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The formula for formulation efficiency factor (ε) is shown in Figure 1.
|Type of Active
|SPF 30 Sunscreen||SPF 50 Sunscreen|
|Formulation||Actives (%)||Sorbeth 2 Hexaoleate (%)||SPF||FEF|
Improving the effectiveness and efficiency of sunscreens continues to be a goal in the cosmetic industry. The US Food and Drug Administration (FDA) uses sun protection factor (SPF) as one of the most important tests related to sunscreen formulations. The FDA's recent Final Rule changes how sunscreens are viewed. For example, sunscreens cannot be labeled as “sunblock, waterproof or sweat-proof.” Also, the FDA has put restrictions on how much actives a sunscreen formulation can contain. These restrictions have left chemists with the quest of improving all aspects of sunscreen formulations (SPF being the FDA standard) including efficiency, effectiveness and feel to the consumer.
The FDA defines SPF as a measure of how much solar energy (UV radiation) is required to produce sunburn on protected skin (i.e., in the presence of sunscreen) relative to the amount of solar energy required to produce sunburn on unprotected skin. As the SPF value increases, sunburn protection increases.
There is a popular misconception that SPF relates to time that skin can be exposed to the sun. For example, many consumers believe that, if they normally get sunburn in one hour, then an SPF 15 sunscreen allows them to stay in the sun 15 hours (i.e., 15 times longer) without getting sunburn. This is not true because SPF is not directly related to time of solar exposure, but rather to amount of solar exposure. Although solar energy amount is related to solar exposure time, there are other factors that impact the amount of solar energy. For example, the intensity of the solar energy impacts the amount. Therefore, solar exposure for one hour at 9 AM or 15 minutes at 1 PM may result in the same amount of solar energy.
Generally, it takes less time to be exposed to the same amount of solar energy at midday compared to early morning or late evening because the sun is more intense at midday relative to the other times. Solar intensity is also related to geographic location, with greater solar intensity occurring at lower latitudes. Because clouds absorb solar energy, solar intensity is generally greater on clear days than cloudy days.
In addition to solar intensity, there are a number of other factors that influence the amount of solar energy that a consumer is exposed to including: skin type, amount of sunscreen applied and reapplication frequency.