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| Fig. 1: The left image shows a man wearing SPF 50 sunscreen on half of his face in visible light and the right side shows him in ultraviolet (UV) light. (Source: Wikimedia Commons) |
Ultraviolet Radiation (UVR) from the sun is known to have clinical effects on the human skin that can be acute or chronic. Acute effects can include tanning, erythema (sunburn), and reduction of blood pressure while chronic effects include skin cancer and photoaging (premature aging of the skin). At a molecular level, these effects are caused by DNA damage, melanogenesis, and etc. [1] UV Radiation is divided into three different bands - UVA (320-400 nm), UVB (290-320 nm) and UVC (200-290) but UVA and UVB have wavelengths with penetration properties that reach and effect the skin. [2]
However each individual also has a different susceptibility to effects from UVR based on his/her skin phototype. For lower number skin types that correlate to whiter skin colors, sunburns happen more readily, tanning occurs minimally, and the likelihood of skin cancer is higher. These sensitivities of skin types can be measured through a "visual assessment of the minimal erythemal dose (MED), which is the amount of UVR needed to induce just perceptible erythema after exposure (typically 24 hours)". [1]
To protect against these harmful effects of UVR, sunscreen products have long been used and applied prior to outdoor activities. How does sunscreen work to protect the skin against UVR and how effective is it?
Many sunscreen products displayed in stores are labeled with SPF values, which are "calculated as the ratio of the doses of solar-simulated radiation causing erythema with sunscreen applied to that without sunscreen". [1] The components of sunscreen are created to protect the skin from these harmful effects of UV rays by absorbing, reflecting, or scattering they rays (refer to Fig. 1). Thus, they are supposed to be helpful in protecting against the development of skin cancer and against photoaging. There are different types of sunscreen, however, and can be divided into chemical and physical. While a chemical sunscreen absorbs the UV rays, physical sunscreens reflect the rays like a coat of armor. Generally, sunscreens are known to reduce skin lesions caused by UV radiation and postpone/ minimize sunburns. [2] However, it is also important that sunscreens do block the benefits of UVR like syntesis of vitamin-D, photoadaptation, induction of innate immunity, and more. [1]
While it is widely known that sunscreen can help with preventing sunburns, etc., its prevention of skin cancer like melanoma is not as widely accepted. In order to test whether sunscreen really is effective in preventing skin cancer, a study conducted in Australia found that 63% of all melanomas and keratinocyte cancers could be caused by high levels of exposure to UVR of the Australian population. However, the number of people diagnosed with squamous cell carcinoma (SCC), a type of skin cancer, had been reduced by 9.3% and those diagnosed with melanoma had decreased by 14% due to regular sunscreen use. [3]
With the wide use of sunscreen, debates about its toxicity in penetrating the skin has also risen. Studies have found toxic effects such as dermatitis and photoallergy linked to ingredients commonly used in sunscreen. Thus, many have been concerned about the toxicity of these ingredients and their possible absorption into the systemic circulation. However, several studies have found that the ingredients did not penetrate significantly. Furthermore, a scientific research tested healthy volunteers with two commercial sunscreen products applied to their forearm for 2 hours. After the two hours, they found that the products did not penetrate beyond the stratum corneum and thus do not have significant toxic effects. [4]
While Vitamin-D is extremely important, and spending time outside and being healthy and active is encouraged. It definitely is also necessary to use sunscreen for people with skin types more prone to sunburns and skin cancer. It is recommended that sunscreen is applied in a short period of time before exposure to the sun rather than longer periods. Most products recommend re-applying every few hours in the sun.
© Amy Kang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. R. Young et al., "Ultraviolet Radiation and The Skin: Photobiology and Sunscreen Photoprotection," J. Am. Acad. Dermatol. 76, S101 (2017).
[2] A. K. Saeed et al., "Ultraviolet Type B-Radiation-Induced Hyperplasia and Seborrheic Keratosis is Reduced by Application of Commercial Sunscreens," Pak. Vet. J. 36, 450 (2016).
[3] C. M. Olsen et al., "Cancers in Australia Attributable to Exposure to Solar Ultraviolet Radiation and Prevented By Regular Sunscreen Use," Aust. Nz. J. Publ. Heal. 39, 471 (2015).
[4] V. K. Tippavajhala et al., "In Vivo Human Skin Penetration Study of Sunscreens by Confocal Raman Spectroscopy," AAPS Pharmscitech 19, 753 (2018).
