Sunscreens are believed to be a valuable tool in providing photoprotection against the detrimental effects of UV radiation, a known carcinogen. However, a number of controversies have developed regarding their safety and efficacy. This review summarizes the relevant studies surrounding these controversies.
Evidence of the prevention of skin cancer, an oft-cited reason for sunscreen use, was examined as it pertains to squamous cell carcinoma, basal cell carcinoma and melanoma. We also reviewed studies examining the effects of sunscreen on the synthesis of vitamin D, an essential nutrient whose role in health and disease continues to grow. Lastly, we analyzed studies surrounding the safety and toxicity of oxybenzone, retinyl palmitate and nanoparticles of zinc oxide (ZnO) and titanium dioxide (TiO2).
The overwhelming majority of available data is drawn from studies conducted using antiquated sunscreen formulations. Nonetheless, our research revealed that topical use of sunscreen protects against squamous cell carcinoma, does not cause vitamin D deficiency/insufficiency in practice and has not been demonstrated to adversely affect the health of humans.
Given the established benefits of UV protection, the use of sunscreens remains an important part of an overall photoprotective strategy. Future sunscreens with improved formulation should ideally offer superior protection. With increased usage of sunscreen by the public, continuous and vigilant monitoring of the overall safety of future products is also needed.
The detrimental effects of UV exposure from the sun have been well described in the literature. Both acute and chronic UV exposure can lead to sunburn, photocarcinogenesis, photoimmunosuppression and photoaging. Current methods of photoprotection include sun avoidance, seeking shade, use of protective clothing and the application of sunscreen. Of these, the use of sunscreens remain the most prevalent protection strategy used by the public.
However, concerns have emerged surrounding the safety and efficacy of sunscreens. Studies have been published assessing whether sunscreens prevent skin cancer, and conversely, confer an increased risk of melanoma, the most fatal form of skin cancer. Similarly, a growing body of literature demonstrating the beneficial effects of vitamin D on health outcomes has called into question the potential for vitamin D deficiency/insufficiency with sunscreen use.
Controversy has also developed regarding the possibility of adverse biological effects from various ingredients in sunscreens. Oxybenzone, an ingredient widely used in sunscreens, is purported to have a potentially disruptive effect on hormonal homeostasis. Retinyl palmitate, a compound used extensively in various cosmetic and personal care products, has received wide attention as a potential photocarcinogen. Lastly, safety concerns have also developed as a result of the inclusion of nanosized inorganic UV filters, zinc oxide (ZnO) and titanium dioxide (TiO2), into sunscreens. In this review, we attempt to clarify the above issues by critically analyzing the available evidence regarding these controversies.
Excerpts from full article:
Prevention of skin cancer
Squamous cell carcinoma (SCC) and basal cell carcinoma (BCC): Routine use of sunscreen has been shown to be effective at reducing the development of both SCC (1, 2) and actinic keratoses (3–5), which are known precursors to SCC (6). However, a statistically significant protective benefit of sunscreen has not been demonstrated against either BCC or malignant melanoma.
Evidence demonstrating the protective effect of sunscreen in the prevention of melanoma is inconclusive. To date, only case–control studies have been published, all of which are subject to recall bias of sunscreen use. At the time of this review, there have been no randomized controlled trials conducted examining the effects of sunscreens in preventing malignant melanoma. As a result, we provide reviews of two meta-analyses published on this topic.
Sunscreen-induced vitamin D deficiency
Considerable overlap exists between the UV absorption profiles of sunscreens and the action spectrum for vitamin D synthesis (54, 55). In theory, correct usage of sunscreens should significantly reduce vitamin D levels. However, this is not the case in practice. In fact, several studies have demonstrated that sunscreens are rarely applied correctly, in the right dosages and with appropriate frequency (13–17, 56). Therefore, under real-world conditions it is likely that the improper use of sunscreen and/or increased exposure time, result in production of vitamin D among sunscreen users. Lastly, it is important to note that the level of cutaneous vitamin D production from UVB exposure is also influenced by seasons, latitude, obesity level and age. Despite the potential inhibition of cutaneous synthesis with sunscreen use, sufficient levels of vitamin D can be achieved from dietary supplementation.
Safety of oxybenzone in sunscreen
Oxybenzone (benzophenone-3) belongs to the class of aromatic ketones known as benzophenones, which provide broad-spectrum UV coverage. The molecule was first approved for use by the Food and Drug Administration (FDA) in the early 1980s and has grown to become one of the most widely used organic UVA filters in US sunscreens today. As a photoprotective agent, it has an absorption profile spanning from 270 to 350 nm with absorption peaks at 288 and 350 nm. Despite established photoprotective effects, a number of important issues have arisen surrounding the use of oxybenzone in sunscreen.
Safety of retinyl palmitate in sunscreens
Over the past 20 years, retinyl palmitate has found widespread use in cosmetic and sunscreen products. As of 2000, data from the FDAs Voluntary Cosmetics Registration Program indicate that the number of retinyl palmitate-containing formulations in the United States has increased to 667, up from 355 in 1992. In addition to cosmetic and sunscreen products, retinyl palmitate has also been approved by the FDA for use in a wide variety of over-the-counter and prescription drugs. It is also worthy to note that retinyl palmitate is commonly found as a food additive in the United States (e.g. to fortify low-fat milk, dairy products and breakfast cereals). Recently, media organizations have touted retinyl palmitate in sunscreen as having photocarcinogenic potential, calling into question the safety of its use in sunscreens. A critical analysis regarding the photocarcinogenic potential of retinyl palmitate has been reviewed by Wang et al. (75), and we will briefly summarize their findings herein.
Safety of nanoparticles in sunscreen
The increasing ubiquity of these nanocompounds in personal care and cosmetic products makes safety research especially relevant. Much concern have been voiced that the integration of nano-material technology into everyday formulations has outpaced the body of research evaluating their safety. Currently, the FDA does not have regulations in place regarding the labeling of products containing nanoparticles of TiO2 and ZnO.
Considerable data assessing the potential toxicity of these materials in sunscreens has been published to date, and the studies referenced above were performed in controlled environments on healthy, undamaged skin. It has been established that the stratum corneum is an effective barrier preventing the entry of nano-ZnO and -TiO2 into deeper layers of the skin. Nonetheless, it remains to be determined whether a greater degree of penetration occurs through skin that is damaged, diseased or otherwise compromised. At the present time, however, the available data do not provide conclusive evidence demonstrating that damaged skin leads to an increased penetration of nanoparticles (105).
Mark E. Burnett, Steven Q. Wang
Volume 27, Issue 2, pages 58–67, April 2011
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