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We receive this question often enough and it's a very valid question. Skin cancer is a #1 cause of mortality. So here is the thing. When you are going to the beach, it's indeed the best if your sunscreen has not reached its expiration date.
Current sunscreens available on the market usually come in 3oz-5oz size and if used as recommended by the FDA, a bottle or tube of 3oz would have about 6 full body applications maximum. And that is for a small body, assuming a child. For an adult, it should last even less. So if you buy a tube of sunscreen sized 3oz, use it. Don't try to make it last 2 years by not using it as recommended. They are inexpensive enough these days to not cause a cost concern.
But even if... in one of your beach bags you run across a tube of sunscreen with an expired date, don't fret... The longevity and efficacy of the sunscreen primarily depends on how it's been stored for all this time. If it's been stored in a cool and dry place, like a closet, it can last for years, even as long as 5 and more years. The standard expiration date stamped on all sunscreens is 2 years. However, experts of the field and chemists insist that this number is overly conservative.
But having in mind the purpose of such product, people often take it to the beach, leave it out in the sun, in hot car or trunk. That is where degradation of the formula accelerates. Prolonged heat and light speed up break down of the formula components. This would apply not only to sunscreens, but most, if not all, skin care products. This is also why we do not recommend keeping your beauty products on a bathroom window seals.
What in fact happens to the ingredients? Mineral sunscreens contain inorganic compounds, such as zinc oxide and/or titanium dioxide, which prevent sunburn by reflecting ultraviolet (UV) radiation that would otherwise penetrate your skin and cause damage. They also contain supporting ingredients that help the product spread better, smell great and nourish your skin. Those ingredients are natural vegetable or fruit oils, essential oils, aloe, and emulsifier. Emulsifier is a special ingredient that helps mixing oils and water together, forming and emulsion. And that is the ingredient that tends to break down first, if a product was not being stored properly. The visual clue of degradation is when a product looks runny or grainy, or would not adhere to the skin as it should. While it is not a desirable outcome, the product is still can be used, if no other alternative is available. All you have to do is give it a good shake and in most of the cases it should return to its creamy form.
Plant oils that often are present in sunscreens can go rancid overtime too and unpleasant smell would be the clue. But if a product is adequately preserved, that is usually not a problem.
However, here is encouraging news. Mineral sunscreens are based on Zinc Oxide and Titanium Dioxide. They come in a form of powder, which does not deteriorate overtime. These minerals have an indefinite shelf life, which means that they are extremely stable and can tolerate high heat, UV light and moisture without disintegration. (Unlike their chemical counterparts) So, in the worst case scenario, even if other ingredients may show an evidence of deterioration, ZnO and TiO2 still continue to perform to a valid degree, even if with some loss of overall product potency. For example, a sunscreen of SPF50, with age can reduce its strength to SPF40 or 30. Or a sunscreen with SPF30, may reduce its potency to SPF20 or 15. That is also important to keep in mind and not forget to apply it more often. So don’t throw away your expired sunscreen. There is still some life in it.
Sweetsation Therapy offers 2 Mineral based sunscreens SPF30.