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Sunscreens containing zinc oxide lose their effectiveness after two hours and can become toxic

July 12, 2022
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Sunscreens containing zinc oxide lose their effectiveness after two hours and can become toxic

Summer, sea and sun bathing. Whether you choose the mountains, the sea, the cities as your holiday destination or prefer to stay at home and go to the swimming pool, protecting your skin from the sun is essential. But beware of how sunscreen is used, especially if it contains zinc oxide, a substance that is gaining ground in several sunscreen products. In fact, these creams can quickly lose effectiveness and become, in some cases, even toxic, as emerges from a recent scientific research published in the journal Photochemical & Photobiological Sciences by an American team of the University of Oregon (Ginzburg et al, Photochemical & Photobiological Sciences, 2021).

Sun creams with physical and chemical filters

The sun is good for you, there is no question about this. The sun's rays are, in fact, our most precious source of vitamin D and also improve mood. However, like everything, the sun should be taken in moderation, avoiding the central hours of the day and in any case using protective filters, the famous sunscreens, to protect the skin from the risk of erythema, irritation, premature aging and tumors. Sun creams contain substances that act as a filter against the sun's UVA and UVB rays. Filters are of two types, chemical and physical. Chemical filters, such as Octyl methoxycinnamate, Octocrylene or Benzophenone-3, absorb solar radiation and release it in the form of heat. Physical filters, on the other hand, reflect radiation. Science is focusing on the study of the various substances used as a filter to understand their stability and possible toxicity in order to put products on the market that are safe for human health and the environment. Among physical filters, zinc oxide is a substance that is attracting a lot of interest. But how does it behave if exposed to solar radiation for a long time? This question was answered for the first time by the scientists of the study we are talking about today and the results are noteworthy.

The problem of zinc oxide, here's what happens after two hours

Scientists have created five mixtures with chemical sunscreen filters, based on substances commonly used in sunscreens marketed in America and Europe. All these mixtures were subjected to solar radiation for two hours. They were then analyzed to assess their stability. All blends were still perfectly stable, with no modifications or traces of toxicity.

Then, the scientists added zinc oxide to all the mixtures, which, as mentioned, is a physical filter that is added in increasing quantities in solar products. Zinc oxide has been included in the formulations in an amount equal to 6%, which is the lower limit recommended by the guidelines, which provide for the use of this substance between 5 and 24%. Well, after two hours of exposure to sunlight, all the mixtures showed significant changes in their structure, with an 80% reduction in the ability to filter UVA rays, which represent 95% of the solar radiation that hits the earth. UVA rays penetrate deep into the skin, causing photoaging, blemishes, wrinkles, dry skin and inflammation that, in the long run, can lead to cell degeneration. What happened is explained by the fact that the zinc oxide, exposed to radiation, broke after about two hours, degrading the mixtures, which thus lost their effectiveness. Not only that, after carrying out tests on zebrafish, which are small fish that, from a genetic, cellular and molecular point of view, are very similar to humans, mixtures with zinc oxide, after two hours of exposure to solar radiation, have shown an important degree of toxicity.


In conclusion, it is essential to protect your skin with sun creams, but care must be taken in the use of these products. First of all, as emerges from the study, it would be preferable to avoid using solar products that have chemical and physical filters at the same time, the latter represented by zinc oxide. Care should also be taken not to apply two sun creams, one above the other, of different types, namely one with chemical filters and one with physical filters with zinc oxide.

In addition to this, the study shows that after two hours of application, zinc oxide loses its stability. The toxicity was found only in combination with other substances that act as a chemical filter, but in any case the results indicate that after two hours of sun exposure, zinc oxide, even if used alone, can lose its effectiveness and therefore it is necessary to make another application of the sunscreen.

In addition to this, the results obtained concerned zinc particles of all sizes, both nano and micro particles.

Therefore, the mere writing nano free on a cream, indicating that nanoparticles are not used, as is often happening now, does not guarantee total safety. Attention should always be paid to the substances contained.

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