Sunscreens work primarily through two mechanisms: (i) scattering and reflection of UV energy, and (ii) absorption of UV energy. Many current sunscreens contain ingredients that work through both mechanisms in terms of UV protection.
The most important assay for determining the effectiveness of a sunscreen is the sun protection factor (SPF). The SPF measures a sunscreen’s ability to prevent development of erythema upon exposure to UV radiation, primarily UVB. The SPF value is defined as the ratio of the UV energy required to produce minimal erythema on protected skin to that required to produce the same erythema on unprotected skin in the same individual. For example, an individual using a sunscreen SPF 4 will take four times as long to develop cutaneous erythema when exposed to UVB radiation, as compared to when that individual has no protection. The Food and Drug Administration (FDA), which oversees the marketing and distribution of sunscreen products in the United States, mandates that a sunscreen agent must provide at least an SPF value of 2. Most commercially available sunscreen products have SPF values that exceed the minimum protection.
Despite attempts by the FDA to educate consumers and promote appropriate branding by manufacturers, sunscreen labeling has its limitations. The complicated names, as well as the variations in names for any given agent, may be overwhelming for the average consumer. The photostability of sunscreens is not quantified or labeled, and varies according to the chemical agent. The SPF value primarily measures a sunscreen’s ability to protect against UVB radiation and does not adequately address the effects of UVA. In addition, SPF readings may also vary for a given agent depending on the light source (33).
Nonetheless, concerted efforts to educate consumers have been the goal of the FDA. Confusing terminology such as “sunblock” and “all-day protection” is prohibited. The term “waterproof’ should be replaced with “water resistant.” The FDA discourages the branding of a sunscreen product as having an SPF of greater than 30. Although values greater than 30 offer increased protection, the risks of providing consumers with a false of security encouraged the labeling to restrict labeling to 30-plus. For sunscreen products making the claim of “water resistant,” the label SPF is the SPF value determined after forty minutes of water immersion, as determined by FDA guidelines.