Why you should apply sunscreen every two hours

With sunny weather and vacations on the horizon, we are once again being given plenty of warnings about the effects of the sun on our skin. It is clear that sunscreen is indispensable, but does it matter which one you use? And why does it stick? PhD candidate Heleen Kibbelaar, who studies complex fluids for her research, answers five questions about the chemistry behind sunscreen. Why does sunscreen stick?. 'Many UV filters, the ingredients in sunscreen that absorb UV light, have a greasy, sticky, oily structure. Therefore, a large amount of UV filters, for higher SPFs, often creates a more 'sticky' texture. Water-soluble filters suffer less from this, and fortunately there is a lot of research being done into new types of UV filters. By adjusting the formulation therefore, more non-sticky sunscreens are coming onto the market.' Should you throw sunscreen away after a year? . 'Sunscreen does not have an unlimited shelf life. On the packaging you will find a symbol of an open jar with a number in it. That number represents the number of months that a product will be usable after opening. UV filters have a certain photostability, which has to do with the stability of sun filters under the influence of (sun) light. A UV filter must therefore be sufficiently photostable. The photostability of all sunscreen products decreases with exposure to sunlight and heat. That's why it is recommended that we still buy a new sunscreen product every year, since we often leave them out in warm places like the beach, garden or patio.' Does it matter if you buy a cheap or expensive sunscreen?