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.
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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.
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. Increasing photoprotective effects against sunburn have been shown with SPF up to 100 or higher. 
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.
Photostability and Toxicity
It 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
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.
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.