UV filters affect the environment around us and can in some cases also affect the person using the product. The higher the molecular weight of a substance, the less likely it is to penetrate the skin. Since UV filters only provide UV protection on the skin's surface, the filters that do not penetrate the skin will work the best. Hence, it is advisable to look for products that are photo-stable, meaning they are not broken down by sunlight. When certain UV filters break down, by-products are formed which can be harmful to the user. For instance, both allergic reactions and endocrine-disrupting effects have been observed to be caused by some UV filters or their degradation products. Also, several studies show that some filters pass through the skin and can then be measured in breast milk and urine. However, further studies are needed to determine if and how the amounts absorbed through the skin affect the user.
Examples of UV filters that have been shown to cause endocrine disruption in animal or in vitro studies are: Benzophenone-3, Homosalate, 4-Methylbenzylidene camphor, Ethylhexyl methoxycinnamate, and Ethylhexyl dimethyl PABA.
Studies also showed that some of these substances, namely Benzophenone-3, 4-Methylbenzylidene camphor, and Ethylhexyl methoxycinnamate, quickly passed through intact skin and could be measured in the bloodstream. (Krause 2012, Maipas 2015)
In 2017, the amount of Benzophenone-3 allowed in sunscreen products, in the EU, was reduced from 10% to 6%. The European Commission's scientific committee for consumer products, Scientific Committee on Consumer Safety (SCCP), in its evaluation of Benzophenone-3, stated that the substance may cause contact allergy and maybe photo allergenic. Since very small amounts of a substance are required to cause allergic reactions, it is advisable to completely avoid this ingredient if you want to avoid the risk of getting these reactions.
The studies available show that physical filters do not break down and are not absorbed by the skin. They can, therefore, be considered less risky to use, but since they often leave a white cast on the skin, many people prefer to use products with chemical filters.
Examples of chemical filters that have not been shown to cause endocrine disruption in animal studies or in vitro: Bis-ethylhexylphenol methoxyphenyltriazine, Diethylamino hydroxybenzoyl hexyl benzoate, Ethylhexyl triazone, Diethylhexyl butamido triazone, Terephtalylidene dicamphor sulfonic acid, Drometriazole trisiloxane och Tris biphenyl triazine.
In the United States, for instance, many of the UV filters that have been shown to cause endocrine-disrupting effects and that penetrate the skin are still widely used in products, while many of the UV filters that have not been shown to be endocrine disruptors are not allowed for use. This can be good to consider before going on vacation or when shopping for imported products.
Sunscreen products also contain a variety of other substances that can, either alone or in conjunction with other products, affect skin absorption of the substances when applied. UV filters used in sunscreen products can also be found in other cosmetic products, packages, clothing, textiles, and furniture. You can, therefore, be exposed to UV filters even when you have not used sunscreen.