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Sun Protection Factor (SPF) Rating System and UV-B Protection
Sunscreens are developed to offer additional protection against UV radiation, thereby protecting our skin and helping to minimize the occurrence of skin cancers and photoaging. These sunscreen products are rated using the Sun Protection Factor (SPF) system, a numeric scale based upon the degree of protection they offer against the sun’s ultraviolet radiation.
SPF < 15: mild protection
SPF 15-30: moderate protection
SPF >30 (30+): high protection
A product’s SPF rating is determined under artificial conditions using a predetermined amount of sunscreen. SPF is defined as the ratio of the amount of energy necessary to produce a minimal erythema (redness) on skin covered by sunscreen, compared to the amount of energy necessary to produce the same degree of erythema (redness) on unprotected skin. If it takes 35mJ/m2 of energy to produce 1 MED in unprotected skin and if it takes 350mJ/m2 of energy to produce 1 MED after applying a sunscreen, the sun protection factor (SPF) of that sunscreen is 10.
350 mJ/cm2 = SPF 10
35 mJ/cm2
The SPF rating system is not a complete determination of a sunscreen’s ability to offer protection from all the sun’s harmful rays. The SPF only measures the protection provided against UV-B radiation (290-320nm); it does not measure protection against UV-A radiation (320-400nm). At this time the FDA does not require that a sunscreen offer UV-A protection, nor does it require any official rating system for UV-A protection. As a result, the vast majority of sunscreens have been formulated to provide UV-B protection, with little attention paid to offering quality UV-A protection.
Studies show that most consumers understand that the higher the SPF number, the more the product protects the skin. Unfortunately, studies also show that people often have the mistaken notion that the higher the SPF number of the sunscreen they use, the longer they can stay - and will stay - in the sun. The amount of protection against UV-B rays does not increase greatly with higher SPF values:
SPF 15 blocks 93.3% of UV-B radiation … or … your skin absorbs 6.7% of what you’d absorb wearing nothing
SPF 30 blocks 96.7% of UV-B radiation … or … your skin absorbs 3.3% of what you’d absorb wearing nothing
SPF 40 blocks 97.5% of UV-B radiation … or … your skin absorbs 2.5% of what you’d absorb wearing nothing
SPF 45 blocks 97.8% of UV-B radiation … or … your skin absorbs 2.2% of what you’d absorb wearing nothing
SPF 50 blocks 98% of UV-B radiation … or … your skin absorbs 2% of what you’d absorb wearing nothing
SPF 60 blocks 98.3% of UV-B radiation … or … your skin absorbs 1.7% of what you’d absorb wearing nothing
Sunscreens that do offer protection against UV-B and UV-A rays are labeled “broad spectrum”, but this term is loosely applied and is therefore often misleading. Certainly the addition of any UV-A protection is beneficial when compared to UV-B protection alone; but the amount of UV-A protection in many so-called “broad spectrum” sunscreens is often minimal in comparison and only for short wave UV-A (UV-AII) rays. In many of these so-called “broad spectrum” sunscreens there is usually very little coverage provided throughout the entire UV-A spectrum. And subsequently, the use of a high SPF product wrongly (encourages)allows people to stay in the sun longer, with less burning, while they receive more damaging UV-A exposure. A claim such as "broad spectrum" on a sunscreen label needs to be supported by evidence that the product provides significant and meaningful protection across the entire UV-B/UV-A spectrum.
Although it is frequently assumed that highly-rated (>15) SPF products offer more UV-A protection, this is not necessarily true. The sun protection provided by specific sunscreen products varies significantly depending upon the type of active sunscreen ingredient(s) [organic chemical- or mineral-based] used, the products chemical composition, and how that material is dispersed on the surface of the skin. In vivo exposure testing of a specific product is the only true way to document a sunscreen’s efficacy, and to determine its “substantivity” (duration of action / the lasting effect of its protection). It is these properties that become important when one is perspiring or swimming.
UV-A protection is also an essential part of preventing increased skin pigmentation in response to sun exposure. Studies have demonstrated that SPF determination cannot be correlated with a product’s efficiency in preventing skin pigmentation. When different products having the same SPF (same protection against erythema) but different levels of UV-A protection are compared, only sunscreen products with a high level of UV-A protection show a similar level of protection against both sunburn and skin pigmentation. Products with low UV-A protection have a lower capacity of preventing induced pigmentation compared with their efficacy against erythema.
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Sunlight & UV rays
SPF Rating
Sunscreen Testing
Critical Wavelength
Sunscreen Ingredients
Photostability
Skin Sensitivity
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