These organic chemicals also explain, to an extent, why sunscreen has to be reapplied. Some of the organic chemicals used will be photostable; that is, they won’t break down when exposed to UV light. Some of them, however, will slowly break down as they absorb UV light over time – avobenzene, shown in the graphic, is one of the prime examples of this. Other chemicals can be added to help slow this breakdown, but it can be one of the reasons why it’s necessary to reapply sunscreen regularly. The more obvious reasons include the fact that even sunscreens that claim to be ‘water-resistant’ will still eventually wash off, which is why regulation now states sunscreens should also specify the length of time to which they remain water resistant.