It's a reflex that many people have: as soon as the first rays of sunshine appear, you pull out your tube of sun cream thinking you're doing the right thing. We coat ourselves with the product, believing that we are protecting ourselves against UV rays. But after spending a year in the back of your closet, your sun cream may no longer have any protection.
This was explained to Numerama by Christine Lafforgue, a biologist at the Paris-Sud University:
You can’t keep sun cream from one year to the next: the molecules will be less effective at protecting the skin.
Without you knowing it, a chemical process occurs in your tube of sunscreen. And time doesn't help it keep its properties.
The ‘classic’ composition of sun cream is as follows: an emulsifying base (like cream), preservatives, stabilisers, anti-free radicals (such as vitamin E or C) and ultraviolet filters. The latter has a crucial role since they serve to prevent ultraviolet B rays that cause sunburn, and ultraviolet A rays that cause premature skin ageing.
After the holiday, throw it away
Inside your tube, the sun cream thus forms a chemical cocktail that protects you thanks to what are called conjugated double bonds. And as in a good mayonnaise, the recipe for success lies in the right dosage of the different ingredients. While it works in a brand new tube, no sun cream is designed to last and be reused from one year to the next.
And for good reason, sunscreens generally suffer from prolonged exposure to the sun and heat, by spending several hours in your beach bag. Despite the preservatives, even the most resistant of creams lose their effectiveness. Laurence Coiffard warns:
If your tube is not empty at the end of the summer, it means you haven't protected yourself enough. Sun cream is a product intended to be used up. As soon as you get back from your holiday, it should go right in the bin.
Check out the video above for more on why you absolutely shouldn't keep using your old sunscreen...