- When applied correctly, sunscreens are efficacious in preventing acute sunburn and tanning.- When applied correctly, sunscreens are efficacious in preventing acute sunburn and tanning.
- They have also been successful in reducing some chroniceffects of UVR irradiation, including immunosuppression,photocarcinogenesis, and photoaging.
- Vehicle type and application patterns
- Sunscreen vehicles often determine product efficacy.
- To maintain the photoprotective properties and photostability of its UV filters, a sunscreen vehicle must minimize interaction of inert and activeIngredients.Vehicle type also determines sunscreendurability and water resistance.
- The FDA designates sunscreens with intact photoprotective properties after two 20 minute exposures to water as ‘‘waterresistant.’’
- The term ‘‘very water-resistant’’ has replaced ‘‘water proof’’ and applies to sunscreens remaining effective after four 20-minute periods of water immersion
- In addition, despite their label,even ‘water resistant, sunscreens can exhibit a gradual loss in SPF withsubsequent water immersions
- Sunscreen vehicles include lotions/creams, water based gels, sticks, sprays, and cosmetics. Lotions and creams, consisting of either oil-in-water or water-in oil emulsions, permit the greatest diversity of formulations and are most commonly used.
- Gels are easily washed away by swimming and sweat, but are preferred by patients with oily skin or recurrent acne.- Sticks are used for protecting small areas like the lips or nose. Sprays are convenient, but are oftenapplied insufficiently.
- Sunscreen agents may also be found in hair care products, such as shampoos and artificial dyes, to protect against UV-induced color change and protein damage.
- Vehicle type often plays a role in cosmetic acceptability,application patterns, and compliance.
- The opacity of inorganic sunscreens and the greasiness of organic agents may contribute to inadequate application and subsequent SPF reduction.
- Sunscreen failure can stem from insufficient application and infrequent reapplication
- Sunscreen use may reduce the risk of developing actinic keratoses (AKs) and NMSC, and regular use of sunscreens has been shown to be cost effective in preventing NMSC and AKs.
The mechanism by which these agents reduce photocarcinogenesis may involve protection against UV-induced reduction in epidermal antigen-presenting cells
- The association between melanoma and sunscreen use is more ambiguous. It was previously thought that sunscreen use could increase the risk of developing melanoma.
However, two separate reviews failed to find correlations between sunscreenuse and melanoma risk.
The multifactorial nature of melanoma may partly explain the inconsistent associations found between sunscreen use and melanoma development.
- Chronically irradiated skin can be metabolically overactive, leading to epidermal hyperplasia, irregular pigmentation, telangiectases, elastosis, reduced collagen, and wrinkles.
One 2-year, randomized trial demonstrated that regular use of SPF 29 sunscreen prevented solar elastosis to a greater degree than placebo.
An additional study showed that daily broad-spectrum sunscreen use prevented changes ,associated with photoaging, including epidermal thickening and decreased collagen.
SAFETY AND ADVERSE EFFECTS OF SUNSCREEN
- While sunscreens can effectively protect against photocarcinogenesis and photoaging, they may also have adverse effects, including contact sensitivity, risk of vitamin D deficiency, and estrogenicity.
Sensitivity and toxicity from systemic absorption- Although infrequent and subjective, stinging and burning are the most common complaints associated with sunscreens
- True allergic contact dermatitis is rarely caused by sunscreen ingredients, althoughits incidence may be underestimated
- PABA and oxybenzone are the most common contact photoallergens in use today, whereas avobenzone, sulisobenzone, octinoxate, and padimate O have fewer reported reactions.
- Patients with photodermatoses and eczema may be predisposed to developing photoallergy and should be counseled appropriately.
- Salicylates, ecamsule, and inorganic agents cannot penetrate the stratum corneum, so photosensitivity to these agents is uncommon.
- nanosized- titanium dioxide can pass through cell membranes- andimpair function ofhuman dermal fibroblast- cultures.61 Polymer-coating these particles may prevent- cell membrane adhesion, thereby preserving cell- function.61 Microfine zinc oxide and titanium dioxide-can induce neural stem cell apoptosis, although this- toxic effect may be more dose dependent than size- dependent.59,62,63
- nanosized- titanium dioxide can pass through cell membranes- andimpair function ofhuman dermal fibroblast- cultures.61 Polymer-coating these particles may prevent- cell membrane adhesion, thereby preserving cell- function.61 Microfine zinc oxide and titanium dioxide- can induce neural stem cell apoptosis, although this- toxic effect may be more dose dependent than size- dependent.59,62,63
- Despite these concerns, the benefits- afforded by proper sunscreen use likely outweigh- the toxicity risks. To definitively determine the risk of systemic absorption of sunscreen ingredients, testing- appropriate filter concentrations and commercial- sunscreen formulations will be helpful, particularly- in children and pregnant women.
- Vitamin D
- Vitamin D synthesis requires exposure to UVB.
At least 90% of an individual’s vitamin D requirement is estimated to be derived in this manner.
- There is ongoing controversy over the effects of sunscreen use on UV-dependent cutaneous synthesis of vitamin D3 (cholecalciferol).
- Because proper use of SPF-15 sunscreen- may reduce vitamin D synthesis by over 98%,- some suggest that regular sunscreen application may- cause vitamin D insufficiency, whereas others have- shown that long-term sunscreen use has little to no- effect on vitamin D levels and function.
- Possible- explanations for these discrepancies include the fact- that a significant portion of vitamin D is obtained- through diet, limited sun exposure is required to- produce adequate vitamin D, most people apply- sunscreen insufficiently, and even when applied appropriately- some UVR still penetrates the skin.
- While- the American Academy of Dermatology (AAD) once- asserted that vitamin D deficiency in healthy individuals- was not related to sunscreen application, it- recently revised its position, stating that regular sunscreen- use may increase the likelihood of developing- vitamin D insufficiency, such that some patients could- require extra vitamin D via diet or supplements.77
- While sunscreen use and photoprotection can- effectively reduce skin cancer risk, individuals must- be aware of their vitamin D status to avoid other- risks.
- Patients at high risk for vitamin D insufficiency- include those who are elderly, darker skinned,- housebound, photosensitive, or obese, and patients- residing in more northern latitudes who regularly- use sunscreens and photoprotective clothing.87,88- The AAD recommends counseling these groups- regarding the U.S. Department of Agriculture- Dietary Guidelines for vitamin D supplementation,- which recommends a daily dose of 1000 IU, obtained- through the diet or supplements.89
- One study reported that- padimate O, octinoxate, homosalate, and oxybenzone- increased in vitro proliferation of MC7 cells, a- human mammary tumor cell line. This study also- noted a dose-dependent increase in the uterine- weight of immature Long-Evans rats when fed- octinoxate and oxybenzone, although its experimental- methods might have been flawed.
- Homosalate and oxybenzone exhibit anti-androgen- and anti-progesterone activity in vitro.
- In one study, oxybenzone and octinoxate were- systemically absorbed after 1 week’s application,- although no significant change in reproductive hormone- levels was observed.
- IMPLICATIONS OF SUNSCREEN USEBehavioral changes- A recent review examined the association between- sunscreen use and sun exposure, concluding- that sunscreen use increased the duration of intentional- sun exposure between 13% and 39%.99- Specifically, use of sunscreens with a higher SPF- appeared to lengthen exposure times.100
- The anticipated- protection from sunscreen likely instills a false- sense of safety, but more importantly, higher SPFs- raise the threshold for sunburn. - Individuals extending- their sun exposure in this manner may in fact be at- increased risk of skin cancer.Recommended sunscreen use and photoprotection
- The AAD recommends regular sunscreen use to- prevent skin cancer. Selecting a sunscreen with broadspectrum- (UVB/UVA) coverage is vital, and daily use- of an SPF30 product is recommended. Sunscreens- must be applied liberally and uniformly, 15 to 30- minutes before exposure. To remain effective, they- must be reapplied often, especially when perspiring- or swimming. However, sunscreens are only a single- aspect of photoprotection.
- Adopting sun-protective- behaviors is the best way to prevent UV-induced skin- damage, such as avoiding the sun between 10 AM and- 4 PM,whenUVR is at its peak. Seeking shade may help,but it is estimated that 50% of UVA exposure occurs in- the shade.104 Sunglasses, wide-brimmed hats, and- sun-protective clothing are alternative ways to shield- skin from the effects of UVR.FUTURE DIRECTIONS
- Octyl triazone (ethylhexyl triazone, Uvinul T150),- enzacamene (methyl benzylidene camphor), and- amiloxate (isoamyl p-methoxycinnamate) are three- UVB filters that will likely be added soon to the FDA- sunscreen monograph.
- The photostable Mexoryl XL (L’Oreal; drometrizole- trisiloxane, silatriazole) filters UVB and UVA2,- acting synergistically with ecamsule to increase dramatically- overall UVA protection.3 Tinosorb S (bemotrizinol)- and Tinosorb M (bisoctrizole) are highly- photostable, broad-spectrum agents
- Tinosorb S is- oil soluble and stabilizes avobenzone and octinoxate.- 33 Tinosorb M consists of microfine particles- (100-200 nm) dispersed within the aqueous phase- of sunscreens and can reflect, scatter, and absorb- UVR.14 Because of their relatively large size,- Tinosorb filters rarely cause allergic contact- dermatitis, systemic absorption, or endocrine-like- effects.26,105
- Uvasorb HEB (diethylhexyl butamido- triazone) is another UVA agent presently under FDA- review. - Neo Heliopan AP (bisdisulizole disodium)- and Uvinul A Plus are other promising prospects- which will likely be considered by the FDA in the- future. Neo Heliopan AP is a water-soluble UVA filter,- while Uvinul A Plus is the anticipated successor to- avobenzone because of its high photostability and- similar absorptive properties
- Topical antioxidants,- including flavonoids, resveratrol, and- green tea extracts, may diminish UV-related skin- damage, although they can be unstable and diffuse- poorly into the epidermis.14,106-109 DNA repair enzymes,- such as T4 endonuclease V and photolyase,- have been shown to decrease DNA damage after UV- exposure.110-112 Similarly, application of thymidine- dinucleotides to human skin before UV exposure- may prime the cellular DNA repair response and- diminish subsequent UV-induced damage.11
- Developed by Sol-Gel Technologies, Ltd, the- microencapsulation approach loads UV filters into- 1-m silica shells.- Because the- active ingredients do not directly contact the- skin, microencapsulation may help to lower the incidence- of contact allergies and systemic sunscreen- absorption. Several products currently take advantage- of this technique, specifically Eusolex UV Pearls- (Merck).
- In summary, the efficacy of sunscreens in reducing- photoaging and skin cancers is widely documented.- UVA- and UVB-blocking products protect against- sunburn as well as more subtle suberythemal skin- damage. While there may be concerns over the longterm- safety of UV filters, the benefits of sunscreens- clearly outweigh their potential risks.
