The term SPF that appears on sunscreen labels stands for Sun Protection Factor, but it is really a sunburn protection factor. Products with a higher SPF allow fewer of the photons that produce sunburn to strike the skin. In simple terms, you can view an SPF 10 sunscreen as allowing 10 out of every 100 photons to reach the skin.
SPF is defined as the UV energy required to produce a minimal erythema (sunburn) dose (MED) on protected skin divided by the the UV energy required to produce an MED on unprotected skin.
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