One ounce of sunscreen protection is worth its weight in cancer prevention. Over the past several decades, skin cancer has been increasingly recognized as a major worldwide public health concern. An estimated 2–3 million cases of nonmelanoma skin cancer (NMSC) and 132 000 cases of melanoma occur worldwide each year and the incidence of both has been increasing over the past decade . In the US alone, over 2 million cases of NMSC are diagnosed each year and melanoma is predicted to account for more than 75 000 cases of skin cancer in 2012 .
Exposure to ultraviolet (UV) radiation is the most significant environmental risk factor for all types of skin cancer. Therefore, prevention of skin cancer focuses on limiting UV exposure by sun-protective behaviors. Although sunscreen application is the most common modality for sun protection , there appears to be widespread confusion about what sunscreens do and mixed perceptions as to the amount of protection they offer, as well as concern over potential adversities associated with their use, such as systemic toxicity and vitamin D deficiency. The unclear labeling of sunscreen products, inconclusive evidence about their protective efficacies, and discrepancies between recommended and typical use of sunscreens has added to this confusion. Skin cancers are largely preventable by following recommended sun-protection guidelines; therefore promoting routine sunscreen use is a critical aspect of public health approaches to reduce the incidence of skin cancer [4▪]. Medical providers should be adequately versed in both the science and practical use of sunscreens to dispel patient and parental fears.
In this article, we review UV radiation's effects on the skin, discuss how sunscreens work to protect us and interpret current sunscreen terminology. The current evidence on sunscreen's potential to cause systemic toxicity and vitamin D deficiency is reviewed and current recommendations on proper use, issues with compliance and consideration for sun-protection interventions among children and adolescents are covered.
Ultraviolet radiation of sunlight is comprised of UVA (315–400 nm), UVB (280–315 nm) and UVC (100–280 nm) radiation, based on wavelength. Whereas the stratospheric ozone layer completely blocks UVC radiation and UVB wavelengths below 295 nm, 90–95% of the UV radiation that reaches the Earth's surface is UVA, with UVB accounting for most of the remainder [5,6]. Depending on the wavelength, UV radiation has diverse effects on the skin, and these effects can be immediate (e.g. sunburn) or long-term (e.g. photoaging, immunosuppression, photocarcinogenicity) .
EFFECTS OF ULTRAVIOLET RADIATION ON THE SKIN
The effects of UV radiation on the skin may be obvious within hours or take years to manifest.
Within seconds of UV exposure, formation of reactive oxygen species and photo-oxidation of pre-existing melanin result in immediate pigment darkening which resolves in just a few hours, whereas more persistent pigment darkening occurs 2–24 h after UV exposure after redistribution of the photo-oxidized pre-existing melanin. The photoprotection gained from the adaptive formation of new melanin in response to UVB does not take place until 3 days after UV exposure [8,9]. This ‘delayed tanning’ can provide mild UVB protection, with a sun protection factor (SPF) of 3 .
Excessive UV exposure leads to sunburn. This is mainly due to the small UVB component of UV radiation, with UVA accounting for only 20% of sunburns . Sunburns are the most common acute skin damage under influence of the sun and are characterized by the appearance of intense redness, along with the sensation of burning and sometimes pain, 4–6 h after sun exposure, peaking at around 24 h.
Intensive intermittent UV exposure in childhood and teen years leading to blistering sunburn is a known risk factor for basal cell carcinoma and malignant melanoma and, in fact, epidemiological data indicate that a history of five episodes of sunburn per decade increases the risk of melanoma approximately three-fold . However, it is important to remember that excessive UV exposure is damaging regardless of whether a sunburn occurs .
Photoaging is a long-term effect of UV exposure and refers to the cumulative, degenerative process of external or premature aging of the skin caused primarily by UVA radiation [11,12].
Exposure to UV radiation is considered to be a major environmental risk factor for all three types of skin cancer: basal cell cancer (BCC), squamous cell cancer (SCC) and malignant melanoma [13–15].
Another important component of UV radiation in the development of skin cancer is the suppression of the immunological response, which indirectly promotes carcinogenesis .
In 2009, the WHO classified sunbeds in the highest category of risk for cancer, alongside tobacco and asbestos . Despite the known risks, nearly 28 million Americans use a sunbed or a sunlamp every year, 2.3 million of whom are teenagers and 70% of whom are white women aged 16–29 years [17,18].
The lamps in tanning beds typically emit 95–99% of their light as UVA and these UVA rays can be 10–15 times stronger than those from mid-day sun, exposing a person to up to 1.2 times the average normal annual exposure from sunlight over the course of 20 sessions [19,20]. Equivalent to UVA radiation from the sun, UVA from tanning beds is a known carcinogen, promoting DNA damage, immunosuppression, and photoaging. Moreover, UVB radiation accounts for 1–5% of light emitted by tanning beds and, with enough exposure, one can still receive a large dose of UVB radiation with tanning bed use.
In its stance against indoor tanning, the American Academy of Dermatology supports a ban on the nonmedical production and sale of indoor tanning devices. In the US, tanning is regulated by the states, and in 2011 California was the first to ban children under the age of 18 from using tanning beds, and, more recently, Vermont became the second . Similarly, the UK has also introduced legislation that bans children under 18 years of age from using tanning beds in Great Britain .
HOW SUNSCREENS WORK
Sunscreens work by containing an active ingredient that absorbs radiation in the range of 290–400 nm. In most countries, these active ingredients are regulated as cosmetics; however, in the US, Canada, and Australia commercial sunscreens are regarded as over-the-counter drugs. In the US, the Food and Drug Administration (FDA) regulates the active ingredients in sunscreen products, determines the methods of testing them, and dictates labeling requirements. Currently, there are 17 FDA-approved sunscreens in the US, compared with at least 34 in Australia and 26 in the European Union. Sunscreens are classified as either ‘physical’ or ‘chemical’ based on their active ingredient. Sunscreen products combine ingredients in a variety of combinations to produce a product that confers stability and optimal UV protection.
In ‘physical’ sunscreens the active ingredient is an inorganic compound that works by physically reflecting or scattering the UV radiation  (see Fig. 1). The most common inorganic agents currently used are zinc oxide and titanium dioxide, both of which offer UVA and UVB protection and, in order to achieve sufficient reflection, require a thick application  (see Fig. 2).
In ‘chemical’ sunscreens the active ingredient is an organic compound that works by absorbing UV radiation and dissipating the energy as heat or light  (see Fig. 1). Most absorb UVB radiation, a few absorb in the UVA2 range (320–340 nm), and there is only one FDA-approved organic sunscreen, avobenzone, that absorbs in the UVA1 range (340–400 nm)  (see Fig. 2).
It is important that physicians correctly interpret the terminology used to classify sunscreens and educate their patients to ensure they get the protection they need.
Sun protection factor
Sun protection factor is a laboratory measure of sunscreen efficacy that is based on erythema. Because UVB radiation is about 1000 times more erythemogenic than UVA radiation, SPF is primarily a measure of UVB protection . It is defined as the ratio of the least amount of UV radiation required to produce minimal erythema on sunscreen-protected skin to that required to produce the same erythema on unprotected skin . The SPF of a product is not related to the duration of UV exposure , despite common misperception. Additionally, the relationship between SPF and UVB protection is not linear, meaning that a sunscreen with double the SPF rating does not necessarily mean one can remain in the sun twice as long before becoming sunburnt.
Given the recent discoveries illustrating the role of UVA radiation in skin cancer development, UVA protection is an important consideration when evaluating a sunscreen's ability to block UV radiation .
Substantivity and photostability
Substantivity refers to the ability of a sunscreen to remain effective in the presence of adverse conditions, primarily water and sweat. The current US FDA final ruling has banned the use of the terms ‘sunblock,’ ‘water proof,’ or ‘sweat proof.’ Instead, the label ‘water-resistant’ (40 min) or ‘water-resistant’ (80 min) is given to reflect the actual water-resistant testing that is required to be done [30,31▪▪]. Furthermore, sunscreens cannot claim to provide sun protection for more than 2 h without reapplication.
New compounds and combinations
Given the high standards to be met by sunscreens, new compounds and combinations have been developed in efforts to prevent photodegradation, as well as to increase SPF levels, provide broad-spectrum UVA and UVB protection, and follow FDA filter maximum limits within sunscreens. An example of this is the addition of octocrylene to prevent the destruction of the photosensitive compound avobenzone. Over the past decade, two newer compounds, Ecamsule (Mexoryl SX, patent-protected by L’Oreal) and avobenzone and oxybenzone (Helioplex, patent-protected by Neutrogena Corp.), have been added to sunscreen formulations, providing broad-spectrum UVA protection and high SPF .
Many controversies surround the use of sunscreens and these concerns have led to misconceptions and confusion among the public. Physicians should be prepared to answer inquiries related to these issues and alleviate concerns and/or confusion.
The observation that sunscreens protect against sunburn led to the common expectation that they also would protect against malignant melanoma. Surprisingly, some case-controlled studies actually showed a higher incidence of melanoma among sunscreen users [5,32]. However, careful analysis found that these studies were flawed due to the failure to adequately control for confounding factors between melanoma and sunscreen use . For example, persons with fair skin are both more likely to use sunscreen to prevent sunburns, and at higher risk of developing skin cancer. Furthermore, these studies neglected to consider SPF or the appropriate application of sunscreens. The more recent meta-analyses of case–control studies have not found any association between sunscreen and the development of melanoma . Furthermore, the recent randomized controlled trial published in 2011 suggests regular use of sunscreen can decrease the incidence of melanoma among white adults [35▪▪].
The active ingredients (UV filters) in sunscreens routinely undergo a rigorous safety assessment to explore the possibility of local and systemic toxicity, such as sensitization, irritation, phototoxicity, and carcinogenicity . Despite this, adverse skin reactions and potential systemic influences have been proposed as risk factors related to the use of sunscreens, and, with the development of nanoparticles, the absorption potential of these smaller particles has been questioned.
Potential systemic influences
Sunscreen products with organic UV filters may penetrate the skin in small amounts (0.1–5% of dissolved filter) , whereas UV filters existing as inorganic filters (e.g. titanium dioxide) do not penetrate the skin, posing no potential risk of systemic toxicity .
Much effort has been made to address the cosmetic shortcomings of inorganic UV filters, zinc oxide, and titanium dioxide. Since the cosmetically driven development of nanoparticles (single particles with a diameter <100 nm) that do not make the skin white and pasty upon topical application and their approval for use in sunscreens by the FDA in 1999, in-vivo studies using both animal and human skin have shown penetration of nanoscale zinc oxide and titanium dioxide is limited to the stratum corneum and, therefore, their in-vitro toxicity profiles are of no consequence to human health [3,39].
Because Vitamin D3, a precursor for the biologically active vitamin D, is formed in the skin after exposure to UVB radiation, there is ongoing concern that sunscreen use could potentially lead to vitamin D deficiency by absorbing UVB radiation and inhibiting vitamin D production. Considering that vitamin D synthesis may be reduced by more than 98% with proper use of SPF 15 sunscreen, some suggest regular sunscreen use may cause vitamin D insufficiency; however, others have shown long-term sunscreen use has minimal to no effect on vitamin D levels and function [3,24,40▪▪,41,42].
The Skin Cancer Foundation advises children and adults under age 70 who regularly practice sun protection to obtain the recommended 600 IU of vitamin D a day from food sources such as oily fish, fortified dairy products and cereals, and supplements. For infants under 12 months old and people 70 and older, the recommended daily dose is 400 and 800 IU, respectively. For those who are vitamin D-deficient, it is acceptable to take 1000–2000 IU doses daily for limited periods .
RECOMMENDED USE OF SUNSCREENS
The failure of sunscreens to prevent sunburn is usually due to the way sunscreens are applied by the consumer.
Despite explicit recommendations and evidence supporting daily sunscreen use, many people do not use sunscreen on a daily basis, and, instead, use sunscreen solely on a short-term basis during outdoor activities, or even to increase duration of UV exposure during tan acquisition. A review examining the association between sunscreen use and sun exposure found that sunscreen use increased the duration of intentional sun exposure between 13 and 39% . The use of sunscreen to extend sun exposure for tan acquisition is considered an abuse of the product and can actually increase the risk of skin cancer and photoaging by exposing a person to increased UVA radiation .
The American Academy of Dermatology (AAD) has developed guidelines to reduce UV exposure including recommendations for the proper sunscreen use  (see below). Despite the reality of their use, sunscreens should be considered a third line of defense after clothing and shade.
- Measures to reduce UV exposure
- Seek shade and avoid the sun between 10 a.m. and 4 p.m. when UV radiation peaks
- Wear protective clothing including long-sleeved shirt, pants, sunglasses, and a wide-brimmed hat
- Be careful in the setting of water, snow, and sand as these surfaces reflect UV rays
- Avoid tanning beds
- Proper use of sunscreens
- Who: Everyone, including people of all skin colors
- What: Broad-spectrum, water-resistant sunscreen with a minimum SPF of 30
- Where: All exposed areas of the body not protected by clothing
- Pay particular attention to vulnerable sites such as the nose, shoulders and dorsal feet
- How: Apply generously and uniformly
- Use enough sunscreen to fill a shot glass (1 oz)
- When: every day and year-round
- 15 min before going outdoors
- Reapply every 2 h especially after swimming or heavy perspiration
Sunscreen and infants
Because the skin of infants and toddlers is thinner, less concentrated with melanin, and immunologically immature, UV radiation can penetrate the skin more deeply and there is less of an immune response mounted against UV damage, putting them at higher risk for cumulative UV damage and skin cancer [47▪]. For these reasons, the American Academy of Pediatrics (AAP) has developed guidelines for UV protection in infants (see Table 1).
Sunscreen and skin color
Photoprotection provided by increased epidermal melanin in darker skin offers inherent SPF and in black skin can filter twice as much UVB radiation compared with Caucasian skin, contributing to the low incidence of cutaneous malignancies in darker-skinned groups. Although skin cancers are not as prevalent in people with darker skin, they can have morbidity and fatalities from remaining undiagnosed for a while . Prevalence of sun-protective behaviors in the US population according to race shows few blacks wear sunscreen and instead seek shade [40▪▪]; however, the use of sunscreen is recommended for people of all skin color, as everyone is at risk for skin cancer .
TARGETING THE YOUNG
Up to 80% of total lifetime sun exposure takes place before the age of 18 years .
Young and at risk
Childhood is a particularly vulnerable time for photocarcinogenic effects of the sun; thus sun protection is of utmost importance during childhood and adolescence. Epidemiologic studies show a higher incidence of malignant melanoma in persons with a history of sunburns during childhood and adolescence [11,49▪▪]. A meta-analysis of 51 studies found that ever reporting a sunburn during childhood almost doubled the risk for development of cutaneous melanoma . Moreover, regular sunscreen use during childhood and adolescence could reduce lifetime incidence of NMSC by approximately 78% [4▪].
Timing of interventions
Adolescents and young adults have the lowest skin-protection rates of all age groups, receive large amounts of UV radiation, and increase their UV exposure habits as they move into adulthood, making this a critical period for skin cancer prevention [4▪]. Evidence from a prospective, population-based study showed that over 50% of peri-adolescent children experience sunburns before age 11 and again 3 years later [49▪▪]. During this same period, liking a tan and spending time outside to get a tan significantly increased and the percentage of those appropriately using sunscreen dropped from 50 to 25% [49▪▪]. Therefore, childhood seems to be the ideal time to intervene in terms of sun-protective behaviors and the targeting of children in pediatric offices and community settings regarding unprotected UV exposure may be a practical approach [49▪▪,50]. In fact, interventions aimed at parents, teachers, and children designed to increase children's use of sun protection have already been shown to be effective .
The major challenges for the future relate to optimizing the use of sunscreens and to motivating the general public, especially adolescents, to achieve UV protection by other means . Whereas there may be concerns regarding the long-term safety of UV filters, the benefits of sunscreens clearly outweigh their potential risks. However, the primary barrier to attaining these benefits is patient compliance. Therefore, our future efforts should focus on compliance, rather than technical issues of sunscreens, so that people want to apply sunscreen appropriately and, as a consequence, benefit from the harm reduction that advancements in technology promise to deliver.
Furthermore, continued public health efforts are needed to facilitate sun protection, prevent sunburn, and avoid increases in skin cancer cases. Future skin cancer prevention campaigns should focus on the proper use of sunscreen, emphasize the importance of wearing clothing and seeking shade, as well as put forth major effort to change adolescents’ attitudes towards sun tanning .
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
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