR below 315 nm (29)
PhotoretinitisPhotochemical injury that may be accompanied by a blind spot (scotoma) and can be sight-threateningRetinaShort wavelength visible light (blue and violet) (29)
Chronic damage
Cortical cataractIncreased opacity of the lens that leads to vision lossLensUVR (30,31)
PterygiumRaised wedge-shaped growth on the conjunctiva that can extend onto corneaConjunctivaUVR (32,33)
Macular degenerationProcess affecting a macula (central portion of retina) leading to loss of central visionRetinaShort wavelength visible light (blue light) (34,35)

Sunglasses can protect the eye from UV damage. Aside from the cosmetic quality, an ideal pair of sunglasses should block all UV rays while not sacrificing the transmission of visible light. Currently in the United States, a standard guideline (American National Standards Institute [ANSI] Z80.3 2008) is issued by the ANSI (a nongovernmental and consensus body) to categorize the different types of sunglasses according to the degree of shading and UV absorption profile (see Table 3). For example, according to this standard guideline, lenses in the cosmetic category require the light transmittance to be >40% while restricting UVB rays to be 12.5% of the light transmittance and <1% of total UVB under normal and prolonged use, respectively. In comparison, special purpose lenses permit 3–8% light transmittance and <1% of total UVB transmission. However, this standard (ANSI Z80.3 2008) is voluntary, and manufacturers are not compelled to build or label their products according to it. The lack of labeling requirement can potentially cause more harm. A pair of sunglasses may provide shade without adequate UV protection. Wearing such a pair of sunglasses diminishes the amount of visible light, which can disable the squinting mechanism and dilate the pupil. These physiological responses can increase UV exposure to the lens and retina, causing cataract and maculopathy, respectively. In sum, finding a pair of sunglasses that provides adequate protection may not be a straightforward task. Price, brands and polarizing lenses do not ensure adequate protection (36,37).

Table 3.  Transmittance properties for nonprescription sunglasses according to ANSI Z80.3 2008
LensLuminous transmittance τvMean UV transmittance
UVBUVA
Normal useProlonged exposureNormal useProlonged exposure
Cosmetic lens (light)>40%≤12.5% τv≤1% UVBτv≤50% τv
General purpose lens (medium to dark)8–40%≤12.5% τv≤1% UVBτv≤50% τv
Special purpose lens (very dark)3–8%≤1% UVB≤1% UVB≤50% τv≤50% τv
Special purpose lens (strongly colored)>8%≤1% UVB≤1% UVB≤50% τv≤50% τv

Although the composition of lenses and the degree of UV protection are important, the shape and coverage of the sunglasses are perhaps more critical. Most ocular damages from UV radiation result from scattered and reflected light from the periphery. In extreme conditions, indirect UV rays bounce off the ground, snow, water, and sand. Hence it is crucial that an ideal pair of sunglasses is wrapped closely to the eye. However, most sunglasses are designed for fashion purposes, and only the anterior and superior sides of the eyes are protected. The lateral and inferior portions are often poorly shielded, which can lead to passage of UV rays from these angles. In fact, both cortical cataract and the pterygium predominately involve the nasal aspect of the eye (38,39) supporting the notion that most of UV damage is induced by oblique peripheral rays (40).

For the same reasons, the time to wear sunglasses should also be emphasized. Skin safety campaigns instruct the public to stay indoor when the sun is overhead and most intense (41) and encourage outdoor play during the morning and evening. By extension, many people suppose that eye protection can be ignored or neglected during these off-peak hours. In fact, most ocular damages occur during the morning and late afternoon when the incident rays are parallel to the pupil axis. During the peak hour, the brow ridge and eyelid shield the eye from the UV rays. In addition, the squinting mechanism minimizes area of exposure on the ocular surface, and pupil contracts to reduce UV transmission. In contrast, during the off-peak hours, the UV incident angle is low and the UV rays bypass the defense of the brow ridge and eyelid. In addition, the squinting and pupillary constriction mechanisms are not initiated because the light intensity is weaker. Hence, to prevent UV damage to the eye, sunglasses should be worn during the mornings and afternoon to ensure adequate protection.

Sunscreens

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

Sunscreens have become a quasi-exclusive mode of protection used by the public when engaged in outdoor activities (42,43), even though seeking shade and wearing protective clothing provide more protection. The popularity may be attributed to the recommendation from physicians (44) and marketing actions from the sun care and cosmetic industry (45). Nevertheless, routine use has been shown to reduce skin cancers and slow the photoaging process. Sunscreen is effective in decreasing the number of actinic keratoses (46) and squamous cell cancers (SCCs) (47). In a subsequent study with more than 8 years of follow up, a more dramatic preventive benefit in the reduction of SCC was observed (48). As for basal cell carcinomas (BCCs), the protective benefit is not conclusive. A statistically significant reduction of BCCs with routine use of sunscreens has not been demonstrated. However, there is an overall trend showing a decreased incidence of BCCs (47,48). As for the protective role of sunscreens in melanoma, there have been intense scientific discussions that periodically spill over and land in the headlines of the general media. The major controversy centers on the debate about whether sunscreen use can lead to an increase in melanoma. Epidemiologic studies are mixed with some showing preventive benefit, whereas others showing an increased risk for developing melanoma. A recent meta-analysis concluded that sunscreens use is safe, and sunscreens use does not increase the risk for developing melanoma (49). Many confounding factors may explain the lack of protective effect with routine sunscreen use. Diffey et al. summarized this well in suggesting that most sunscreens in the past offered no or little UVA protection (50). It is foreseeable that future studies involving the use of sunscreen with more balanced coverage may show the protective benefit for melanoma and BCCs.

Aside from skin cancer prevention, sunscreens offer other medical and cosmetic benefits. Many tell-tale signs of aging, such as the formation of wrinkles, appearance of pigmentations, dilation of blood vessels, and loss of collagen, are accelerated by UV exposure. Routine use of sunscreen can attenuate and slow down this process (51). In addition, many first line treatments of photodermatoses involve the use of sunscreens. For example, it is recommended that patients with polymorphic light eruption use broad spectrum sunscreens to reduce the frequency of flares (52). Similarly, broad spectrum sunscreens can attenuate the immunosuppressive effects of UV rays (53) and reduce the severity of cutaneous lupus erythematosus (54).

In the United States, sunscreens are regulated by the Food and Drug Administration (FDA). At the moment of completing this review, there are only 16 active ingredients that are approved (Table 4), and it is expected that additional actives, and new combinations of actives, may be accepted once the much anticipated sunscreen monograph is passed by the FDA. For ease of understanding, the discussion on sunscreen actives is divided into two groups: inorganic and organic.

Table 4.  Sunscreen active ingredients currently approved in the Food and Drug Administration monograph
Active ingredientsMaximum concentrationPeak absorption λ (nm)UV action spectrum
  • a

    Only as a component of certain approved sunscreen formulations approved under the new drug application.

  • b

    Absorption varies depending on the particle size.

Organic filters   
UVA filters   
Benzophenones   
 Oxybenzone (benzophenone-3)6%288, 325UVB, UVA II
 Sulisobenzone (benzophenone-4)10%366UVB, UVA II
 Dioxybenzone (benzophenone-8)3%352UVB, UVA II
Dibenzoylmethanes   
 Avobenzone (butyl methoxydibenzoylmethane, Parsol 1789)3%360UVA I
Anthralates   
 Meradimate (menthyl anthranilate)5%340UVA II
Camphors Ecamsulea (terephthalylidene dicamphor sulfonic acid, Mexoryl SX)10%345UVB, UVA
UVB filters   
Aminobenzoates (PABA derivatives)   
 PABA (p-aminobenzoic acid)15%283UVB
 Padimate-O (octyl dimethyl PABA)8%311UVB
Cinnamates   
 Cinoxate (2-ethoxyethyl p-methoxycinnamate)3%289UVB
 Octinoxate (octyl methoxycinnamate)7.5%311UVB
Salicylates   
 Octisalate (octyl salicylate)5%307UVB
 Homosalate (homomenthyl salicylate)15%306UVB
 Trolamine salicylate (triethanolamine salicylate)12% UVB
Others   
 Octocrylene10%303UVB, UVA II
 Ensulizole (phenylbenzimidazole sulfonic acid)4%310UVB
Inorganic filters
Titanium dioxide25% UVB, UVAb
Zinc oxide25% UVB, UVAb

Inorganic sunscreens

Inorganic sunscreens work by both scattering and absorbing UV rays. Currently, in the United States, there are only two inorganic filters, titanium dioxide (TiO2) and zinc oxide (ZnO), approved by the FDA. Compared with the organic actives, TiO2 and ZnO offer a number of advantages. Both actives are photostable, yielding sunscreen products with predictable degrees of photoprotection after UV exposure. By contrast, some organic actives, such as avobenzone, are photolabile and may lose 50% of photoprotective property after 1 hour of UV exposure if not stabilized properly. Both TiO2 and ZnO have low allergenic potential and low rates of sensitization. Finally, ZnO offers protection extending to the UVA I (up to 380 nm) range (55), but the magnitude of UV protection from ZnO is low compared with other organic UV filters.

Despite these advantages, the public has always resisted embracing inorganic sunscreens, especially the early generation of these products. The reluctance is largely caused by the large particle size and high refractive indices of both TiO2 (refractive index = 2.6) and ZnO (refractive index = 1.9), which result in unsatisfactory whitening appearance. With a higher refractive index, TiO2 products appear whiter than ZnO products when applied to the skin. Aside from the cosmetic shortcomings, large particles and poor dispersion also create a gritty sensation when the sunscreens dry. Lastly, the opaque nature and occlusive qualities of these products are comedogenic.

To address these shortcomings, enormous progress has been made in the past decade to reduce the particle size of the ZnO and TiO2. Decreasing the size leads to less scattering of visible light and improvement in the cosmetic appearance. However, in the progress toward micronizing these particles, there is a growing trend to incorporate nano-scaled TiO2 and ZnO in the sunscreens. Conventionally, nanoparticles are defined as particles with dimensions less than 100 nm. Partially because of the massive increase in surface area-to-volume ratio, nanomaterials exhibit new optical, mechanical and electrical properties that are vastly different from their conventional-sized counterparts. These new properties can also lead to unpredictable outcomes when they interact with biological tissues. For this very reason, there is a growing concern regarding the safety profile of personal care products containing nanomaterials.

In the case of sunscreen, nano-sized ZnO and TiO2 provide superior UV protection while further eliminating the unsightly white residues. Titanium dioxide particles with size in the 20- to 30-nm range provide the optimal UV absorption/scattering properties, whereas zinc oxide particles with size from 60 to 120 nm offer the most favorable UV protection. Each year, thousands of tons of these nanomaterials are produced for sunscreen formulation. Along with the rising popularity, there is also an increased scrutiny from the nonprofit organizations, scientific communities, and governmental regulatory agencies regarding safety issues, specifically relating to the skin penetration and toxicity profiles of these newer formulations.

Regarding the issue of penetration, the major concern is that these nanoparticles can penetrate the skin barrier with relative ease and interact with living cells in the lower portion of the epidermis. However, a large number of in vitro and in vivo studies using murine, porcine, or human skin have shown that the nano-sized TiO2 and ZnO remain at the stratum corneum level (56–65). There is no increased level of penetration when compared to the macro-sized counterparts. In studies that showed penetration of these nanoparticles, most of these nanomaterials are found in the pilosebaceous openings and superficial portion of the follicles. A number of factors may explain the poor penetration through the stratum corneum. From research in the field of transdermal drug delivery, it is known that the penetration of molecules through the stratum corneum depends on a number of factors, such as the concentration and molecule size. Small compounds with a molecular weight between 163 and 357 Da (molecular size between 0.8 and 1.6 nm) can penetrate the skin barrier with ease. Nanoparticles often form larger aggregates and conglomerates once incorporated in the final sunscreen formulation. The sizes of these aggregates often exceed 100nm, which is 10–100 times larger than the desired 1.6 nm (molecular weight 163 to 357 Da). Lastly, it is worth mentioning that most studies are focused on healthy skin with intact stratum corneum. There may be an increase in penetration of nanomaterials through compromised or diseased skin. However, to date, a review of the literature suggests that there is no conclusive evidence showing compromised skin always lead to greater penetration (66). In some diseases, such as psoriasis, hyperkeratosis leads to reduced penetration.

The toxicity profile of nanoparticles after topical application depends on exposure to living cells and intrinsic toxicity of TiO2 and ZnO. From the discussion earlier, there is no evidence of showing increased penetration into living cells with the use of nanoparticles in sunscreens. TiO2 and ZnO have low intrinsic toxicity profile. Both compounds have been on the market for many decades, and both have low or no incidence of adverse skin or systemic effects. Furthermore, Zn is considered as an essential nutrient with known health benefits, and Ti has been used in many food additives. The toxicity concern for nanoparticle TiO2 and ZnO arose following a study showing TiO2 were photogenotoxic in mouse lymphoma and Chinese hamster lung cells (67). The presumed mechanism is the production of hydroxyl radicals from TiO2 after UV exposure, which leads to DNA strand breakage (68). To demonstrate the safety profile, producers of TiO2-based sunscreens tested 10 different sunscreen grade TiO2, including nano- and micro-sized particles, for genotoxicity (Ames test, clastogenicity in mammalian cells), photo-genotoxicity (photo-Ames test in mammalian cells), and cytotoxicity (CHO and V-79 cells). Their conclusion suggested these TiO2 particles were not cytotoxic, phototoxic, genotoxic, or photogenotoxic. Furthermore, there was no significant difference between the nano- and micro-sized particles (69). The toxicity issue was also examined by the European Commission where sunscreen preparations containing nano-sized TiO2 and ZnO were reviewed. The results suggested that these materials are not toxic, irritating, sensitizing or photosensitizing after topical application (70,71).

In summary, inorganic sunscreens with nano- or micro-sized TiO2 and ZnO are effective in providing UV protection while preserving the cosmetic elegance. Current studies suggest that these nanoparticles do not show an increased penetration and have a good safety profile. Currently, there is no regulation in the United States regarding the testing and labeling standards for sunscreens with nano-TiO2 and ZnO. This deficiency will not be addressed in the new FDA sunscreen monograph scheduled to be released in the end of 2009.

Organic sunscreens

In contrast to inorganic sunscreen agents, organic chemicals absorb UV radiation through their conjugated aromatic ring structures. Upon UV exposure, electrons of these compounds are energized and jump to an excited and unstable state. In time, the electrons return to their ground and stable state and release the energy in the form of heat (10). For the ease of understanding, the following discussion of organic actives is divided into UVB and UVA filters. Table 4 showcases the complete list of all the organic filters approved in the United States.

UVB filters

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

Aminobenzoates

Para-aminobenzoic acid (PABA) was the first UVB filter widely used in the United States (72). For historical reasons, this compound is included in this discussion. The peak absorption wavelength is 283 nm. The agent is not soluble and binds tightly to the keratinocytes via hydrogen bond. This latter quality makes this agent an ideal candidate for producing water-resistant sunscreen. However, PABA became unpopular as consumers dislike the yellow discoloration that stains the clothing (73). More critically, 4% of the population have photoallergic reaction to the chemical (74), and there are concerns regarding the carcinogenic potential of this agent (75).

Cinnamates

Within this class of UVB compounds, there are two members: octinoxate (octyl methoxycinnamate [OMC]) and cinoxate (2-ethoxyethyl-p-methoxycinnamate). OMC is the most potent UVB absorber widely used in the United States and worldwide. It absorption profiles spans from 270 to 328 nm with the peak absorption at 320 nm. The efficacy of OMC can be further increased when encapsulated in polymethylmethacrylate microsphere (76). Other attributes that make this chemical appealing to the sunscreen formulators are its water resistant property and low skin irritancy potential (77). One major drawback is its incompatibility with avobenzone, a UVA filter widely used in the United States. The combination of OMC and avobenzone is photolabile and compromises the overall UV protection (78).

Salicylates

Salicylates are another group of UVB filters available in the United States. Currently, octisalate, homosalate, and trolamine salicylate are approved in the FDA monograph. As a group they filter UV radiation from 290 to 315 nm. Octisalate has a maximum absorption at 307 nm and homosalate at 306 nm. They are considered to be weak UVB absorbers, and high concentrations of compounds are needed to meet the SPF requirement (79). On the other hand, there are certain advantages that make them useful. They do not penetrate the stratum corneum and have low sensitizing potential. Both octisalate and homosalate are water insoluble that leads to their high substantivity, ability to retain its effectiveness after exposure to water and perspiration. Trolamine salicylate is water soluble and has been used in hair products (80). These compounds can also play a minor role to stabilize and prevent photodegradation of other sunscreen ingredients like oxybenzone and avobenzone (80).

Octocrylene

The absorption profile of octocrylene (2-ethylhexyl-2cyano-3,3-diphenylarylate) spans from 290 to 360 nm with peak absorption at 307 nm. The compound has an excellent safety profile with low irritation, phototoxicity and photoallergic potential (77). Octocrylene has low substantivity and loses its efficacy when exposed to water or sweat. In the past, this agent was not widely used because of its cost and difficulty in formulation. However, because it is regarded as the best photostabilizer for avobenzone available in the United States, octocrylene has become increasingly popular among the sunscreen manufacturers.

UVA agents

Currently there is no global consensus on the labeling and testing standards for measuring UVA protection with each country and region adopting its own set of guidelines. In Australia, the UVA test is performed with an in vitro methodology, and uses a pass/fail system in which only SPF ≥ 15 products that pass the UVA test can be labeled as “broad spectrum.” The United Kingdom also uses an in vitro method. It calculates the ratio of mean UVA absorbance to mean UVB absorbance and translates the value into a Boots Stars rating. In Japan, UVA protection is measured via persistent pigment darkening (PPD), an in vivo method alike that of the SPF testing. The sunscreen products are then labeled with a PA+, PA++, or PA+++ category. In the European Union, the Commission Recommendation was published in September 2006. It adopted the PPD in vivo testing method, but encouraged the use of in vitro methods. It required products to have a minimum UVA protection factor (UVAPF) of 1/3 of the SPF.

In the United States, currently there is no testing or labeling guideline. In a proposed rule released in August 2007, the FDA recommended a combination of in vivo PPD and in vitro methods to measure UVA protection. The in vitro method uses the ratio of mean absorbance of UVA1 (340 to 400 nm) to mean absorbance of UV (290–400 nm). The overall UVA protection category for use in product labeling will be the lowest category determined by the in vitro and in vivo test results. A total of five categories (i.e., none, low, medium, high, and highest) will be assigned to products that fulfill the criteria. This proposal has not yet been approved. Despite these diverse standards across the globe, there is a growing convergence towards the general principle of testing and labeling for UVA protection, and in many cases, products that fulfilled the criteria in one region would also satisfy the criteria in other countries or regions (81).

Benzophenones

Benzophenones are a group of aromatic ketones with a relative broad spectrum of UV coverage, and it is widely used by the US sunscreen manufactures. Oxybenzone (benzophenone-3) is a member of this class of UVA filters. It was approved in the early 1980s and has an absorption profile spanning from 270 to 350 nm. The two absorption peaks are at 288 and 325 nm. As a UVA filter, it mainly offers protection for the UVA II rays. Although oxybenzone is widely found in US sunscreens, it has a number of shortcomings. First, this active has been regarded as the most allergic agent and has been implicated for many contact and photoallergic dermatitis (82,83). Second, the compound has systemic absorption and is detected in urine and blood stream (84). Growing concerns raised by non-profit organizations have suggested the endocrine and carcinogenic effect of this compound, even though current scientific studies have not supported these fears (84). Lastly, oxybenzone is not photostable and can generate oxygen radicals upon UV exposure (85). Most European sunscreens have replaced this filter in the formulation, as it has to be specifically mentioned on the package “contains oxybenzone,” but it is also important to note that European sunscreen manufacturers have superior and more filters at their disposal.

Avobenzone

Avobenzone (butyl methoxydibenzoylmethane) is the second organic filter widely used in the United States. It has an absorption profile ranging from 310 to 400 nm. The molecule has two structural isomers, each with its own absorption peak. In general, the peak absorption is centered around 360 nm. In the United States, this is the most powerful organic filter that extends protection into the long-range UVA I rays. Despite its efficacy and broad spectrum, avobenzone is intrinsically photo unstable. Significant photodegradation with a loss of 50 to 90% of molecules can occur after one hour of UV exposure (86). Furthermore, avobenzone should not be combined with octinoxate, the most powerful UVB filter available in the United States. After absorbing UV radiation, avobenzone molecules jump from its stable and ground state to the excited and unstable state. At its excited state, avobenzone can react with octinoxate to form a new compound. In this process, both avobenzone and octinoxate are destroyed, which lead to loss of both UVA and UVB protection (87). To increase the photostability of avobenzone, other molecules are added to facilitate or expedite the transition of avobenzone from the excited/unstable to ground/stable state. Some of these stabilizers include UV absorber (e.g., octocrylene) and non-UV filter (e.g., diethylhexyl 2,6-napththalate). The combination of avobenzone, octocrylene, oxybenzone, and diethylhexyl 2,6-napththalate is patented by Neutrogena and marketed under the trade name of Helioplex.

Ecamsule

Ecamsule (Mexoryl SX or terephthalylidene dicamphor sulphoic acid) is a broadband UVA filter with an absorption profile ranging from 290 and 390 nm. Its peak absorption is at 345 nm. The compound is photostable, water resistant and has low systemic absorption. Currently, ecamsule has been approved by the FDA as an active filter via the NDA process and can thus only be used in certain formulations. Currently, ecamsule is marketed in sunscreen brands such as Anthelios or Vichy, combined with avobenzone, octocrylene, and titanium dioxide.

Pending Sunscreen Filters in the United States

Compared with Europe and Australia, US sunscreen manufacturers have fewer numbers of actives, especially UVA filters. In addition, the allowable concentration of avobenzone is 3% compared with 5% in Europe. Lastly, avobenzone cannot be combined with inorganic filters. These limitations pose a challenge for formulators to design products with improved UVA protection. Some of these limitations will hopefully be addressed when the new FDA sunscreen monograph is passed. Currently, there are a number of active filters waiting for the approval from the FDA via the TEA process (material time and material extent application). Under the TEA procedure, the FDA can approve the formulations and ingredients with 5 years of foreign marketing experience. Many of these potential new compounds have fulfilled the time requirement, as they have been marketed in Europe, Asia, and Australia. Please review Table 5 for all the potential new filters that may be approved in the near future, and below are three possible compounds that may receive the FDA approval via the TEA process in the near future. The addition of these new filters will give formulators more options and in theory enable creation of superior products.

Table 5.  Active ingredients pending FDA approval via the TEA process
Active ingredientsMaximum concentrationPeak absorption λ (nm)UV action spectrum
  • a

    Under the proposed rule 2007, these agents are expected to receive approvals.

Methylene-bis-benzotriazolyla tetramethylbutylphenol (Tinosorb M; bisoctrizole)10%305, 360UVB, UVA
Bis-ethylhexyloxyphenola methoxyphenyl triazine (Tinosorb S; bemotrizinol)10%310, 343UVB, UVA
Ethylhexyl triazonea (octyl triazone)5%314UVB
Isoamyl methoxycinnamate* (amiloxate)10%310UVB
Methylbenzylidene camphor* (enzacamene)4%300UVB
Diethylhexyl butamido triazone (iscotrizinol)3%312UVB
Terephthalylidene diacamphor sulfinic acid (Ecamsule)10%345UVB, UVA

Bisoctrizole (methylene-bis-benzotriazolyl tetramethylbutylphenol)

This compound is marketed under the trade name of Tinosorb M (Ciba Specialty Chemicals, Basel Switzerland, now part of BASF SE), and approved in Europe, Australia, and Japan. The maximum allowable concentration in Europe is 10%. This molecule offers a broad spectrum coverage with two absorption peaks at 303 and 360 nm (88). It is intrinsically photostable and will help to stabilize other UVA filters, such as avobenzone. The large size of the molecule (molecular weight = 659 Da) also reduces the risk of skin penetration and systemic absorption. Lastly, although the molecule behaves like an organic filter, it also shares some of the inorganic property. By micronizing the size of the particles, the filter also blocks UV rays through scattering.

Bemotrizinol (bis-ethylhexyloxyphenol methoxyphenyl triazine)

This is another broad spectrum filter, produced and marketed by Ciba Specialty Chemicals under the trade name of Tinosorb S. Like Tinosorb M, it is approved in Europe, Japan and Australia. In Europe, the maximum allowable concentration is 10%. The absorption spectrum spans from 280 to 380 nm with two absorption peaks at 310 and 343 nm. The molecule (molecular weight = 629 Da) has two hydroxyl groups in the ortho-position, which permits phototautomerism and facilitates the compound's return to its ground and stable state by rapid release of energy. This factor contributes to its inherent photostability quality.

Octyl triazone (ethylhexyl triazone)

This is a UVB filter marketed under the name of octyl triazone, and it is widely used in Europe, Australia, and Japan. The molecule (molecular weight = 823 Da) has been on the European market for more than 10 years, and it currently awaits approval via the TEA procedure that was filed in 2003.

Antioxidants (AOs) and Repair Stimulators

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

Public health campaigns emphasize the importance of photoprotection and stress the need for avoiding excessive sun exposure, wearing clothing/hats, and applying sunscreens. These passive and preventive actions should only be viewed as the first-line of defense, and active protections that reverse or repair damage from UV rays offer other desired and supplemental benefits. Current photoprotection strategies are not widely adopted by the general public, and even those who use sunscreens do not achieve complete protection, which results in subclinical and clinical damages to the skin. In this section, topical AOs and other agents with capacity to repair or protect cellular damages will be discussed. These agents have been incorporated in many cosmetic products marketed to reverse signs of aging. However, the public and medical community do not regard these actives as a part of photoprotection agents. Perhaps, it is time to rethink the scope of photoprotection, and include these agents as a part of overall discussion.

UVA damages the skin tissue through increased ROS production, and these radicals are neutralized by the body's innate defense mechanism through a series of enzymatic (e.g., superoxide dismutase, catalase, glutathione reductase, and peroxidase) and nonenzymatic AOs. When the buildup of ROS from UV and environmental pollutions exhausts the enzymatic machinery and depletes the AOs reservoir, damage to the DNA, lipid membrane, and protein can occur. Topical AOs exert their effect inside the cells and can reverse this shortage. Furthermore, once penetrated through the stratum corneum, they may remain active for several days (89).

Although AOs can be supplemented through diet and oral supplements, physiological barriers limit adequate level to be delivered to the skin. In order to attain the claimed benefits, any topical formulations with AOs must fulfill a number of requirements. First, the AO agent must have potent antioxidative capacity to quench the reactive oxygen species. Second, the AOs must not be converted into reactive species that initiate a cascade of radical chain reactions. Third, AOs in the final formulation must be stable. However, AOs are inherently unstable, and if not properly formulated, the protective properties are quickly lost. Fourth, high concentrations and correct forms of AOs are needed to achieve deeper penetration and attain the optimal efficacy. Unfortunately, high concentrations may also irritate the skin. Lastly, the formulation needs to be cosmetically elegant. Many AOs are deeply colored, making them cosmetically unacceptable. A number of well studied AOs and are discussed in detail in the following section, and Table 6 showcases other AOs that have been incorporated in topical formulations.

Table 6.  Benefits of natural antioxidants in topical formulations
Antioxidant compoundsSourceEnd pointStudyReference
HumanAnimal
  • a

    Reduction of post-carbon dioxide laser resurfacing erythema.

Flavonoids     
 GenisteinSoy, red clover, ginkgo bilobaErythema++(90,91)
Photoaging +(90)
Immunosuppression +(92)
Photocarcinogenesis +(90)
 EquolMetabolite of daidzeinErythema, inflammation +(91,92)
Photoaging +(93)
Immunosuppression +(92)
Photocarcinogenesis +(94)
 ApigeninFruits and leafy vegetables, tea, winePhotocarcinogenesis +(95)
 DaidzeinSoybeans and plant, red cloverErythema +(91)
 Silymarin/silibininMilk thistle/bioactive component of silymarinImmunosuppression +(96)
Photocarcinogenesis +(97,98)
 Caffeic, ferulic acidsVegetables, olive, olive oilErythema+ (99)
Polypodium leucotomos extractTropical fern plant Polypodium leucotomosErythema+ (100)
Photoaging +(101)
Photocarcinogenesis +(101)
PycnogenolExtract from the bark of the maritime pine treeInflammation +(102)
Immunosuppression +(102)
Photocarcinogenesis +(102)
ResveratrolSkin and seeds of grapes, nuts, fruits, red wineEdema +(103)
Photocarcinogenesis +(104)
Vitamin C (L-ascorbic acid)Most fruits and vegetablesErythema++(105)a, (106)
Immunosuppression +(107)
Photoaging +(108)
Photocarcinogenesis +(108)
Vitamin E (α tocopherol)Plant oilsErythema+ (109)
Immunosuppression +(110)
Photoaging +(108)
Photocarcinogenesis +(111)
Green tea polyphenols (epigallocatechin-3-gallate), (epicatechin-3-gallate), (epigallocatechin), (epicatechin)Polyphenolic fractions isolated from teaErythema++(112,113,114)
Immunosuppression +(115,116)
Photocarcinogenesis +(117–119)
Photoaging +(114)

Vitamin C

Vitamin C (L-ascorbic acid) is the predominant AOs in the skin with concentration reaching 15 times that of glutathione, 200 times that of Vitamin E and 1000 times that of ubiquinol/ubiquinone (120). It is a water soluble and low molecular compound. Aside from the AO property, L-ascorbic acid is an essential cofactor for collagen synthesis (121) and can reduce pigment darkening by inhibiting tyrosinase (122). Lastly, the molecule can also improve epidermal barrier function. Topical application L-ascorbic acid has shown to protect UV-related damage as measured by erythema or sun burn cells (106).

Topical formulation of L-ascorbic acid ensures the delivery of high concentration into the skin. Special formulation that involves removing of the ionic charge from the molecule is needed to enhance penetration (89). A maximum level in the skin can be achieved after 3 days of application with a formulation that is comprised of 15%. Because of the unstable nature, derivatives of L-ascorbic acid have been used in different formulations. Common substitutes include magnesium ascorbyl phosphate and ascorbyl-6-palmitate, both of which can be converted into the active L-ascorbic acid in cell cultures (123). However, the AO property of both derivatives is much lower than that of L-ascorbic acid.

Vitamin E

Vitamin E is another important AO in the skin. Although there are eight major forms, four tocopherols and four tocotrienols, humans mainly use the α-tocopherol. The major function of vitamin E is to quench peroxyl radicals that oxidize and destroy the structural integrity of cellular lipid membranes. The lipophilic nature of the molecule allows high penetration and delivery through the skin. Topical application of α-tocopherol has demonstrated a number of protective effects including reduction in erythema (124), photoaging (125), photocarcinogenesis (111), and immunosuppression (110). Additionally, it also showed the ability to inhibit melanin formation, a useful action to reduce the rate and intensity of pigmentations. Vitamin E works well in conjunction with other AOs, and in the absence of other AOs is depleted quickly. The combination of 15% L-ascorbic acid and 1% α-tocopherol provided a fourfold increase in protection against UV-induced erythema and thymine dimer formation (126) as the depleted vitamin E is regenerated by vitamin C. Aside from combination with other AOs, the correct form of vitamin E also matters. Attempts have been made to stabilize α-tocopherol by converting the hydroxyl group on the ring with an ester. Common substitutes include α-tocopheryl acetate and α-tocopheryl succinate. However, both substitutes are less effective than α-tocopherol in protecting the skin against UV-induced erythema,photoaging, immunosuppression, and carcinogenesis (124,125,127).

Selenium

Selenium is an essential element for the optimal activity of glutathione peroxidase and thioredoxin reductase, two enzymes that play a key role in the defense against oxidative stress. Unlike vitamins C and E, selenium activity level in the skin can be increased via oral supplementation, and a number of studies have shown its protective effect for UV-induced damage ranging from DNA oxidation to lipid membrane destruction (128,129). Topical supplementation of selenium can be delivered in the form of L-selenomethionine, which has shown to increase the minimal erythemal dose in human subjects (130).

Silymarin

Silymarin is derived from the milk thistle plant, Silybum marianum. It contains a mixture of three flavonoids: silybin, silydianin and silychristine. Of these, silybin constitutes the major component with highest biological potency. This plant-derived flavonoid has strong AO effects capable of scavenging ROS and preventing lipid and lipoprotein oxidation. Topical silymarin has been shown to inhibit UVB-induced sunburn cells (97), prevent UVB-induced pyrimidine dimers (131), and reduce the number of UVB-induced tumors in mice (97).

Green tea polyphenols

Green tea contains a rich level of polyphenols in the forms of epicatechin, epicatechin-3-gallate, epigallocatechin, and epigallocatechin-3-gallate. As AOs, tea polyphenols are more potent than vitamins C and E (132). They are capable of scavenging singlet oxygen, superoxide radicals, hydroxyl radicals, peroxyl radicals and hydrogen peroxide. Like vitamin C, these tea polyphenols can regenerate oxidized vitamin E. Aside from the AO functions, tea polyphenols, specifically epigallocatechin-3-gallate, also have anti-inflammatory and anticarcinogenic effects (112,117) and can inhibit collagenase activity. However, it is important to note that not all products containing green tea extracts exhibit the same level of AO properties. Unfermented green tea extract has a very high antioxidative activity, but oxidative fermentation occurs from the raw material storage to production and shipping of the final products. Along the manufacturing steps, the antioxidative property of green tea extracts may diminish significantly. Hence, like other AOs, tea polyphenols are inherently unstable and a large portion of their biological activity is lost over a short duration. The addition of butylated hydroxytoluene may help to stabilize these overly reactive AOs (133).

Conclusions

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References

UV radiation is associated with a variety of negative health effects. Multiple photoprotective methods can be utilized to provide protection against harmful exposure. Physical barriers such as clothing and sunglasses can serve as an effective way to shield UV rays from the skin and eyes, respectively. Topical sunscreens filtering UVB and UVA spectrum are also available to optimize protection. In comparision to old generations of sunscreens, modern products offer more balanced protection from UVB and UVA, but significant progress to improve UVA protection is still needed in the United States. The impending passage of the new FDA guideline on testing and labeling UVA protection and the potential approval of new filters have the potential to stimulate the sunscreen industry to produce better products. Lastly, alternative agents such as AOs, which work by a different mechanism, can be used to supplement and augment photoprotection provided by sunscreens.

References

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Environmental Factors
  5. Clothing and hats
  6. Sunglasses
  7. Sunscreens
  8. UVB filters
  9. Antioxidants (AOs) and Repair Stimulators
  10. Conclusions
  11. References