Encouraging photoprotection is the leading preventative health strategy employed by physicians involved in skin care. Although sun avoidance is most desirable, outdoor occupations and lifestyles make total avoidance impossible for many individuals. The regular use of sunscreens represents a practical compromise in this regard. Sunscreens prevent the formation of squamous cell carcinomas in animals. In humans, the regular use of sunscreens has been shown to reduce actinic keratoses, solar elastosis, and squamous cell carcinoma. [1, 2] The routine use of sunscreens may also reduce melanoma risk.  Drug photosensitization and photo-induced or photo-aggravated dermatoses may be prevented with sunscreen use, especially with products that offer better blockage in the UV-A range.
See the image below.
Additionally, the Medscape article Sunburn may be of interest.
UV radiation (UVR) that reaches the Earth's surface can be divided into UV-B (290-320 nm) and UV-A (320-400 nm). UV-A can be further subdivided into UV-A I, or far UV-A (340-400 nm), and UV-A II, or near UV-A (320-340 nm).
The US Food and Drug Administration (FDA) regulates sunscreen products as over-the-counter drugs. The Final Over-the-Counter Drug Products Monograph on Sunscreens (Federal Register 1999: 64: 27666-27963) established the conditions for safety, efficacy, and labeling of these products. The sun protection factor (SPF) is defined as the dose of UVR required to produce 1 minimal erythema dose (MED) on protected skin after the application of 2 mg/cm2 of product divided by the UVR required to produce 1 MED on unprotected skin. A proposed amendment (Federal Register 2007: 72: 49070) recommended a maximum designation of SPF 50 plus.
A Final Rule has been issued (Federal Register 2011: 76: 35620-35673) that further elaborated on UV-A testing. A “broad spectrum” sunscreen provides protection through the entire spectrum of both UV-B and UV-A as measured by the Critical Wavelength Method. A “water-resistant” product maintains the SPF level after 40 or 80 minutes of water immersion.
In Europe, sunscreen products are considered cosmetics, their function being to protect the skin from sunburn. The Third Amendment of the European Economic Community Directive provides a definition and lists the UV filters that cosmetic products may contain. The European Union allows several ingredients not available in the United States.
Active Sunscreen Ingredients
Sunscreens have traditionally been divided into chemical absorbers and physical blockers on the basis of their mechanism of action. Chemical sunscreens are generally aromatic compounds conjugated with a carbonyl group. These chemicals absorb high-intensity UV rays with excitation to a higher energy state. The energy lost results in conversion of the remaining energy into longer lower energy wavelengths with return to ground state. Inorganic particulate sunscreens, zinc oxide, and titanium dioxide, can reflect or scatter UVR. Micronized forms of physical blockers may also function by absorption. Allowable ingredients and maximum allowable concentrations, as listed in the FDA monograph, are shown in the Table below. Europe, Australia and Japan allow additional active ingredients. In 2014, the US Congress passed the Sunscreen Innovation Act to establish an expedited review process to allow approval of additional sunscreen ingredients. To date, no new ingredients have been approved, several of which have been used in Europe for over a decade.
Sunscreen ingredients can also be classified by the portion of UVR that they effectively absorb.
Table. FDA Sunscreen Final Monograph Ingredients (Open Table in a new window)
|Drug Name||Maximum Concentration, %||Absorbance|
|Dioxybenzone||3||UV-B, UV-A II|
|Oxybenzone||6||UV-B, UV-A II|
|Sulisobenzone||10||UV-B, UV-A II|
|*Only available in United States in patented products.|
Para- aminobenzoic acid (PABA) was one of the first chemical sunscreens to be widely available. Several problems limited its use. It required an alcoholic vehicle, it stained clothing, and it was associated with a number of adverse reactions. Ester derivatives, mainly padimate O or octyl dimethyl PABA, became more popular, with greater compatibility in a variety of cosmetic vehicles and a lower potential for staining and adverse reactions. Because of problems with PABA formulations, manufacturers emphasized the PABA-free claim, and now both PABA and padimate O are less frequently used. Padimate O is the most potent UV-B absorber. The decline in its use, along with the demand for higher SPF products, has led to the incorporation of multiple active ingredients into a single product to achieve the desired SPF, replacing single PABA esters.
The cinnamates have largely replaced PABA derivatives as the next most potent UV-B absorbers. Octinoxate or Octyl methoxycinnamate is the most frequently used sunscreen ingredient. Octinoxate is an order of magnitude less potent than padimate O.
Octisalate or octyl salicylate is used to augment the UV-B protection in a sunscreen. Salicylates are weak UV-B absorbers, and they are generally used in combination with other UV filters. Other salicylates must be used in higher concentrations. They all have a good safety profile.
Octocrylene may be used in combination with other UV absorbers to achieve higher SPF formulas. Octocrylene used in combination with other sunscreen ingredients, such as avobenzone, may add to the overall stability of these ingredients in a specific formula.
Most chemical sunscreen ingredients are oils that are soluble in the oil phase of emulsion systems, accounting, in part, for the heavy, greasy aesthetics of many of these products. Ensulizole or phenylbenzimidazole sulfonic acid is water soluble, and it is used in products formulated to feel lighter and less oily, such as daily use cosmetic moisturizers. It is a selective UV-B filter, allowing almost all UV-A transmission.
Although benzophenones are primarily UV-B absorbers, oxybenzone absorbs well through UV-A II. Oxybenzone can be considered a broad-spectrum absorber. It significantly augments UV-B protection when used in a given formula.
Anthranilates are weak UV-B filters, and they absorb mainly in the near UV-A portion of the spectrum. Anthranilates are less effective in this range than benzophenones, and they are less widely used.
Often referred to by its trade name, Parsol 1789, butyl methoxydibenzoylmethane or avobenzone provides superior protection through a large portion of the UV-A range, including UV-A I. A significant addition to sunscreen products for true broad-spectrum UV protection, concerns have been raised regarding its photostability and its potential to degrade other sunscreen ingredients in products in which it is used. This photoinstability can be offset by combining avobenzone with octocrylene or with other nonsunscreen ingredients such as diethylhexyl 2,6 napthalate.
Terephthalylidene dicamphor sulfonic acid or Mexoryl SX provides protection within the near UV-A range (320- to 340-nm). It is only available in select patented sunscreen products with the trade name Antihelios. Mexoryl SX is water soluble, making it less water resistant. Also subject to photoinstability, it is combined with octocrylene to increase its photostability. In 2015, the FDA proposed preventing this ingredient from being used in the US market unless proven safe and effective by manufacturers seeking to use this sunscreen in products.
Some of the original sunblocks were opaque formulations reflecting or scattering UVR. During World War II, red petrolatum was extensively used by the military. Titanium dioxide and zinc oxide also functioned in this fashion. Poor cosmetic acceptance had limited the widespread use of the latter 2 ingredients until microsized forms became available, also known as inorganic particulate sunscreens.
The ideal sunscreen agent would be chemically inert, safe, and absorb or reflect through the full UV spectrum. Titanium dioxide meets these criteria limited only by aesthetics. By decreasing the particle size of this pigment to microsize or ultrafine grades, thereby making it less visible on the skin surface, some of these advantages could be used. This ingredient can be classified as a broad-spectrum agent.
Despite advances in the technology, formulating products with this ingredient that do not whiten the skin secondary to pigment residue is difficult. Adding other pigments that simulate flesh tones may partially camouflage this effect. The net effect may be that the user is inclined to use less of the product (a light application), effectively lowering the SPF. Hybrid products that use a combination of chemical UV absorbers with inorganic particulate sunscreens may represent a practical compromise.
Having been used for many years in opaque blocks, zinc oxide has recently been approved by the FDA as an allowable active ingredient in sunscreen products. Like titanium dioxide, microsized or ultrafine grades of this ingredient have been developed, offering some of the same advantages and disadvantages described above, including the ability to provide more full-spectrum protection. Zinc oxide is less whitening in this form than titanium dioxide and provides better UV-A I protection. 
Arguably, a SPF 15 sunscreen provides full UV-B protection for healthy individuals. A SPF 15 product filters out more than 93% of UV-B radiation, and a SPF 30 product filters out less than 97%.  The difference of 4% would not seem significant to most individuals. Product application technique outside the laboratory alters the SPF. As previously noted, the standard FDA method for SPF testing involves a sunscreen application thickness of 2 mg/cm2. Several studies indicate that under in vivo, real-world conditions, application thickness more likely approximates 0.5-1.0 mg/cm2, lowering the effective SPF of the product. When SPF testing is conducted outdoors, the efficacy of products is found to be lower than in the laboratory.
Erythema, the key measurement in the SPF assay, is a relatively crude biologic endpoint. A comparison of a SPF 15 sunscreen versus a SPF 30 sunscreen showed subclinical damage (sunburn cell formation) in the former without visible erythema.  The SPF 30 product provided significantly greater protection. Other forms of subclinical damage may occur with a SPF 15 formulation. Although UV-A protection may be less than desirable with all sunscreen products, the UV-A protection is better with a higher SPF, particularly in the UV-A II (320-340 nm) or shorter UV-A range.
Although sunscreens provide excellent UV-B protection, they lack in UV-A protection, particularly UV-A I. With the availability of higher SPF products allowing individuals to spend greater amounts of time in the sun without burning, concerns have been raised about the adequacy of the UV-A protection of these products. In fact, individuals relying on sunscreens as their sole form of photoprotection may now be subject to greater cumulative sun exposure, including UV-A radiation.
No consensus exists about the best method for measuring UV-A protection. A variety of methods have been proposed. In vivo methods have been developed on the basis of direct UV-A erythema, persistent pigment darkening, and photosensitization with psoralens. A detailed discussion is beyond the scope of this review.
At best, each method has its limitations and indications for a particular clinical situation or skin type. An in vitro method relying on transmittance through a thin substrate, a thin film, is currently used in Europe. The FDA Final Rule also relies on an in vitro assay known as the Critical Wavelength Method (see Definitions). The critical wavelength (CW) is determined to be the wavelength below which 90% of the total area of the UV absorbance resides. A “broad spectrum” sunscreen has a CW of greater than or equal to 370 nm. If protection from UV radiation into the UV-A I range is desired, the formula should contain either avobenzone or an inorganic particulate sunscreen as an active ingredient.
Vehicle type is critical for determining sunscreen efficacy and aesthetics.  Ingredients, such as solvents and emollients, can have a profound effect on the strength of UV absorbance by the active ingredients and at which wavelengths they absorb. Film formers and emulsifiers determine the nature of the film that forms on the skin surface. Higher SPF products require a formula that provides a uniform and thick sunscreen film with minimum interaction of inert ingredients with the actives. Durability and water resistance are obviously vehicle dependent. Lastly, product aesthetics play a large role in patient compliance with specific sunscreen recommendations.
The most popular sunscreen vehicles are lotions and creams. Two-phase oil-in-water or water-in-oil emulsion systems allow for the widest variety in formulation. Most sunscreen ingredients are lipid soluble and are incorporated into the oil phase of the emulsion. Higher SPF products may contain 20-40% sunscreen oils, accounting for the occlusive greasy feel of many of these products. Dry lotions, often marketed as sport lotions, represent the formulator's attempt to provide a less oily product. Newer "ultrasheer" products further refine these qualities with the use of silica as a major vehicular component.
Other vehicles for organic sunscreen ingredients include gels, sticks, and aerosols. Water- or alcohol-based gels provide less greasy aesthetics, but they rely on the more limited number of water-soluble sunscreen ingredients and are less substantive with a greater potential for irritation. Sticks readily incorporate lipid-soluble sunscreens thickened with waxes and petrolatum and are heavier on application, but they are useful for protecting limited areas, such as the lips, the nose, or around the eyes. Sprays or aerosols provide convenience on application, but they may be difficult to apply evenly, resulting in a discontinuous film. The FDA Final Monograph has not approved sprays as a dosage form pending further considerations and testing.
Sunscreens have been incorporated into a broad range of consumer products, including daily-use cosmetics. The FDA monograph now distinguishes between beach and nonbeach products. The availability of sunscreens in this fashion offers many advantages.
Daily protection is facilitated for a large segment of the population. UV protection is encouraged by the glamour image associated with cosmetic use. Sunscreens/moisturizers are available year-round, as opposed to seasonal beach products. Moisturizers that incorporate sunscreens are generally oil-in-water emulsions. Water-soluble sunscreen ingredients are often used to decrease the oil phase and to increase the cosmetic elegance. Foundation makeup without sunscreen generally provides a SPF of 3 or 4 by its pigment content. By raising the level of pigments, including inorganic sunscreen particulates, titanium dioxide and zinc oxide, higher SPF can be achieved with or without the use of organic chemical sunscreens. Makeup with sunscreen has intrinsic full-spectrum UV-A protection based on opacity. Chemical sunscreens are generally added to lipsticks to provide enhanced SPF protection.
Photostability and Toxicity
Photostability refers to the ability of a molecule to remain intact with irradiation. Photostability is potentially a problem with all UV filters because they are deliberately selected as UVR-absorbing molecules. This issue has been raised specifically with avobenzone, with photolysis demonstrated, especially in in vitro systems, that simultaneously irradiate and measure transmittance in situ. This effect may degrade other sunscreens in a formulation. This change has also been observed with octyl methoxycinnamate and octyl dimethyl PABA, while oxybenzone was shown to be relatively stable.
Higher SPF sunscreen products have led to the use of multiple individual sunscreen agents used in combinations at maximum concentrations that may interact. The photostability of the molecules also depends on the solvent or the vehicle used. Certain ingredients may have a stabilizing effect on others; octocrylene has been shown to photostabilize avobenzone. Other ingredients may be added to the sunscreen formulation to provide photostability or raise SPF.  The relevance of these observations to the in vivo situation remains unclear. Much work remains to be done in this area.
Subjective irritation associated with burning or stinging without objective erythema is the most common sensitivity complaint from sunscreens.  This irritation is most frequently observed in the eye area. Persistent objective irritant contact dermatitis is more common than and may be difficult to distinguish from true allergic contact dermatitis, although true allergy to sunscreen ingredients is uncommon. 
Fragrances, preservatives, and other excipients account for many of the allergic reactions that occur with sunscreens.  Virtually all sunscreen ingredients reported to cause contact allergy might be photoallergens. Although still relatively uncommon, sunscreen actives seem to have become the leading cause of photocontact allergic reactions. Individuals with preexisting eczematous conditions have a significant predisposition to sensitization associated with their impaired cutaneous barrier. Most individuals who develop photocontact dermatitis to sunscreens are patients with photodermatitides.
Organic sunscreens, specifically PABA and its derivatives, have been the subject of extensive in vitro photochemical and cytologic studies that suggest that organic sunscreens, such as PABA, interact with DNA following UV radiation and might potentiate photocarcinogenesis. Both acute and chronic in vivo animal studies show sunscreens to be protective for both UV-induced DNA damage and skin tumor formation. Most significantly, routine sunscreen use in humans has been shown to reduce solar elastosis, actinic keratoses, and squamous cell carcinomas. The in vivo data would seem to eliminate concerns related to photocarcinogenicity with the use of organic chemical sunscreens. 
Sunscreens containing inorganic particulates (titanium dioxide and zinc oxide) provide a good option for individuals with sensitive skin because these ingredients are not associated with irritation or sensitization. Absent demonstrable dermal penetration, concerns raised about toxicity with the use of nanotechnology would seem unfounded with these ingredients. 
Regular sunscreen use can diminish UV-dependent cutaneous synthesis of vitamin D. Elderly persons are particularly susceptible to the consequences of vitamin D deficiency, including osteopenia and bone fractures. Under conditions of actual usage, clinical trials have shown that individuals instructed in carefully applying sunscreens still receive enough sunlight to maintain normal vitamin D levels. Maximal vitamin D synthesis through UV exposure is obtained with relatively low doses of erythemogenic UV achievable with sunscreen usage.
Optimal vitamin D levels have not been defined. Ample vitamin D levels can be obtained from incidental sun exposure, diet, and supplements.  For individuals practicing rigorous photoprotection, daily intake of 600 IU of vitamin D through diet or supplements is recommended; for persons aged 71 years or older, 800 IU or higher may be recommended.
Sunscreens alone may provide insufficient protection from UVR. Sunscreens function best to prevent sunburn from UV-B radiation. They provide more limited protection from UV-A radiation. Sole dependence on sunscreens can have the unwanted effect of increasing outdoor exposure times, particularly in those individuals who burn easily and tan poorly. Sun avoidance remains the most desirable form of sun protection.
Simply staying indoors is obviously the best way of avoiding the sun. However, encouraging individuals to time outdoor exposure to avoid the hours when the sun is at its zenith is more practical. Trying to schedule activities before 10 am and after 4 pm (daylight savings time) avoids solar exposure at times of peak intensity. Individuals need to be reminded that on cloudy days as much as 80% of UVR may still penetrate the cloud cover. Shade availability in recreational areas is also desirable despite difficulty in accurately estimating the protective effects with varying reflection and penetration in different environments. Window glass absorbs most of the radiation below 320 nm; however, considerable amounts of UV-A radiation may still pass through glass. Special plastic films containing UV-A shields as an interleaf or overlay are available.
Clothing can be an excellent form of sun protection. The most important determinant is tightness of the weave. Fabric type is less important. Thickness is also less important than regular weave. Protection drops significantly when the fabric becomes wet. Color plays a minor role, with dark colors protecting better than light colors. A crude test of clothing is to hold it up to visible light and observe the penetration. The FDA defines clothing with a SPF rating as a medical device. One approved line of clothing with a rating of SPF 30 or greater is Solumbra (1-800-882-7860). Hats are the most important articles of clothing. A 4-inch wide circumferential brim is required to cover the entire face and neck.
Because UV light can have both acute and chronic adverse effects on ocular tissue, sunglasses provide important protection. National standards are in place for Europe, Australia, and the United States. Compliance with the standard in the United States is voluntary.
Sunscreen should be applied 15-30 minutes prior to sun exposure to allow sufficient time for a protective film to develop. Sunscreen should be reapplied after prolonged swimming or vigorous activity. Under conditions of continuous UVR exposure, they should be reapplied every couple of hours. Sunscreen needs to be applied liberally. As much as 1 oz may be needed to cover the entire body. Particular attention needs to be paid to the back of the neck, the ears, and the areas of the scalp with thin hair. Sunscreens represent only one component of a total program of photoprotection.