Sunscreens are topical formulations developed to protect human skin from the harmful effects of ultraviolet radiation (UVR). These products act as part of a broader photoprotection strategy, aiming to reduce risks associated with sun exposure, such as erythema, photoaging, and skin cancers. Despite their intended protective function, questions regarding sunscreen efficacy, safety, and environmental impact persist.
Ultraviolet Radiation and Its Effects
UVR constitutes approximately 5% of the total solar radiation reaching Earth and is categorized into three types based on wavelength: UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm). While UVC is largely absorbed by the ozone layer, UVA and UVB reach the surface and impact the skin. UVB primarily causes sunburn and contributes to DNA damage in the epidermis, while UVA penetrates deeper into the dermis, causing collagen degradation and photoaging. Both UVA and UVB are associated with mutagenesis and carcinogenesis.
Historical Development of Sunscreens
The use of topical agents for sun protection dates back to ancient civilizations such as the Egyptians and Greeks. The commercial development of sunscreens accelerated in the early 20th century, particularly during World War II. Early formulations were primarily physical barriers, later supplemented with chemical agents like PABA. The recognition of UVA-induced photoaging in the 1960s spurred the inclusion of UVA filters such as avobenzone in subsequent decades.
Types of Sunscreen Filters
Sunscreens are classified by their active ingredients:
Inorganic (Physical) Filters: These include zinc oxide and titanium dioxide. They primarily reflect and scatter UVR. Newer formulations with micronized particles offer improved cosmetic acceptability.
Organic (Chemical) Filters: These absorb UVR and convert it into heat. Examples include avobenzone, octinoxate, and oxybenzone. Some organic filters have raised health concerns due to their potential for systemic absorption and hormonal disruption.
Regulatory Classification
Regulations vary internationally. In the United States, sunscreens are considered over-the-counter drugs regulated by the FDA, whereas in Europe they are categorized as cosmetics. Consequently, fewer UV filters are approved in the U.S. compared to Europe. Regulatory differences also affect labeling, testing standards, and permitted ingredients.
Sun Protection Factor (SPF) and UVA Ratings
SPF primarily measures protection against UVB-induced erythema. An SPF 15 sunscreen blocks approximately 93% of UVB rays, while SPF 30 blocks around 97%. However, the efficacy is highly dependent on application thickness. UVA protection is indicated through systems such as the UVA star rating, persistent pigment darkening (PPD), and the PA system.
Application and Effectiveness
Effective photoprotection requires the application of 2 mg/cm² of sunscreen. Studies have shown that users typically apply less than this amount, leading to reduced protection. Formulations (e.g., cream, lotion, stick) influence user behavior and application uniformity. Uniform coverage is critical to maintain labeled protection levels.
Potential Health Concerns
Some chemical filters have been detected in human tissues and fluids, raising concerns about systemic absorption. Reported adverse effects include contact dermatitis and possible endocrine-disrupting activity. Recent investigations have also explored the correlation between sunscreen use and conditions like frontal fibrosing alopecia, although causality has not been established.
Vitamin D and Cardiovascular Considerations
UVB exposure is necessary for endogenous vitamin D synthesis. Concerns exist about whether sunscreen use inhibits this process. However, real-world studies suggest that regular sunscreen use does not significantly impact vitamin D levels. Additionally, some evidence suggests that UVR exposure may influence blood pressure, prompting discussions about balancing sun exposure and protection.
Environmental Impact
UV filters have been detected in water bodies, including treated wastewater, raising concerns about aquatic toxicity and bioaccumulation. Certain compounds, such as oxybenzone and octinoxate, have been associated with coral reef damage and have been banned in specific regions. However, the overall contribution of sunscreens to coral bleaching remains under investigation.
