|Year : 2018 | Volume
| Issue : 2 | Page : 78-82
Sunscreens: Time to think beyond UV rays
Chitralekha Keisham1, Nelson Elangbam2, Rashmi Sarkar3
1 Department of Dermatology, JNIMS, Porompat, India
2 Medical Officer, State Health Service, Manipur, India
3 Department of Dermatology, MAMC, Delhi, India
|Date of Web Publication||14-Dec-2018|
Dr. Chitralekha Keisham
Department of Dermatology, Jawahar Lal Nehru Institute of Medical Sciences, Imphal
Source of Support: None, Conflict of Interest: None
It has been known to us that solar radiation contributes to photoaging. Until recently, it was thought to be due to ultraviolet rays alone. However, a growing number of evidence confirms that visible and infrared (IR) rays also contribute to extrinsic aging. Visible and IR rays account for 50% and 45% of the solar radiation reaching the earth. Ultraviolet A induces retrograde mitochondrial signal, thus leading to induction of matrix metalloproteinase. Ultraviolet B and IRC cause heat-related generation of free radicals and destruction of collagen and elastin. Exposure to visible light induces cytokines, free-radical formation, and pigmentary changes in human skin. The end result of solar radiation is generation of free radicals and ultimately oxidative damage, photoaging, and photocarcinogenesis. The present broad spectrum sunscreen does not provide complete protection of the human skin from oxidative insult. So, a combination of a sun protection factor active component along with an antioxidant is the ideal way of photoprotection. Till date, a number of antioxidants have been tried in human and animals which have shown to be an effective photoprotective agent, though few studies have failed to prove the same. Even with conflicting reports, effect of antioxidants on human skin needs to be explored more. A good study design with a large sample size in humans must be conducted as visible light and IR rays contribute significantly to photodamage.
Keywords: antioxidants, infrared, sunscreens, UV rays, visible light
|How to cite this article:|
Keisham C, Elangbam N, Sarkar R. Sunscreens: Time to think beyond UV rays. Pigment Int 2018;5:78-82
| Introduction|| |
Sunscreens are a group of chemicals used for protection of human skin from various acute and chronic side effects of sun exposure. Acute exposure to sun rays causes sunburn and delayed tanning. Chronic sun exposure is associated with photoaging, actinic keratosis, and squamous cell carcinoma. Intermittent sun exposure is associated with basal cell carcinoma and melanoma. The first ultraviolet (UV)B filter para-aminobenzoic acid (PABA) was patented in 1943 and the first UVA filter benzophenone was introduced in 1962. In 1982, Lorraine Klingman supported the role for infrared (IR) radiation (760–4000 nm) in premature skin aging. Haywood showed that visible light (400–700 nm) also contributed to skin damage via induction of radical formation. Although initially used mainly for protection against UV wavelength, in the past decade, it has come to the knowledge that wavelength beyond UV rays, that is, visible and IR rays, also contribute to skin damage and photoaging in particular. And since then, this has prompted the development of novel products for photoprotection.
Ours was an evidence-based review. Study data were searched in PubMed for articles related to the use of sunscreens and newer agents for photoprotection that would include protection against visible light and IR rays. There was no limit in the search timeframe. Keywords used were sunscreens update, polyphenols, and antioxidants. The search was further extended to the keywords solar protection, IR radiation, visible rays, ectoin, oxothiazolidine, and Uvinul A plus (BASF, Ludwigshafen, Germany).
Solar radiation and skin
The optical spectrum of sunlight consist of UV rays, visible, and IR rays which accounts for 5%, 50%, and 45% of the total spectrum. Among the UV rays, UVC is filtered by the ozone layer. UVB rays are 5% of the solar UV radiation and are mainly responsible for a variety of skin diseases, including nonmelanoma and melanoma skin cancers. UVB (280–315 nm) is partially filtered by ozone layer, absorbed by melanin, and it penetrates up to the basal layer of the epidermis, producing reactive oxygen and nitrogen species (ROS and RNS). UVB rays are responsible for cutaneous inflammation, sunburn, aging, formation of cyclobutane pyrimidine dimers and photoadducts. Accumulation of mutation in skin further due to excision repair failure leads to development of UV-associated cancers. UVA comprises 90% to 95% of the solar UV spectrum, and it is considered as an aging ray. UVA (315–400 nm) is less energetic than UVB, but it is present is a larger amount. It is absorbed by melanin, riboflavin-containing flavin adenine dinucleotide, and flavin mononucleotide. UVA reaches up to the dermis. It also generates ROS and RNS leading to oxidized deoxyribonucleic acid (DNA) base, hence causing premature aging and risks of cancer., Because UVA produces only a small number of pyrimidine dimers in skin, it is assumed that much of the mutagenic and carcinogenesis of UVA radiation is mediated through reactive oxygen species.
Visible light accounts for 50% of the solar light. It penetrates into tissue, and 20% of it reaches the hypodermis. Visible light is absorbed by it, schromophore which are hemoglobin, melanin, bilirubin, riboflavin, and porphyrins. This was followed by generation of ROS, inflammatory cytokines, and matrix metalloproteinase (MMP) enzymes in human epidermal equivalents., In ex vivo skin explants, ROS produced were 4% for UVB, 46% for UVA, and 50% for visible light. Visible light produces DNA damage in the form of oxidized DNA. Visible-light-induced pigmentation on skin types IV to VI was darker and lasted longer as compared to irradiation with long UVA. Pigmentation with visible light was induced with 415-nm wavelength and not 630-nm radiation on skin types III and IV. The pigmentation lasted as long as 3 months.
IR rays consist of IRA (700–1400 nm), IRB (1400–3000 nm), and IRC (3000 nm–1 mm). Of these IR rays, IRA penetrates skin deeply. IRA is 30% of all IR radiation and 65% of it reaches the dermis and 20% the hypodermis. The action of IRA is mainly mediated through interaction with cytochrome C oxidase as a chromophore, leading to dysfunction of mitochondrial electron transport and production of ROS and triggering retrograde mitochondrial signaling. Retrograde signaling leads to modulation of genes involved in photo aging, that is, MMP-1 and type 1 procollagen. IRA radiation directly stimulates dermal fibroblast to produce MMP-1, and this was not mediated through heat. IRB and IRC induce MMP and cytokines production through heat-sensitive receptors., This leads to generation of ROS, an important step in tropoelastin expression. There is an imbalance with increased tropoelastin production and decreased fibrillin-1, leading to decreased tropoelastin deposition on microfibril causing abnormal elastic fiber formation. Moreover, MMP destroys newly formed tropoelastin and fibrillin. As a result, IR rays cause degeneration of elastin and collagen leading to photodamage.
Sunscreens-related indices have been formulated by various in vitro and in vivo methods to determine their efficacies. They are developed for both UVA and UVB spectrum. Various indices are described below.
SPF for UVB
It refers to the measurement of protection of skin from the harmful effects of UVB radiation. It is defined as the ratio of minimal erythema dose (MED) of photoprotected skin to MED of unprotected skin.
Grades of sun protection factor (SPF) are
- Low SPF: 2 to 15.
- Medium SPF: 15 to 30.
- High SPF: 30 to 50.
- Highest SPF: 50+.
- Japanese standard (persistent pigment darkening): It is an in vivo method and calculated as UVA dose required to produce the effect with the sunscreen agent to that produced without an agent after 2 to 24 h.
- Australian/new standard: An 8-μm layer of the product should not transmit more than 10% of radiation of 320 to 360 nm or a 20-μm layer of the product should not transmit more than 1% of radiation of 320 to 360 nm.
- European Union guidelines: UVA protection factor (persistent pigment darkening) = 1/3 of SPF and critical wavelength = 370 nm.
- Boot star rating system: It is used in the United Kingdom and in an in vitro measurement of a product’s UVA (340–400 nm) absorbance over its UVB (290–320 nm) absorbance. Product with better UVA absorbance has higher Boot star rating.
Classification of sunscreen
Sunscreens have been mainly classified on the basis of blocking the UV radiation conventionally. There are reports of both visible light and IR radiation having detrimental effects on human skin. The last food and drug administration (FDA) approval was given for 16 sunscreens in 1999. There are three nomenclatures for sunscreen agents in the world. These are the International Nomenclature Cosmetic Ingredient name, US adopted name, and trade name.
Sunscreens consist of both organic and inorganic filters protecting against mainly UVA and UVB. But again the issues of stability, safety, and broader spectrum arose, and new agents having more photostability and broader spectrum covering both UVA and UVB were added. As of now considering all the factors, newer sunscreens are the need of the hour covering all the three spectrums, namely, UV radiation, visible light, and IR radiation. The agents protecting against the visible light and IR are at the experimental stage. The nonmicronized optically opaque zinc oxide, titanium dioxide, and iron oxide are able to block visible light. The antioxidants, namely, grape seed extract including flavonoids, procyanidins, phenolic acids, etc., and extracts of Scutellaria baicalensis and Polygonum aviculare (high phenolic and flavonoid contents), may be able to act against the IR radiation.,, Newer agents such as ectoin are also coming up with protection against UV rays and beyond. A brief classification is proposed from the available data which is described in [Table 1].
Role of antioxidants in photoprotection
The end point of all different solar wavelength on human skin is the increased production of reactive molecule species and increased oxidative stress. The present broad spectrum sunscreen does not protect human skin from 94.2% of solar radiation (comprised of visible and IR) or from heat accumulation damage. At present, only opaque filters such as nonmicronized form of zinc oxide, titanium dioxide, and iron oxide are able to block visible light. Unfortunately, these compounds are matte white or red, water insoluble, and leave a tinted coating on the skin, which are cosmetically unacceptable to the patients. So, the need of the hour for sunscreen is an SPF active ingredient along with antioxidants. The antioxidants must be stable to solar radiation, heat, and as well as highly potent in neutralizing ROS while promoting tissue repair.
There are numerous antioxidants that have shown efficacy in preventing photodamage. Among them, polyphenols have gained importance in the last decade. Polyphenols are a group of naturally occurring plant products that are widely distributed in plant foods including fruits, vegetable, nuts, seeds, flowers, and bark. Most of the natural polyphenols are pigments, typically yellow, red, or purple, and can absorb UV rays. They can absorb the entire UVB spectrum and part of UVC and UVA spectra. Antioxidants mainly polyphenols in oral and topical formulation, in both human and animal studies, have shown anti-inflammatory, antioxidant action against UV rays., Green tea polyphenols also have ability to repair photodamaged DNA as seen in epigallocatechin-3-gallate-induced DNA repair by nucleotide excision repair as shown by Meeran et al. in human fibroblast. Various animal models have been able to show the antiphotocarcinogenic effects of polyphenols such as oral green tea polyphenols, topical epigallocatechin, oral proanthocyanidins, topical resveratrol, and silymarin.,,, However, this studies are limited by their small sample size and poor study designs. Contradictory to this, a randomized, placebo controlled trials in 50 volunteers by Farrar et al. using systemic green tea failed to demonstrate protection from UV-ray-induced sunburn. Similarly oral green tea in randomized controlled trial in 50 healthy adults did not provide protection from direct DNA damage induced by higher dose solar simulated radiation.
Topical use of ẞ-carotene (2 mg/cm2) was protective for human skin against IR radiation in a study by Darwin et al. on healthy volunteer. The in vivo cutaneous carotenoid concentration, measured by resonance Raman spectroscopy showed that free radicals produced due to IR radiation can be effectively neutralized by topically applied antioxidants. The relative degradation rate for a definite IR radiation dose was identical for all volunteers, independent of their initial carotenoid level. This means that individual living on a healthy diet rich in fruit and vegetable are better protected than those living on antioxidant poor nutrition and a stressed lifestyle. Another study also showed the efficacy of a topical mixture of antioxidants (vitamin C, vitamin E, ubiquinone, and grape seed extract) in preventing IRA-induced MMP-1 expression in human skin. Importantly when the above same mixture of antioxidants were added to a SPF 30 sunscreen, there was a significant reduction in MMP-1 messenger ribonucleic acid expression as compared to SPF 30 sunscreen alone in healthy volunteers. However, data analysis of commercially available combined sunscreen and topical antioxidants available in market showed no antioxidant power or low power. Contradictory to this, a study by McDaniel et al. using a combination of sunscreen along with antioxidants demonstrated improvement in lines and wrinkle after 4 weeks of once daily use in patients of moderate-to-severe photodamage. Efficacy was also seen in vitro and ex vivo studies for the same. These results show the effect of antioxidants to repair the existing damage. Although some data on topical sunscreen combined with antioxidants are conflicting, future prospect into such a formulation must be explored widely as it is much needed.The effectiveness of oral antioxidants depends on its metabolism and bioavailability. For topical formulation of antioxidants, it requires a suitable formulation for enhanced penetration to provide maximum action. For sunscreen to provide the mentioned protection, they have to be applied adequately. Unfortunately, sunscreens have been underapplied most of the time. This could be the reason why sunscreens with antioxidants fail to show results on actual human use. In addition, there is the issue of penetration and bioavailability of the active component of topical formulation of sunscreens and antioxidants.
| Conclusion|| |
The harmful effect of solar radiation is not due to UV rays alone. Both visible light and IR rays are responsible for oxidative stress and aging. These have been proved by in vitro studies and clinical studies. So, a broad solar protection requires an SPF active component with a potent efficacious antioxidant. Moreover, well-designed human studies of topical as wells as oral antioxidants on a larger sample size must be undertaken. Presently, the combination of sunscreens with antioxidants has abundantly reached the consumer level. As of now, there are no well-defined criteria for the photoprotective effect given by antioxidants. Despite the emerging trend to use a novel sunscreen protecting against all components of the solar spectrum, the need to use protective clothing and shade seeking behavior have to be emphasized as sunscreens are incorrectly used most of the time.
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Conflicts of interest
There are no conflicts of interest.
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