The Science of Sunscreen

• Sunscreen, also known as sunblock, is a topical product such as lotion, spray, or gel.
• It absorbs or reflects the sun's ultraviolet (UV) radiation to protect the skin.
• SPF stands for Sun Protection Factor.

The Effect of Sunscreen on the Skin

• Acute exposure of unprotected skin to UV radiation produces physiological effects, including sunburn.
• Lifetime exposure to solar UV leads to skin cancers and visible signs of skin aging.

UVA

• Longer wavelengths of UVA range from 320-400 nm, penetrating deeper into the skin, reaching the dermis.
• UVA is associated with skin aging, wrinkling, and long-term skin damage.
• UVA contributes to tanning but can indirectly damage DNA, increasing skin cancer risk and are present all day and can penetrate windows.

UVB

• UVB includes shorter wavelengths (290-320 nm) that mainly affect the outer layer of the skin (epidermis).
• UVB is the primary cause of sunburn, damages DNA directly, and is a significant factor in skin cancer risk.
• UVB rays are more intense at midday, high altitudes, or closer to the equator.

Ultraviolet Radiation Skin Effects of UVR

• UVB is the main cause of acute sunburn and tanning, making sunscreen efficacy directed against UVB radiations.
• Exposure to UVB radiation is immunosuppressive, mutagenic, and carcinogenic.

Importance of the Biological Effects of UVA

• The biological effects of UVA have been recognized.
• UVA induces photobiological reactions, requiring oxygen, such as immediate and delayed tanning and new melanin formation.
• UVA contributes to long-term degenerative changes in the skin, such as premature skin aging and cancer formation, and UVB-induced carcinogenesis.

Free Radicals, Free Radical Scavengers, and Antioxidants

• Photochemical reactions from UVR are intricately coupled with the chemistry of free radicals.
• The photostability of UV filters is one aspect of the reaction in cosmetic products applied on sunlight-exposed skin.

The Role of Reactive Oxygen Species(ROS)

• ROS and free radicals may cause skin damage, where exposure to UVR generates a multitude of ROS.
• ROS cause injury by reacting with molecules like lipids, proteins, and nucleic acids, and deplete skin's natural defenses.
• Molecules that control ROS in sunscreens are beneficial in terms of photodamage or actinic damage of UV-exposed skin.

Active Ingredients

• Most sunscreen products are similar to lotions or creams, with 4% to 40% sunscreen actives.
• UV filters can be categorized into four groups: polar oils, oil-soluble crystalline solids, water-soluble salts, and insoluble powders/particulates.
• Polar oils include octinoxate, octisalate, homosalate, and octocrylene.
• Oil soluble crystalline solids include avobenzone and benzophenones.
• Water soluble salts include ensulizole.
• Insoluble powders/particulates include zinc oxide and titanium dioxide.

Types of Sunscreen

• Polar oils can make the product feel greasy and oily at high concentrations.
• Oil soluble crystalline solids need high levels of oily solvents/emollients to dissolve, which can make the skin feel greasy and oily.
• Water soluble salts reduce the capability of aqueous polymeric thickeners, leading to higher polymer levels, making the product feel sticky and heavy.
• Insoluble powders/particulates can make the product feel dry and draggy and lead to a white appearance on the skin.

Specific Legislation Concerning Sunscreens

• Sunscreens are subject to specific regulations in almost every country.
• UV filters allowed/maximal concentrations are in regulatory documents by authorities, but regulations vary between countries.
• In the United States, UV filters are over-the-counter drugs; the FDA's monograph of sunscreen drugs specifies safety, efficacy, and labeling conditions; went into effect in 2001.
• In the EU, sunscreens are regulated as cosmetic products.
• In Canada, sunscreens are regulated by Health Canada as drugs.

Improving Sunscreen Product Aesthetics

• The primary strategy for sunscreen formulation is to use a lower concentration and number of UV filters to achieve the most efficient sunscreen systems.
• There are many approaches to manage the trade-offs of sunscreen actives.
• Cosmetic powders reduce the greasiness of oily UV filters or solvents, to give the product a drier skin feel.
• Oil-soluble film-forming polymers thicken the oily sunscreen actives and solvents/emollients, reducing slick/oily/greasy feel on the skin, increasing efficiency by improving skin coverage.
• Incorporating silicone emollients reduce the draggy, dry skin feel of zinc oxide and titanium dioxide sunscreen actives.
• Alternative product forms minimize product skin feel negatives, such as rub-free sprays.
• These types of technologies improve efficiency and promote aesthetics, which have allowed new generations of sunscreens, even allowing products with SPF greater than 15 to be formulated as daily use products such as moisturizers.

American Academy of Dermatology

• The American Academy of Dermatology's Guidelines/Outcomes Committee developed "Guidelines of Care for Photoaging/Photodamage."
• These guidelines state that there's no scientific evidence that contradicts the relation of sun exposure to the development of skin cancer and the undesirable results of photoaging and photodamage.
• Early recognition and treatment of photodamaged and photoaged skin will decrease the incidence of premalignant and malignant skin lesions.
• Photodamage and photoaging are partially reversible with photoprotection, and sunscreen use is encouraged.

American Cancer Society

• Long-term exposure can cause skin changes, including aging, wrinkles, loss of elasticity, dark patches (lentigos, or "age spots"), and actinic keratoses.

American Society for Photobiology and European Society for Photobiology

• The American Society for Photobiology (ASP) concerns the interaction of light and living things, including the harmful effects of UV on humans.
• Sunlight is implicated in several skin diseases, including premature aging and skin cancer.
• Skin sensitivity to sunlight is controlled by an individual's genetic ability to produce melanin, the pigment that helps protect the skin.
• Topical and systemic sunscreen agents prevent the acute and chronic effects of sunlight allowing outdoor activities with a reduced risk of sun-induced injury.
• Damage that absorbed light creates in the skin, such as aging, is preventable by using water- and sweat-resistant sunscreens.

Conclusion

• Exposure to solar UV damages human skin.
• This can be in the form of acute over-exposure resulting in sunburn or subtle subclinical damage.
• Repeated exposure manifests as photoaging and skin cancers after many years.
• Sunscreens were probably of benefit in reducing squamous cell carcinoma but there was not enough evidence supportive of protection against basal cell or melanoma skin cancers.