Selection of sunscreen and sun-protective measures

INTRODUCTION — Sunlight is essential for synthesis of vitamin D and has beneficial effects on mood. However, ultraviolet (UV) radiation (290 to 400 nm (figure 1)) is responsible for several acute and chronic detrimental effects on human skin, including sunburn, photoaging, and skin cancer.

Ultraviolet B (UVB; 290 to 320 nm), which represents only 5 percent of the UV radiation reaching the earth's surface, includes the biologically most active wavelengths. UVB is responsible for sunburn, inflammation, hyperpigmentation, and photocarcinogenesis.

Approximately 95 percent of the UV radiation reaching the earth surface is ultraviolet A (UVA; 320 to 400 nm). Ultraviolet A2 (UVA2; 320 to 340 nm), which constitutes approximately 25 percent of the UVA band, produces similar effects on the skin as UVB. Ultraviolet A1 (UVA1; 340 to 400 nm) is less potent than UVA2 and has a decreased ability to induce erythema. UVA contributes to photoaging, has a major role in pigment darkening, and may be involved in skin carcinogenesis [1-3].

Photoprotection, which includes sun avoidance during peak hours, sun-protective clothing, and sunscreens, is crucial to prevent or reduce the potential harms associated with UV exposure.

Sunscreens and sun-protective clothing are reviewed here. Sunburns, photosensitivity disorders, and photoaging are discussed separately. (See "Sunburn" and "Photosensitivity disorders (photodermatoses): Clinical manifestations, diagnosis, and treatment" and "Photoaging".)

TYPES OF SUNSCREENS — Sunscreens are topical preparations containing filters that reflect or absorb radiation in the ultraviolet (UV) wavelength range (figure 1). In the United States, where sunscreens are considered to be over-the-counter drugs, 17 different UV filters are approved by the US Food and Drug Administration (FDA) as active ingredients (table 1) [4]. Many other UV filters are available in Europe, Canada, and Australia [5-8].

Sunscreens are classified as organic (formerly known as chemical sunscreens) and inorganic (formerly known as physical sunscreens). Broad-spectrum sunscreens are generally combinations of sunscreen products that are able to absorb both ultraviolet B (UVB) and ultraviolet A (UVA) radiation.

Organic (chemical) filters — Organic filters (table 1) include a variety of aromatic compounds that absorb UV radiation and convert it to a negligible amount of heat [9]. Some organic agents such as cinnamates and salicylates specifically absorb UVB. Among cinnamates, octinoxate is the most widely used UVB filter worldwide.

Benzophenones provide UVB and short-wavelength UVA (ultraviolet A2 [UVA2]) protection. Oxybenzone (benzophenone-3) is the most commonly used UVA filter worldwide. Avobenzone is an effective filter for long-wavelength UVA radiation (ultraviolet A1 [UVA1]) but is extremely photolabile and needs to be stabilized with other compounds such as octocrylene, a weak UVB absorber.

Broad-spectrum sunscreens are generally produced by combining filters with varying UV absorption spectra. As an example, avobenzone can be combined with UVB filters such as homosalate and octisalate to yield broad-spectrum coverage. Broad-spectrum organic filters that have been developed in Europe include methylene bis-benzotriazolyl tetramethylbutylphenol (Tinosorb M) and bis-ethylhexyloxyphenol methoxyphenol triazine (Tinosorb S), terephthalylidene dicamphor sulfonic acid (Mexoryl SX), and drometrizole trisiloxane (Mexoryl XL) [10].

Mexoryl SX and Mexoryl XL are photostable broad-spectrum agents absorbing UVB and short wavelength UVA (UVA2). Both prevent UV-induced pigmentation and show a synergistic effect when used in combination. Mexoryl SX has been available in the United States since 2006 in combination with avobenzone and octocrylene.

Inorganic (mineral) filters — Inorganic filters (table 1) are mineral compounds such as zinc oxide and titanium dioxide that are believed to reflect and scatter UV light over a wide range of wavelengths. However, studies have shown that these compounds, and in particular micronized preparations, absorb rather than reflect UV radiation [11]. Inorganic sunscreens are stable, and in contrast to organic filters, have a low irritating and sensitizing potential. Inorganic sunscreens offer broad-spectrum protection against UVB, short-wavelength UVA (UVA2), and long-wavelength UVA (UVA1).

Older inorganic sunscreens contained large-size particles and formed a cosmetically unacceptable and occlusive white film on the skin. To overcome this cosmetic shortcoming, nanotechnology methods have been used to produce titanium dioxide or zinc oxide particles in the size range of 5 to 20 nm. Nanoparticle products form a more transparent film on the skin while providing effective UVA and UVB protection [12]. However, the utilization of titanium dioxide and zinc oxide nanoparticles into sunscreens has raised questions regarding possible alteration of their absorption spectrum and the potential for percutaneous penetration and toxicity [13]. (See 'Safety' below.)

It should be noted that some sunscreens labeled as "mineral" may contain chemical UVB and UVA filters described on the label as stabilizers; examples include butyloctyl salicylate (in the same family as homosalate) and ethyl ferulate, an analog of octinoxate [14].

Tinted (colored) sunscreens — Tinted sunscreens contain, in addition to UV filters, a combination of non-nanosized titanium dioxide pigment (white) and iron oxides (yellow, red, and black) and provide protection against UV radiation, including UVA1 (340 to 700 nm) and visible light (400 to 700 nm) [15]. Tinted sunscreens are widely available on the market in different shades for all types of skin phototypes.

Visible light, alone or in combination with UVA1 radiation, has been shown to induce pigmentation in persons with highly pigmented skin (Fitzpatrick phototypes IV to VI) [16]. Products containing a concentration of iron oxides of at least 3% are especially useful for patients with hyperpigmentation disorders. (See "Acquired hyperpigmentation disorders" and "Melasma: Management".)

LABELING AND REGULATIONS — The sun protection factor (SPF) measures the sunscreen's ability to protect against a sunburn reaction, which is primarily due to ultraviolet B (UVB). The SPF refers to the ratio of the minimal dose of solar radiation that produces perceptible erythema (minimal erythema dose) on sunscreen-protected skin compared with unprotected skin. The SPF is assessed in experimental conditions using a light source that simulates the solar radiation on the skin of lightly pigmented volunteers who have applied an amount of sunscreen corresponding to 2 mg/cm². (See 'SPF' below.)

The SPF does not adequately measure protection from ultraviolet A (UVA), which encompasses 95 percent of the total ultraviolet (UV) radiation that reaches the earth at sea level. The lack of adequate information on the degree of UVA protection provided by sunscreens prompted the US Food and Drug Administration (FDA) to issue new regulations for labeling sunscreen products, which became effective in 2012. Under the new regulations, only sunscreen products that pass the FDA's test for protection against both UVA and UVB rays will be labeled as "broad spectrum."

Broad-spectrum sunscreen products that contain an SPF of 15 or higher may have the following statement on the label: "If used as directed with other sun-protection measures, decreases the risk of skin cancer and early skin aging caused by the sun." Those that fail the broad-spectrum test or have an SPF <15 must add the following to their label: "Skin cancer/skin aging alert: Spending time in the sun increases your risk of skin cancer and early skin aging. This product has been shown only to help prevent sunburn, not skin cancer or early skin aging."

The FDA no longer allows sunscreen products to be labeled "sweat proof" or "water proof." Sunscreens can be labeled as "water resistant" or "very water resistant" if they maintain their SPF after 40 or 80 minutes of swimming or sweating, respectively.

The UVA protection is not rated in the United States. In Europe, where a number of UVA sunscreen agents (eg, Tinosorb M, Tinosorb S, Mexoryl XL) have been available for many years, a standardized logo (the letters UVA in a circle) and a 5-star rating system for UVA protection has been recommended to the industry by the European Commission. The UVA protection level refers to the ratio of UVB/UVA protection and is required to be at least one-third of the labeled SPF.

WHO SHOULD USE SUNSCREEN? — All individuals, regardless of skin phototype (table 2), are subject to the potential adverse effects of ultraviolet (UV) radiation and will benefit from sunscreen use. However, sunscreens are especially useful for individuals with light skin (phototypes I, II, and III) who are more susceptible to the acute (sunburn) and chronic (photoaging, skin cancer) adverse effects of excessive sun exposure.

Individuals with lightly pigmented skin should regularly use broad-spectrum sunscreens with a sun protection factor (SPF) of 30 or higher when performing outdoor activities in sunny weather, especially in regions with a high level of insolation. Because individuals typically do not apply sunscreen in the recommended amount (approximately 1 to 1.5 ounces or 6 to 9 teaspoons per total body application), they may benefit from a higher SPF [17-19]. This is consistent with the recommendations of the American Academy of Dermatology.

SELECTION OF SUNSCREEN PRODUCTS — The American Academy of Dermatology recommends the choice of sunscreen products that have sun protection factor (SPF) of 30 or higher, broad-spectrum coverage, and water or sweat resistance. In addition to SPF and performance characteristics, the cosmetic appeal of sunscreen products appears to be an important selection criterion used by consumers [20].

SPF — The sun protection factor (SPF) value primarily measures the level of protection against ultraviolet B (UVB) and ultraviolet A2 (UVA2) and is based on the ratio of the minimal erythema dose on sunscreen-protected skin compared with unprotected skin. The relationship between SPF and absorption of UVB radiation is not linear. In fact, when applied in sufficient amount, the amount of UVB radiation absorbed by SPF 15, 30, and 50 sunscreen products is 93, 97, and 98 percent, respectively. However, since the amount of UVB delivered to the skin through a sunscreen is reduced in a nearly linear way with increasing SPF, a sunscreen with SPF of 30 is twice as protective as one of 15 in preventing erythema.

A split-body, randomized trial comparing the efficacy of SPF 50+ and SPF 100+ sunscreens used under real-life conditions at the beach in preventing sunburn found that SPF 100+ sunscreen was more effective than the SPF 50+ one [21]. After five consecutive days of sun exposure between 10:00 AM and 3:00 PM, 31 of 55 participants (56 percent) had more sunburn (erythema score ≥2) on the side of the body protected with SPF 50+ sunscreen, whereas only 4 of 55 participants (7.3 percent) experienced more sunburn on the SPF 100+ side.

Sunscreen products with SPF 15 are generally recommended for daily use. Sunscreen-containing cosmetics (eg, facial moisturizers, foundations) may improve the photoprotection compliance [22]. Most cosmetic products are formulated to provide a SPF of 15 to 30 and may or may not be labeled as broad spectrum. Cosmetics providing broad-spectrum protection should be preferred to those containing only UVB filters.

Broad-spectrum sunscreen products with SPF 30 or higher are recommended for individuals performing outdoor work, sports, or recreational activities.

Spectrum — Broad-spectrum sunscreens that offer sufficient protection against UVB, UVA2, and ultraviolet A1 (UVA1) are preferred to products containing only UVB filters, since both UVA and UVB are involved in skin photoaging and photocarcinogenesis. Although most UVB filters, particularly those with a high SPF, provide some protection against UVA2 radiation, the only three ingredients that satisfy the definition of UVA1 protection are:

●Avobenzone (organic)

●Zinc oxide (inorganic)

●Titanium dioxide (inorganic)

Avobenzone is typically combined with UVA2 and UVB organic filters (eg, cinnamates, benzophenones, or octocrylene) or inorganic filters to achieve broad-spectrum protection and photostability (table 1). Sunscreens containing only zinc oxide and/or titanium dioxide in adequate concentration provide protection against the entire UV spectrum. However, inorganic filters are often combined with organic filters to maximize efficacy and cosmetic acceptability.

Formulations — Research has focused on the development of cosmetically appealing sunscreens that ensure compliance. A common reason for noncompliance is the stickiness of the product, which is greater with higher SPF [23]. A survey of the top sunscreen products on an online retailer found that cosmetic elegance, including texture, absorbability, absence of greasiness, and pleasant smell, was the most commonly positive feature of a sunscreen product cited by consumers [20].

Since sunscreen ingredients are oil-soluble, the most widely commercialized sunscreen products (eg, lotions and creams) are oil-in-water emulsions, in which microscopic drops of oily materials are dispersed in a continuous water phase that typically contains also other polar ingredients like glycerin or glycols [24]. Lotions are thinner and less greasy than creams and are generally preferred for application over large body areas.

Ethanol/oil-based products in combination with polymeric thickeners are used to manufacture liquid, spray, or gel sunscreens. These formulations are fast-drying and leave a cooling sensation on the skin due to their quick evaporation but may be irritating. Less occlusive gels and sprays are preferred by patients with acne or by individuals who work or perform sports activities outdoors in hot weather. However, ethanol-based sunscreens may leave an uneven product film on the skin after evaporation, providing an inconsistent SPF performance.

Water resistance — Water (and sweat) resistance is a key feature to consider when selecting a sunscreen product to use while working or practicing sports or recreational activities outdoors [25]. The terms "water-resistant" and "very water-resistant" mean that the SPF is maintained after 40 or 80 minutes of activity in water or sweating, respectively. The water resistance depends on the vehicle in which the sunscreen is formulated as well as the active ingredient itself. Some types of sunscreens require several minutes after the application to allow the product to form a water-resistant film.

For infants and young children — The American Academy of Pediatrics recommends avoiding the use of sunscreen products in infants younger than six months. However, when adequate clothing and shade are not available, a minimal amount of sunscreen with at least 15 SPF can be applied to small areas, such as the infant's face and the back of the hands.

Since infants have an immature skin barrier, sunscreen products should be nonirritating to the skin and eyes and have a low sensitization potential [26]. Although the frequency of adverse reactions to sunscreens in children has not been determined, photosensitivity and allergic contact dermatitis from organic filters have been reported [27-29]. (See 'Irritant and allergic reactions' below.)

Oil-based emulsions of inorganic filters (ie, titanium dioxide, zinc oxide) are preferred to organic filters for infants and children because they offer broad-spectrum protection and have minimal irritation, sensitization, and skin penetration potential. (See 'Inorganic (mineral) filters' above.)

PROPER USE OF SUNSCREENS — Sunscreens must be applied liberally, repeatedly, and to all sun-exposed parts of the skin to provide effective protection. To achieve the full sun protection factor (SPF) value on the product label, the amount (2 mg/cm²) of sunscreen necessary to cover sun-exposed skin of an average-sized adult when on a beach vacation is approximately equivalent to the amount required to fill a 1-ounce (30 mL) shot glass or to 6 teaspoons. However, larger amounts, up to 45 mL or 9 teaspoons, may be needed for optimal coverage [30].

Most people do not apply such a large amount [31]. Because of the nonlinear relationship between effective SPF and the amount of sunscreen applied, using only half of the proper amount (1 mg/cm²) would provide approximately one third of the SPF [32,33]. Therefore, SPF 30 or greater sunscreens should be recommended to compensate. As an alternative, double application has been proposed as an effective method to ensure that the correct amount per surface unit is applied [34].

The "teaspoon rule" — Health care providers can instruct patients to adopt simple application techniques that ensure an even application of adequate amounts of sunscreen to the exposed areas [35,36]. One of these is the so-called "teaspoon rule." It involves the application of approximately 1 teaspoon (visually measured) of sunscreen to the face and neck area, a total of 2 teaspoons to the front and back torso, 1 teaspoon to each upper extremity, and 2 teaspoons to each lower extremity [30].

Timing — Timing of application is important. Sunscreens should be applied 15 to 30 minutes before sun exposure to allow the formation of a protective film on the skin. It is recommended to wait for at least a few minutes (ideally, 10 to 20) following sunscreen application before dressing [37]. Reapplication at least every two hours is necessary. Because all sunscreens are washed off upon swimming or sweating, reapplication after each water exposure (swimming) is needed even for sunscreen products labeled as "water resistant" (SPF maintained after 40 minutes of activity in water or sweating) or "very water resistant" (SPF maintained after 80 minutes of activity in water or sweating). (See 'Labeling and regulations' above.)

BENEFITS OF SUNSCREENS

Skin cancers — There is evidence from observational studies and randomized trials that sunscreens prevent the development of actinic keratoses and squamous cell carcinomas [38-41]. However, the extent to which the use of sunscreens reduces the incidence of basal cell carcinoma remains uncertain [40]. A follow-up study of a community-based, randomized trial in Queensland, Australia, provided strong evidence that the regular use of sunscreen reduces the incidence of melanoma [42]. In this study, 10 years after the conclusion of the original four-year trial, participants in the intervention group had developed 50 percent fewer primary melanomas compared with the control. In addition, in the same trial the rates of squamous cell carcinomas were decreased by nearly 40 percent among regular users of sunscreen. (See "Cutaneous squamous cell carcinoma: Primary and secondary prevention", section on 'Sun protection' and "Treatment of actinic keratosis", section on 'Prevention' and "Primary prevention of melanoma", section on 'Sunscreen'.)

Photoaging — Skin damage from ultraviolet (UV) exposure accumulates over time. In individuals with lightly pigmented skin, a substantial amount of photodamage manifests by age 40 [43]. Sunscreens may prevent skin changes such as pigmentation and wrinkling that are attributable to chronic photodamage or photoaging [44]. (See "Photoaging", section on 'Photoprotection'.)

Photodermatoses — Broad-spectrum sunscreens with high sun protection factor (SPF) are generally used for the prevention of photodermatoses, which can be elicited by either ultraviolet B or ultraviolet A (UVA). (See "Photosensitivity disorders (photodermatoses): Clinical manifestations, diagnosis, and treatment".)

A small study including patients with polymorphous light eruption found that a sunscreen with high SPF and high UVA protection factor containing photostable UV filters (Tinosorb M and Tinosorb S) and photostabilized avobenzone prevented the development of lesions after standardized photoprovocation [45].

PATTERNS OF USE — Although regular sunscreen use is a key message of sun safety campaigns worldwide, data on the pattern of sunscreen use in the general population are limited [46-48]. In the United States, a survey of over 4000 adults aged 18 years and older found that approximately 14 percent of men and 30 percent of women regularly used sunscreen on the face and other exposed skin when outside in the sun for more than one hour [49]. Regular use of sunscreen was associated with having sun-sensitive skin, higher annual household income, performing aerobic activity, and having children younger than 18. However, the use of sun protection measures other than sunscreen (eg, wearing a hat or protective clothing) was not investigated. The results of this study indicate a need for more effective sun safety interventions targeting men, individuals with a lower perceived susceptibility to sun damage, and those for whom cost may be a barrier to sunscreen use.

Of major concern is the low level of sunscreen use among children. In a series of more than 10,000 children of nurses from the Nurses' Health Study, only 34 percent of children aged 11 to 18 were using sunscreen, and 83 percent had at least one sunburn during the previous summer [50]. In a Netherlands survey of 1103 parents of children 6 to 12 years of age, approximately 30 percent regularly applied sunscreen to their children's skin during spring and summer [51]. A similar frequency of use (36 percent) was reported by primary and secondary school children in Switzerland [52].

Implementing sun protection education among children and their parents or caregivers may minimize sun damage at a young age and possibly promote a lifelong sun protection behavior. The efficacy of a multicomponent sun protection program, involving the distribution of a read-along book and swim shirt and weekly text-message reminders, was evaluated in a four-week randomized trial including 300 caregiver-child pairs [53]. At the end of the study, sun protection behavior scores were significantly higher in the intervention group, compared with the control group. Moreover, children in the intervention group did not show significant pigmentary changes compared with baseline, whereas a significant increase of pigmentation was noted in the control group.

SAFETY

Systemic absorption and toxicity

Inorganic (mineral) filters — The available evidence suggests that most inorganic (mineral) sunscreens have an excellent safety profile and are without significant systemic absorption [54]. However, the integration of titanium dioxide and zinc oxide nanoparticles into sunscreens has raised questions regarding their potential percutaneous penetration and toxicity [55,56].

Although some in vitro studies indicated that nanoparticles of titanium dioxide may induce the generation of reactive oxygen species with and without ultraviolet (UV) irradiation [13,57,58], several in vivo and in vitro studies have provided evidence that nanoparticulate titanium dioxide and zinc oxide applied to intact skin do not penetrate beyond the stratum corneum or at most show an insignificant dermal penetration [59-62].

A 2013 literature review performed by the Australian Government Therapeutic Goods Administration concluded that titanium dioxide and zinc oxide nanoparticles do not penetrate the underlying layers of skin, with penetration limited to the stratum corneum, and neither of them is likely to cause harm when used as ingredient in sunscreens [63]. Subsequent in vivo studies in various experimental conditions also support the safety of mineral sunscreens using nanoparticle technology:

●An in vivo study using multiphoton tomography with fluorescence imaging found that zinc oxide nanoparticles did not penetrate into the epidermis of either intact skin or skin with impaired barrier after a six-hour application with or without occlusion [64].

●Another study using multiphoton tomography with fluorescence imaging microscopy found that after repeated daily applications to the skin over five days, zinc oxide nanoparticles accumulate on the skin surface and skin furrows but do not penetrate or cause toxicity in the viable epidermis [65]. Exposing the skin to pool, sea, or showering water after sunscreen application does not seem to facilitate the penetration of zinc oxide nanoparticles into the skin [66].

●In another study involving healthy volunteers, a sunscreen formulation containing uncoated titanium dioxide nanoparticles was applied once daily on intact skin for three to eight consecutive days [67]. A skin biopsy examination using scanning electron microscopy and energy dispersive X-ray spectroscopy showed the presence of titanium dioxide nanoparticles only in the dermis surrounding hair follicles. However, the detectable numbers of nanoparticles were several orders of magnitude lower than those in the applied doses of sunscreen (approximately 0.00014 percent).

Organic (chemical) filters — Information on systemic absorption of organic sunscreen ingredients is limited. To understand whether and to what extent the sunscreen ingredients are absorbed into the body after topical application, the US Food and Drug Administration (FDA) sponsored an open-label, randomized, four-group, parallel study to determine the systemic exposure of active ingredients (avobenzone, oxybenzone, octocrylene, and ecamsule) present in four commercially available sunscreen products of different formulation types (spray, lotion, or cream) under maximal usage conditions [68]. In this study, 24 adult volunteers applied 2 mg of sunscreen per cm² to 75 percent of the body surface area four times per day for four days and each provided 30 blood samples over seven days for measurement of plasma concentrations of the active ingredient. All four sunscreen-active ingredients tested resulted in exposures largely exceeding 0.5 ng/mL, which is the threshold established by the FDA for waiving some nonclinical toxicology studies for sunscreens. The authors concluded that while these findings support the need for further studies to determine the clinical significance of systemic exposure to sunscreens, they do not suggest that individuals should refrain from using sunscreen.

A subsequent randomized trial assessed the systemic absorption and pharmacokinetics of six active ingredients (avobenzone, oxybenzone, octocrylene, homosalate, octisalate, and octinoxate) in four sunscreen products under conditions mimicking real-life usage [69]. In this study, 48 adult healthy volunteers applied sunscreen (2 mg of sunscreen per cm² to 75 percent of the body surface area) only once on day 1 and then every two hours for four times on days 2, 3, and 4, after a morning shower. Mean maximum plasma concentrations of all six active ingredients were greater than 0.5 ng/mL in all measurements, with this threshold surpassed as early as on day 1, after a single application. Plasma levels of active ingredients remained above the 0.5 ng/mL threshold at seven days in most participants. All active ingredients were detectable in the skin following tape stripping at days 7 and 14.

It is important to note that under real-life conditions most individuals likely apply only 25 to 50 percent, or even less, of the amount of sunscreen used for product testing [17,70-72] and do not always apply sunscreens every two hours. Thus, until further data on potential harms from systemic absorption of chemical sunscreen agents are available, recommendations for sunscreen use remain unchanged. For those concerned about systemic absorption of organic sunscreens, sunscreens that contain inorganic constituents (zinc oxide, titanium dioxide) can be recommended.

Irritant and allergic reactions — Adverse reactions from sunscreen ingredients have been reported and include allergic and irritant contact dermatitis, phototoxic and photoallergic reactions, contact urticaria, and rare anaphylactic reactions [73-76]. Most of the UV filters known to be contact sensitizers such as para-aminobenzoic acid (PABA), amyl-dimethyl-PABA, or benzophenone-10 are now rarely used in sunscreen manufacture.

Oxybenzone (benzophenone-3), the most widely used ultraviolet A filter worldwide, is the most frequent cause of sunscreen-induced photoallergic contact dermatitis [73]. However, the estimated rate of contact sensitization to oxybenzone-containing products is <0.1 percent [77]. In a study of approximately 24,000 patients patch tested by the North American Contact Dermatitis Group between 2001 and 2010, only 0.9 percent were sensitized to sunscreens [78]. The top three allergens in sunscreens were benzophenone-3, DL-alpha-tocopherol, and fragrances.

Hormonal effects — Some UV filters (eg, octinoxate, oxybenzone) have been found to have estrogenic effects in vitro or in animal models. In one study, oral oxybenzone induced an uterotrophic effect in immature rats [79]. The hormonal effects of sunscreens in humans are uncertain. In one study involving 15 young men and 17 postmenopausal women, the daily whole-body topical application of benzophenone-3, octyl-methoxycinnamate, and 3-(4-methyl-benzylidene) camphor resulted in detectable plasma and urine levels of all three sunscreens but did not alter the follicle-stimulating hormone and luteinizing hormone levels [57]. In another study involving 501 couples that discontinued the use of contraceptive to achieve pregnancy, male partners' urine concentrations of benzophenone-2 and 4-hydroxybenzophenone above the 75^th percentile were associated with a longer time to pregnancy [80]. However, this finding must be interpreted with caution because of the lack of control for other potential confounders.

Benzene contamination — Analytical studies have shown that several sunscreens are contaminated by benzene, a known carcinogen, some over three times the limit set by the FDA [81]. However, evaluation of 10,861 adults from the National Health and Nutrition Examination Survey found no elevation of benzene levels in the blood [82].

SUNSCREENS AND VITAMIN D — Since sun exposure is necessary for the synthesis of 25-hydroxyvitamin D in the skin, there has been concern that photoprotective measures could result in vitamin D deficiency. There is no evidence from randomized trials or longitudinal studies in real-life scenarios that sunscreen use significantly suppresses cutaneous vitamin D production [83-85]. In experimental settings, the production of vitamin D appears to be greatly reduced by the application of adequate amounts of sunscreen before ultraviolet B exposure [83,86-90].

In patients who need to adopt strict photoprotective measures, oral vitamin D supplementation is a safe, well-tolerated, and inexpensive alternative to sun exposure to achieve adequate vitamin D levels [91]. (See "Vitamin D deficiency in adults: Definition, clinical manifestations, and treatment" and "Vitamin D insufficiency and deficiency in children and adolescents".)

ENVIRONMENTAL EFFECTS OF SUNSCREENS — Chemical sunscreens are potential environmental contaminants because they are measurable in many aquatic ecosystems [92-94]. Direct effects of sunscreen run-off on coral reefs are limited, although in vitro studies have shown that some ultraviolet (UV) filters, in particular oxybenzone, are capable of disrupting the ecologic balance in corals, resulting in their bleaching [95-97]. The mechanisms of oxybenzone toxicity are incompletely understood. A simulation study in artificial sea water found that sea anemones, which are closely related to corals, metabolize oxybenzone into water-soluble glucoside conjugates that act as strong oxidants when exposed to UV light, resulting in bleaching and the death of these organisms [98]. However, it has not been determined to what degree this phenomenon occurs in nature, where the amount of UV effectively reaching marine life is highly variable.

PHOTOPROTECTIVE CLOTHING — Wearing clothes is important for sun protection. The degree of protection provided by clothes is defined by the ultraviolet protection factor (UPF), which indicates how effective a fabric is at blocking out solar ultraviolet (UV) radiation. The Australian Radiation Protection and Nuclear Safety Agency introduced a standardized UPF testing method for fabrics in 1996 and established rules for appropriate labeling of clothes claiming to offer sun protection [92].

Factors that contribute to the UPF rating of a fabric are:

●Composition of the yarns (cotton, polyester, etc)

●Tightness of the weave or knit (tighter improves the rating)

●Color (darker colors are generally better)

●Stretch (more stretch lowers the rating)

●Moisture (many fabrics have lower ratings when wet)

●Condition (worn and faded garments may have reduced ratings)

●Finishing (some fabrics are treated with UV-absorbing chemicals)

Additional information on sun-safe clothing is provide by the United States Skin Cancer Foundation.

The UPF classification is certified by national and international organizations (eg, American Sun Protection Association, Skin Cancer Foundation). The categories are UPF 15 to 24 for good protection, 25 to 39 for very good protection, and 40 to 50 for excellent protection. The tightness of the weave probably contributes more than other factors to the UPF of a garment [99]. A garment's photoprotective capacity may also be enhanced by washing with detergents containing optical whitening agents. Shrinkage from repeated washing and drying may actually improve the UPF.

SUMMARY AND RECOMMENDATIONS

●Definition – Sunscreens are topical preparations containing inorganic (mineral) or organic substances that reflect and scatter or absorb ultraviolet (UV) radiation in the ultraviolet B (UVB) and ultraviolet A (UVA) wavelength range (table 1). Broad-spectrum sunscreens are generally combinations of sunscreen products that absorb both UVB and UVA radiations. (See 'Inorganic (mineral) filters' above and 'Organic (chemical) filters' above.)

●Who should use sunscreens? – Everyone, regardless of skin phototype (table 2), is subject to the potential adverse effects of UV radiation and may benefit from sunscreen. However, sunscreens are especially useful for individuals with light skin (phototypes I, II, and III) (table 2) who are more susceptible to sunburn, photoaging, and skin cancer. (See 'Who should use sunscreen?' above.)

●Which type of sunscreen should be used? – Broad-spectrum products with a sun protection factor (SPF) of 30 or higher are recommended for all individuals performing outdoor work, sports, or recreational activities. (See 'Selection of sunscreen products' above.)

●How to apply sunscreens – Sunscreens must be applied liberally, repeatedly, and to all sun-exposed parts of the skin to provide effective protection. Simple practical rules, such as the "teaspoon rule," may be used to determine the adequate amount of sunscreen to apply to certain body areas. Sunscreen should be applied 15 to 30 minutes before sun exposure to allow the formation of a protective film on the skin and reapplied every two hours. (See 'Proper use of sunscreens' above and 'The "teaspoon rule"' above.)

●Safety of sunscreens – Sunscreens generally have an excellent safety profile, and there is no evidence of systemic adverse effects related to percutaneous absorption. For those concerned about systemic absorption of organic sunscreens, sunscreens that contain inorganic constituents (zinc oxide, titanium dioxide) can be recommended. Organic sunscreens may rarely induce allergic and irritant contact dermatitis, phototoxic and photoallergic reactions, or contact urticaria. (See 'Safety' above and 'Systemic absorption and toxicity' above.)

●Sun-protective clothing – Wearing clothes is important for sun protection. The degree of photoprotection provided by clothes is defined by the ultraviolet protection factor (UPF), which indicates how effective a fabric is at blocking out solar ultraviolet radiation. Yarn composition, tightness of the weave, and dark colors increase a garment's UPF. (See 'Photoprotective clothing' above.)