The role of sunscreen in preventing skin cancer

Introduction:

Skin cancer is the most frequent type of cancer, and both clinical and epidemiological studies have linked cumulative solar exposure and the frequency of sunburns to skin cancer. Every year, more than 5.4 million new cases of skin cancer are detected, resulting in considerable health and financial costs. Sunscreen, sun avoidance, and protective gear are all recommended skin cancer prevention techniques.(1)

Skin cancer's broad occurrence imposes major health and financial consequences; from 2007 to 2011, treatment cost an average of $8.1 billion per year.(2) Furthermore, both melanoma and non-melanoma skin cancer (NMSC) have lethal consequences.(3)

The majority of sun protection research focus on photodamage to skin deoxyribonucleic acid (DNA) as the primary endpoint, with fewer studies evaluating the use's effectiveness in preventing skin cancer as an outcome metric. Wearing sun-protective clothes and avoiding direct sunlight minimises exposure to ultraviolet (UV) radiation and the risk of sunburn.(4) Sunburn is frequently employed as a biomarker of skin cancer risk, and studies show that sunburn at any age increases skin cancer risk, implying that wearing protective clothes and seeking shade reduces skin cancer risk.(5) However, evidence that these preventative actions diminish skin cancer is sparse.(6)

Sun avoidance, or seeking shade and avoiding exposure during peak hours of 10 a.m. to 3 p.m., is another popular way of sun protection.(7) However, while this technique reduces total UV exposure, its efficiency in lowering skin cancer risk is unknown, and studies show that seeking shade alone does not reduce the risk of skin cancer.(8) Furthermore, few adults regularly employ more than one preventative approach, and it is possible that those who avoid peak sun exposure are less likely to combine this technique with a physical barrier or sunscreen, reducing its efficiency.(9)

How Do Sunscreens Work?

Sunscreen formulations incorporate either chemical (organic) or physical (inorganic) components that serve to obstruct ultraviolet (UV) radiation, which comprises light with wavelengths shorter than visible light. This UV spectrum is further divided into ultraviolet A (UVA1, UVA2), ultraviolet B (UVB), and ultraviolet C (UVC) rays. Generally, shorter wavelengths pose a higher risk of causing biological harm. Sunscreen filters effectively block UVA1, UVA2, and UVB radiation.(10)

Chemical filters, such as oxybenzone, avobenzone, octocrylene, and ecamsule, consist of aromatic compounds that absorb intense ultraviolet radiation, resulting in excitation to higher energy states. Upon returning to their ground states, these molecules release absorbed energy in the form of lower-energy wavelengths, such as infrared radiation (heat).

On the other hand, physical sunscreen filters like titanium dioxide and zinc oxide work by reflecting or refracting ultraviolet radiation away from the skin. However, research indicates that when particle sizes are extremely small, as in micronized sunscreens, their mechanism of action resembles that of chemical filters. Specifically, micronized zinc oxide and titanium dioxide exhibit behavior akin to semiconductor metals, absorbing ultraviolet light across a significant portion of the electromagnetic spectrum.(11)

How effective are sunscreens in preventing photoaging and skin cancer?

Sunscreens have demonstrated significant effectiveness in preventing both photoaging and skin cancer. Photoaging refers to premature skin aging caused by UV radiation exposure, leading to wrinkles, fine lines, and sagging skin. By blocking or absorbing UV rays, sunscreens help mitigate this damage, slowing down the aging process.(12)

Moreover, UV radiation is a primary risk factor for various forms of skin cancer, including non-melanoma (like basal cell carcinoma and squamous cell carcinoma) and melanoma. Sunscreens reduce the skin's exposure to UV radiation, thereby lowering the risk of DNA damage and mutations associated with cancer development.

Research studies consistently support the effectiveness of sunscreens in these areas. For instance, a study in the Journal of the American Academy of Dermatology highlighted that regular sunscreen use significantly decreases the risk of melanoma, the most lethal skin cancer. Similarly, findings published in the Journal of Investigative Dermatology indicate that daily sunscreen application can prevent photoaging and diminish the appearance of aging signs.(13)

However, it's important to integrate sunscreens into a comprehensive sun protection regimen, which includes seeking shade, wearing protective clothing, and minimizing sun exposure during peak UV hours. Moreover, proper and regular application of sunscreen is crucial for maximizing its protective benefits.(14)

Who should use sunscreen?

According to the American Academy of Dermatology, regular sunscreen application with a Sun Protection Factor (SPF) of 30 or higher is recommended for individuals of all skin types. However, it's worth noting that skin cancers are more commonly found in individuals with lighter skin tones compared to those with darker skin tones. While there haven't been specific studies examining the effectiveness of regular sunscreen use in reducing the risk of skin cancers among individuals who are not White, the general recommendation for sunscreen usage applies to everyone regardless of skin color.(15)

For children younger than 6 months, caution is advised regarding sunscreen use due to the theoretical risk of increased absorption of sunscreen ingredients. This risk arises from higher body surface-to-volume ratios and thinner epidermis in infants. Instead, the primary focus of sun safety for infants involves sun avoidance and the use of protective clothing. If sunscreen is deemed necessary for infants, experts recommend washing it off as soon as it is no longer needed and preferring physical sunscreens over chemical varieties.(16)

How should sunscreen be applied?

Proper application of sunscreen is crucial for effective sun protection. Studies have found that consumers often apply sunscreen inadequately, typically using only 20% to 50% of the recommended amount. To compensate for underapplication, using sunscreens with higher Sun Protection Factors (SPFs) may be advisable. For instance, a sunscreen with an SPF of 50 applied under real-world conditions may offer an effective SPF of only 25.(17)

Although sunscreen labels often advise applying the product 15 to 30 minutes before sun exposure, recent research indicates that immediate protection against ultraviolet radiation occurs after application. However, if water resistance is required, waiting 15 to 30 minutes before exposure may be prudent.

Experimental studies suggest that sunscreen maintains its Sun Protection Factor (SPF) for up to 8 hours after a single application, challenging the previous recommendation to reapply every 2–3 hours, especially during physical activity. Nonetheless, reapplication is recommended after activities that may remove sunscreen, such as sweating, swimming, or friction from clothing or sand.(18)

Cream-based sunscreens are preferred over spray-on varieties for several reasons. Sprays can result in uneven application due to wind dispersal, and their fast-drying nature may make it difficult to ascertain if coverage is uniform. Additionally, aerosolized sunscreens pose risks of flammability, with reported incidents of skin combustion after exposure to flames even after drying. Inhalation risks associated with aerosolized sunscreens have not been thoroughly studied.(19)

Key Safety Concerns:

Skin Reactions: Common adverse reactions to sunscreens include subjective irritation (e.g., stinging, burning) without a rash, irritant contact dermatitis, and comedogenicity. Rarely, chemical sunscreen ingredients may lead to allergic contact dermatitis and photoallergic contact dermatitis, with octocrylene, oxybenzone, and octyl methoxycinnamate being the most commonly implicated allergens.(20)

Absorption of Sunscreen: A small randomized controlled trial (RCT) sponsored by the United States Food and Drug Administration (FDA) in 2019 showed systemic absorption of four sunscreen ingredients (oxybenzone, avobenzone, octocrylene, and ecamsule) when applied under maximal use conditions over four consecutive days.(21) Blood levels of these compounds exceeded FDA guidelines, with long half-lives suggesting potential accumulation within the body. Further research is needed to determine the clinical significance of absorption and whether it poses any health risks, although the known protective effects of sunscreen are still emphasized. In contrast, physical sunscreens are not systemically absorbed.(22)

Endocrine Effects: Concerns have been raised regarding the potential estrogenic and antiandrogenic effects of chemical sunscreens, although evidence is of low quality. Studies on animals and humans have yielded inconsistent results, with some suggesting possible adverse reproductive effects, particularly with oxybenzone exposure during pregnancy. However, the heterogeneity of the studies limits the conclusive interpretation of these findings.(23)

Environmental Impact:

 Recent studies have indicated that chemical sunscreen ingredients are detectable in various water sources and may persist even after wastewater treatment. There are concerns about the bioaccumulation and biomagnification of sunscreen filters in fish tissues. Additionally, the effects of sunscreen ingredients on coral reefs are being investigated, with oxybenzone being implicated in coral reef bleaching. Some jurisdictions have responded to these findings by banning oxybenzone and octinoxate.(24)

Additional Photoprotective Measures:

Sunscreens are just one component of a comprehensive photoprotection strategy. Patients should be counseled on behaviors to avoid UV radiation, including wearing wide-brimmed hats, UV-protective sunglasses, and seeking shade during peak UV hours. Clothing, especially those with tighter weaves and darker colors, can also offer significant protection, with some clothing designed specifically for sun protection with a high Ultraviolet Protection Factor (UPF). Additionally, some fabrics may lose their photoprotective properties when wet or stretched.(25)

Potential New Sunscreen Technologies:

Emerging technologies in photoprotection include topical photolyases and antioxidants such as vitamin C, vitamin E, selenium, and polyphenols found in green tea extracts.(16) While antioxidants cannot yet be stabilized in sunscreen formulations to maintain their biological activity, oral photoprotective agents like niacinamide and Polypodium leucotomos extract are being explored. Studies have shown promising results for Polypodium leucotomos extract in increasing the minimal erythema dose and managing dermatologic conditions induced by UV radiation. Nicotinamide, a form of vitamin B3, has also shown potential in enhancing DNA repair and reducing the formation of DNA damage caused by UV radiation. However, further research is needed to fully understand the efficacy and safety of these emerging photoprotective agents.(26)

Conclusion:

 Exposure to ultraviolet radiation poses direct harm and is strongly linked to the development of skin cancers, which are prevalent in Canada. Robust evidence supports the effectiveness of sunscreen in reducing the risk of both melanoma and nonmelanoma skin cancers. Therefore, it is imperative for physicians to educate patients on comprehensive photoprotection strategies, encompassing avoidance of midday sun, seeking shade, wearing protective clothing, and utilizing sunscreen when direct sun exposure cannot be avoided.

While chemical sunscreens have demonstrated efficacy, there is emerging concern regarding their systemic absorption and potential environmental impact. Individuals wary of these effects may opt for physical sunscreens as an alternative. Ongoing research is dedicated to assessing the safety and efficacy of existing sunscreen formulations and exploring novel agents for photoprotection. By staying informed and implementing evidence-based strategies, both patients and physicians can work together to mitigate the risks associated with UV radiation exposure and promote skin health.

References:

1. Hung, M., Beazer, I. R., Su, S., Bounsanga, J., Hon, E. S., & Lipsky, M. S. (2022). An Exploration of the Use and Impact of Preventive Measures on Skin Cancer. Healthcare (Basel, Switzerland), 10(4), 743. https://doi.org/10.3390/healthcare10040743.

2. American Cancer Society Cancer Facts & Figures 2022. 2022. [(accessed on 1 April 2022)]. Available online: https://www.cancer.org/content/dam/cancer-org/research/cancer-facts-and-statistics/annual-cancer-facts-and-figures/2022/2022-cancer-facts-and-figures.pdf.

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5. Dennis L.K., Vanbeek M.J., Beane Freeman L.E., Smith B.J., Dawson D.V., Coughlin J.A. Sunburns and risk of cutaneous melanoma: Does age matter? A comprehensive meta-analysis. Ann. Epidemiol. 2008;18:614–627. doi: 10.1016/j.annepidem.2008.04.006.

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9. Sanchez G., Nova J., Rodriguez-Hernandez A.E., Medina R.D., Solorzano-Restrepo C., Gonzalez J., Olmos M., Godfrey K., Arevalo-Rodriguez I. Sun protection for preventing basal cell and squamous cell skin cancers. Cochrane Database Syst. Rev. 2016;7:CD011161. doi: 10.1002/14651858.CD011161.pub2.

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13. Hughes MCB, Williams GM, Baker P, et al. Sunscreen and prevention of skin aging: a randomized trial. Ann Intern Med 2013;158:781–90.

14. Silva ES, Tavares R, da Paulitsch FS, et al. Use of sunscreen and risk of melanoma and non-melanoma skin cancer: a systematic review and meta-analysis. Eur J Dermatol 2018;28:186–201.

15. Sunscreen FAQs. Des Plaines (IL): American Academy of Dermatology Association; 2020. Available: www.aad.org/media/stats-sunscreen (accessed 2020 Aug. 16).

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17. Williams JD, Maitra P, Atillasoy E, et al. SPF 100+ sunscreen is more protective against sunburn than SPF 50+ in actual use: results of a randomized, double-blind, split-face, natural sunlight exposure clinical trial. J Am Acad Dermatol 2018;78:902–910.e2.

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20. Rodríguez E, Valbuena MC, Rey M, et al. Causal agents of photoallergic contact dermatitis diagnosed in the national institute of dermatology of Colombia. Photodermatol Photoimmunol Photomed 2006;22:189–92.

21. Matta MK, Zusterzeel R, Pilli NR, et al. Effect of sunscreen application under maximal use conditions on plasma concentration of sunscreen active ingredients: a randomized clinical trial. JAMA 2019;321:2082–91.

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