Sunny Solutions: Unmasking the UV Filters

Sunlight has several beneficial effects in human, such as the production of vitamin D, and induction of β-endorphin expression, which improve well-being. However, excessive sunlight exposure is associated with photo-induced skin damage, namely solar sunburn, hyperpigmentation, photoaging, and the promotion of skin cancers, such as basal cell carcinoma and malignant melanoma. The factor responsible for all the above-mentioned effects (both good and bad) of sun exposure is ultraviolet (UV) radiation.

Sunscreens are the guardians of our skin form harmful effects of UV radiation. But have you ever wondered what is hidden in these skin saviours we call a sunscreens? Get ready for an ingredient safari during which we will explore the world of UV filters and discover what should we pay attention to when choosing an appropriate sunscreen.

The ABCs of UV radiation

UV radiation is one part of the electromagnetic spectrum of radiation that reaches Earth from the sun. It covers the wavelength range of 100–400 nm, which is a higher frequency and lower wavelength than visible light. UV radiation comes naturally from the Sun, but it can also be created by artificial sources used in industry, commerce and recreation.

The UV region is divided into three bands

• UVA (315-400 nm),
• UVB (280-315 nm),
• UVC (100-280 nm (WHO 2024).

As sunlight passes through the atmosphere, all UVC and approximately 90% of UVB radiation is absorbed by ozone, water vapour, oxygen and carbon dioxide. UVA radiation is less affected by the atmosphere. Therefore, the UV radiation reaching the Earth’s surface is largely composed of UVA with a small UVB component.

What is the difference between UVA and UVB radiation and their effects on our skin ? In this article we will demonstrate how straightforward it is to distinguish and remember this difference.

UVA radiation “ages” the skin. Due to its longer wavelengths, UVA radiation reaches into the dermis, inducing indirect photosensitizing reactions and damaging DNA via reactive oxygen species (ROS). It is responsible for skin carcinogenesis, immunosuppression, hyperpigmentation, and skin aging.

UVB radiation “burns” the skin. It is predominantly absorbed by the epidermis and has higher photon energy, and hence can induce potentially mutagenic DNA photoproducts and contribute to the formation of erythema, skin pigmentation, photoimmunosuppression, and skin cancers.

Exposure to solar UV radiation is the most important preventable risk factor leading to skin cancer. About 80–90% of skin cancers are linked to UV exposure (Sander 2020). In order to reduce the damage UV radiation inflicts on the skin, thorough and consequent photoprotection habits are essential, especially in children, as they are most vulnerable. There are multiple effective photoprotection strategies, including avoiding solar exposure or wearing protective clothing. In the US and Europe, topical application of sunscreen remains the most common method of sun protection and skin cancer prevention (Saewan 2015).

UV Filters: The Guardians of Our Skin

Sunscreens are a complex mixture of active and inactive ingredients, and can be found in various forms (e.g. lotions, sprays, roll-ons). The effectiveness of a sunscreens depends upon its UV filters and inactive ingredients.

Let us concentrate on the essential components of sunscreens – ultraviolet (UV) filters. UV filters are the key ingredients in sunscreens that provide protection against UV radiation. All sunscreens contain UV filters and their role is to reduce the amount of UV radiation that lands on our skin.

There are two types of UV filters in sunscreen products:

• organic (chemical),
• inorganic (physical, mineral).

Which one will provide the best sun protection? Let’s take a closer look at the mechanisms of their action.

Organic UV filters absorb highly energetic UV rays and turn them into non-damaging wavelengths of light or heat, which are then released from the skin. Most organic UV filters are organic compounds containing one or more aromatic rings. In the ground state, their conjugated (delocalized) electrons are at the lowest possible energy levels. After absorbing a distinct amount of energy, electrons may jump into an excited state, leading to the transitory polarization of the molecule. The excited molecule then spontaneously returns to the ground state by emitting heat or long-wavelength radiation (Bens 2014). The absorption maximum is molecule-specific and is usually within the spectrum of either UVA or UVB.

Commercially available sunscreen products contain a combination of various organic UV filters to provide broad-spectrum UV protection. Organic UV filters must also meet requirements beyond effective light-absorbing capacity. The excited state is highly energetic, which may cause phototoxic effects due to chemical reactions with adjacent tissue. Therefore, the excited state should be transient, and its chemical structure optimized for sufficient tissue tolerance. Many common organic UV filters that meet the abovementioned criteria are derived from aminobenzoic acids, ethyl cinnamates, salicylic acids and benzophenones.

TIP FOR THE READER: Organic UV filters should be applied at least 15 min before immersion in water to allow sufficient absorption into the skin.

Physical UV filters have a different mechanism for UV protection, namely reflecting and scattering UVA and UVB radiation. They are usually metallic compounds, such as titanium dioxide (TiO₂) and zinc oxide (ZnO) (Bennett 2022). In contrast to organic UV filters, the molecules comprising physical UV filters are inert, hence not posing the risk of photoallergic or phototoxic side effects and being less irritative for the eyes. The formula of sunscreen containing physical UV filters is often thicker and the large particle size and high refractive indices of both TiO₂ and ZnO leave a white, often undesirable residue on the skin. However, engineering these compounds into nanoparticles leads to less scattering of visible light and improvement in cosmetic appearance, resulting in improved consumer tolerability.

TIP FOR THE READER: The full protection of mineral filters is available immediately after application. However, because the molecules do not penetrate the skin, water resistance is often inferior compared to organic UV filters.

There is a plethora of UV filters available on the market. The list of UV filters approved for use in cosmetics in the European Union (EU) is stated in Annex VI to the Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November on cosmetic products, which also includes their maximum allowed concentrations. An individual UV filter, on its own, does not provide protection against the entire UV wavelength range. Each UV filter offers the most protection within a certain UV wavelength range, based on which it is classified as a UVA (further divided into UVA1 and UVA2) and/or UVB filter. Therefore, sunscreen products contain typically combination of UV filters (so-called mixed sunscreens) to provide broad-spectrum UV protection.

TIP FOR THE READER: Only mineral UV filters can provide broad-spectrum coverage on their own, and of them, only ZnO provides both UVB, and UVA1 and UVA2 coverage.

Since we learned something about the mechanism of action of UV filters, it’s time to find out which sunscreen product will be the best (the safest) for your skin.

Hunting for the best sunscreen product

In our search for the optimal sunscreen product, we focus especially on parameters such as:

• effectiveness,
• safety,
• environmental friendliness.

First of all, we have to remember that the sunscreen product should have broad-spectrum UV protection. How to identify which filters are in my sunscreen? The ingredient lists are presented on the sunscreen labels. Basing on them it is possible to identify the type of UV filters and assess whether the product provides UVA, UVB, or UVA+UVB coverage. The summary of most popular UV filters with their range of UV protection is presented in the Table below.

Secondly, we should pay attention to the Sun Protection Factor (SPF) which was developed in 1974 to standardize the measurement of sunscreen efficacy. SPF is defined as “the ratio of the smallest dose of UVB radiation required to produce minimal erythema on sunscreen-protected skin compared to the necessary dose of UVB to produce the same amount of erythema on non-protected skin”. In fact, the minimum erythema dose (MED) is individual, however for the purpose of researches, the average of the test subjects’ results is calculated. SPF is measured using the following formula:

SPF = MED of protected skin / MED of unprotected skin

In practice, this means that the applied product with SPF 50 will protect the skin until it is exposed to 50 times more UVB radiation than required to burn unprotected skin.

In the EU, producers of cosmetics with UV protection must provide the UVB protection index, i.e. the SPF value. Next to the SPF value, the label contains information relating to the level of protection:

• low (SPF 6, 10),
• medium (SPF 15, 20, 25),
• high (SPF 30, 50),
• very high (SPF 50+).

The sunburn protection of a sunscreen increases with a higher SPF. The American Academy of Dermatology advises using water-resistant sunscreen with broad-spectrum protection, including UVA and UVB coverage, and at least SPF 30 for optimal safety. It is recommended to apply 2 mg of sunscreen per square centimeter of skin. This amount can be achieved either by the teaspoon rule, using one teaspoon to cover the face, or by reapplication within an hour.

UV filters – ingredients that may cause allergic reactions

There have been multiple reports on allergic contact dermatitis (ACD) and photoallergic contact dermatitis (PACD) to UV filters. Some organic UV filters, e.g., oxybenzone, avobenzone, octocrylene or ethylhexyl dimethyl PABA, under UV radiation, generate photodegradation products and ROS, causing phototoxicity and/or photoallergic processes in the skin (Nash 2014, Kawakami 2015). There are no reports of ACD or PACD to the inorganic UV filters. Oxybenzone is the most frequently reported contact and photocontact allergen compared with all other UV filters. Moreover, various organic UV filters are absorbed systemically and may therefore affect endocrine processes, hence being classified as endocrine active chemicals (EACs). Some research suggests that homosalate may act as an endocrine disruptor and the Scientific Committee on Consumer Safety (SCCS) states homosalate is safe for consumers when used in the final product at concentrations up to 7.34% (SCCS 2021). The SCCS has raised also concerns about the endocrine-disrupting properties of 4-methylbenzylidene camphor (4-MBC), including both the thyroid and oestrogen systems (SCCS 2022). Similar recommendations regarding concentration in final product i.e. up to 6% in face cream, hand cream, and lipsticks were issued by the SCCS for oxybenzone which may disrupt the endocrine system. However, it should be noted that these limitations are based primarily on data from animal studies. Regarding human data, Janjua et al. (2007) concluded that the UV filters used in their study – 4-MBC and oxybenzone – had not been capable of disturbing the homeostasis of human thyroid hormones, in contrast to the data derived from the animal models. Four further studies (case–control, cross-sectional, prospective cohort and retrospective cohort) by Aker et al.(2016), Przybyla et al.(2018) and Kim et al. (2017) found either no significant or contradictory data on the association between oxybenzone and thyroid hormone levels in humans. Hence, the data on the endocrine effects of organic UV filters in humans are currently inconclusive.

Summarizing, if you have sensitive skin, best of you avoid the following potentially irritating UV filters or chose a product with low concentration of:

• oxybenzone,
• avobenzone,
• octocrylene,
• homosalate.

We must not forget about the remaining ingredients which can also be responsible for skin irritation. The list of prohibited and restricted substances for use in cosmetics in the EU is stated in Annex II and III the Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November on cosmetic products, respectively. Sunscreen products contain various ingredients, such as fragrances, emollients, preservatives, emulsifiers, and thickeners. The most common inactive ingredients in sunscreens that have been implicated in allergic contact dermatitis include preservatives, fragrance, lanolin, and antimicrobial agents.

Safety of UV filters and their impact on the environment

UV filters are present in several environmental compartments, including surface water, groundwater, wastewater, sediments and biota (Carstensen 2022), and can expose the organisms inhabiting them to potentially disruptive or damaging effects. Due to their extensive use in cosmetics, plastics, paints, textiles and many other industrial products, organic UV filters occur ubiquitously in fresh and marine aquatic systems (Ozáez 2016). They enter aquatic environments through wash-off during showering, through the laundering of garments via wastewater treatment plants (indirect input) and through swimming and bathing in bodies of water (direct input) (Balmer 2005). It is estimated that more than a quarter of applied sunscreen washes off during activities in the water.

Organic UV filters have been detected in numerous water systems, including marine sediments, all around the globe inter alia the Okinawa islands (Japan), Chesapeake Bay (the largest estuary in the United States), Lac Bay (Southern Caribbean), the Canary Islands (Spain), the Baltic Sea (Germany), the Mediterranean Sea (Spain), the Yellow Sea (China), Oslofjord (Norway) and the Adriatic Sea (Italy). Moreover, organic UV filters have been demonstrated to induce the lytic viral cycle in symbiotic zooxanthellae with latent infections, resulting in complete and rapid bleaching of hard corals in the Atlantic, Indian and Pacific Oceans and the Red Sea, even at low concentrations. To counter this, Hawaii, the U.S. Virgin Islands, and Palau took precautionary measures in this regard and withdrew the use of preparations containing oxybenzone and ethylhexyl methoxycinnamate (octinoxate).

No data on the occurrence and distribution of mineral UV filters that might pose a threat to coastal ecosystems have been found.

Long-term sun exposure is associated with photo-induced skin damage, namely solar sunburn, hyperpigmentation, photoaging, and skin cancers. Topical application of sunscreen remains the most common method of sun protection and skin cancer prevention. Any sunscreen is better than wearing no sunscreen at all. And remember – sunscreen products should be not only girl’s (but boys too) best friend. Now that you know the most essential features of a good sunscreen product, it is on you to make smart choices.

Literature:

1. Aker A.M., Watkins D.J., Johns L.E., Ferguson K.K., Soldin O.P., Anzalota Del Toro L.V., Alshawabkeh A.N., Cordero J.F., Meeker J.D. Phenols and Parabens in Relation to Reproductive and Thyroid Hormones in Pregnant Women. Environ. Res. 2016;151:30–37.
2. Balmer M.E., Buser H.-R., Müller M.D., Poiger T. Occurrence of Some Organic UV Filters in Wastewater, in Surface Waters, and in Fish from Swiss Lakes. Sci. Technol. 2005;39:953–962.
3. Bennett S.L., Khachemoune A. Dispelling Myths about Sunscreen. J. Dermatol. Treat. 2022;33:666–670.
4. Bens G. Sunlight, Vitamin D and Skin Cancer. Springer; New York, NY, USA: 2014. Sunscreens; pp. 429–463.
5. Carstensen L., Beil S., Börnick H., Stolte S. Structure-Related Endocrine-Disrupting Potential of Environmental Transformation Products of Benzophenone-Type UV Filters: A Review. Hazard. Mater. 2022;430:128495.
6. Kawakami, C.M.; Gaspar, L.R. Mangiferin and naringenin affect the photostability and phototoxicity of sunscreens containing avobenzone. J. Photochem. Photobiol. B Biology 2015, 151, 239–247.
7. Kim S., Kim S., Won S., Choi K. Considering Common Sources of Exposure in Association Studies-Urinary Benzophenone-3 and DEHP Metabolites Are Associated with Altered Thyroid Hormone Balance in the NHANES 2007–2008. Environ. Int. 2017;107:25–32.
8. Nash, J.F.; Tanner, P.R. Relevance of UV filter/sunscreen product photostability. Photodermatol. Photoimmunol. Photomed. 2014, 30, 88–95.
9. Ozáez I., Aquilino M., Morcillo G., Martínez-Guitarte J.-L. UV Filters Induce Transcriptional Changes of Different Hormonal Receptors in Chironomus Riparius Embryos and Larvae. Pollut. 2016;214:239–247.
10. Przybyla J., Geldhof G.J., Smit E., Kile M.L. A Cross Sectional Study of Urinary Phthalates, Phenols and Perchlorate on Thyroid Hormones in US Adults Using Structural Equation Models (NHANES 2007–2008) Res. 2018;163:26–35.
11. Saewan N., Jimtaisong A. Natural Products as Photoprotection. Cosmet. Dermatol. 2015;14:47–63.
12. Sander M., Sander M., Burbidge T., Beecker J. The Efficacy and Safety of Sunscreen Use for the Prevention of Skin Cancer. Med. Assoc. J. 2020;192:E1802–E1808.