How Sunscreen Works (and Why You Shouldn’t Reuse an Old Tube)

In order to protect your skin from sunburn, the use of sunscreen is recommended. How exactly does this product work? Is it necessary to buy a new tube every year?

Your tube of sunscreen may have waited a year, quietly in the back of a closet, for the return of summer. By wishing to reduce your waste, you preferred to reuse it rather than buy it again — you shouldn’t waste the product that remains. Unfortunately, that’s not a very good idea.

We do not keep a sunscreen from one year to the next: the molecules will be less effective in protecting the skin », explains Christine Lafforgue, biologist and chemist, specialist in the formulation of cosmetic products at the University of Paris-Sud. What molecules are hidden in our sunscreens? And how do they protect against first degree burns such as sunburn?

What chemical process is hidden in this tube of cream?

A chemical process is at work in every tube of sunscreen. The product ” contains molecules that absorb wavelengths of the UV spectrum », sums up Christine Lafforgue. UV refers to ultraviolet rays: whether they are of natural or artificial origin, they can be the cause of skin cancers (such as melanomas), underlines the National Health Security Agency (handles). Ultraviolet B (UVB) rays mainly cause sunburn, while ultraviolet A (UVA) rays can lead to premature aging of the skin.

How Sunscreen Works (and Why You Shouldn't Reuse an Old Tube)
Spray sunscreen. // Source: Pxhere/CC0 Public domain (cropped photo)

The cream is created to create a protective film on the surface of the skin “, notes Christine Lafforgue. Inside the product, the molecules that absorb ultraviolet rays have what are called ” conjugated double bonds », explains Laurence Coiffard, doctor of pharmacy and professor at the University of Nantes, specialist in formulation chemistry. This phrase refers to a bond between the atoms of the molecule : a first atom linked by a double bond (stronger than a single bond) to a second, itself linked to a third atom by a single bond, this third atom being linked to a fourth by a double bond.

There are 28 regulated filters that go into the composition of sunscreens. 26 are said to be organic and 2 others (titanium dioxide and zinc oxide) mineral. The former absorb UV radiation, while the latter reflect it. Both can exist in nanometric form (the packaging must mention it in the list of ingredients with the indication ” [nano] “).

Other ingredients go into the composition of the product, such as oil, fats, water, preservatives, perfume and sometimes (surprisingly) alcohol. ” It’s a mixture like when you make mayonnaise: it’s about measuring out the right proportions of ingredients “, notes Laurence Coiffard.

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Sunscreen on a nose // Source: Pixabay

How to choose between these creams?

Not all sunscreens protect against UV rays in the same way. To differentiate them, you can use what is called the “sun protection factor”, known by the abbreviation ” FPS “. It serves ” to know the effectiveness of the protection of the product “, explains Christine Lafforgue. You can find 8 different ones on sunscreen labels.

  • Low protection : FPS 6 and 10,
  • Medium protection : FPS 15, 20 and 25,
  • High protection : FPS 30 and 50,
  • Very high protection : FPS 50+.

When you buy two creams from different brands, which have the same SPF index, they do not necessarily contain the same molecules but they provide equivalent protection. “, notes the biologist. It is not easy to understand how these different SPFs are established. As Christine Lafforgue explains to us, you can remember that ” they denote a ratio of time before the appearance of a sunburn. Generally, when you have fair skin, you should use an index 50 and reapply every two hours.. »

On my tube (new), it says ” reapply frequently, especially after swimming “. Even though most sunscreens are water resistant, maintaining this balance is not so easy. ” We cannot fix the product on the skin, we can only measure what will remain on the skin after passing through the water. It also depends on when you put your cream on: shortly before or long before swimming. remarks Christine Lafforgue.

Even mineral filters can pollute

This sunscreen, which risks being deposited in the ocean, worries some consumers who are trying to make a move to pollute the planet a little less. ” The environmental dimension is increasingly part of the act of buying sunscreenrecognizes Christine Lafforgue. What is certain is that it is not recommended to make your sunscreen at home. » By remaining on the surface of the water, the sunscreens risk « alter water light for marine flora and fauna. Mineral particles, they can settle to the bottom », Explains the biologist.

Mineral filters, often present in creams presented as organic, are no less polluting. They are also less effective, observes Laurence Coiffard. In organic sunscreens, they do not make it possible to obtain an SPF 50 index, contrary to what one can read on the packaging “, adds the specialist.

Anyway, no sunscreen is planned to be reused the following year. ” If your tube is not empty at the end of the summer, it means that you have not protected yourself enough. To determine the effectiveness of a sunscreen, it is applied at a rate of 2 milligrams per cubic centimeter to the skin of a man of 1 meter 80, who exposes himself for a day. You can imagine that he would need several tubes a day smiles Laurence Coiffard.

Exposure to heat (on the beach, when you forget your cream in your car in direct sunlight, etc.) and successive openings of the tube prevent it from being stored from one year to the next without affecting its effectiveness. ” Sunscreen is a product intended to be eliminated. As soon as we come back from vacation, it’s directly garbage “, concludes the doctor of pharmacy.

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Corals.  // Source: Pexels

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