SPF is based on the quantity of solar exposure.

SPF值根据的是太阳曝光的量。

As vitamin D is synthesised in the skin in the presence of certain forms of UV radiation, it is a reliable indicator of solar exposure.

由于维生素 D 是在某些形式的紫外线照射下在皮肤中合成的，因此它是太阳照射的可靠指标。