Up-to-Date Overview of the Use of Natural Ingredients in Sunscreens

1. Introduction

Naturally occurring ultraviolet radiation (UVR) is divided into three regions, classified according to their wavelength, called UVA (315-400 nm), UVB (280-315 nm), and UVC (100-280 nm) [1]. The ozone layer acts as a filter for UVR, absorbing all UVC and 90% of UVB when sunlight passes through the atmosphere. However, some UVB and UVA radiation is not filtered by the atmosphere, reaching the Earth and the sea, and causing a harmful effect on both terrestrial and aquatic organisms [2]. Excessive solar exposure is visually characterized by the swelling and redness of the affected area, well-known as sunburn or solar erythema. Nonetheless, the deleterious effects of excessive exposure are beyond what is seen, since UVR interferes in biological and metabolic processes, triggering a cascade of reactions that cause skin photodamage, photoaging, and photocarcinogenesis [3]. These skin damage events are provoked by UVR, which alters DNA, cellular antioxidant balance, signal transduction pathways, immune system, and the extracellular matrix [4]. UVB radiation directly affects the DNA by inducing apoptosis or errors in the DNA replication, activating inflammatory processes, photo-immunosuppression, melanogenesis, and skin cancer [5,6,7]. On the other hand, DNA damage, due to overproduction of reactive oxygen species (ROS), cross-linking of collagen and elastin fibers, premature skin aging, dryness, wrinkles, hyperpigmentation, and skin sensitization are some of the consequences of excessive sunlight exposure associated with UVA radiation [5,8,9]. DNA damage is the main event that occurs at a cellular level, as a result of UVR exposure. Depending on the wavelength and energy profile of the radiation, the kind of lesions produced might be attributed to direct or oxidative DNA damage [10]. Some of these lesions can evade the endogenous DNA repair mechanisms, thus persisting and even accumulating with chronic exposure, contributing to skin photodamage [10].

Ancient evidence from paintings suggests that clothing covering the body, veils and large brim hats were used by ancient Greeks to protect themselves from solar exposure, and that umbrellas existed in ancient Egypt, Mesopotamia, China and India [11]. More recently, at the turn of the century, various plant extracts were used in folk medicine as sunscreens. One of the most effective was a chestnut extract from which aesculin was derived (1911). Later, several chemicals were introduced as UV filters, such as 2-naphthol-6,8-disulfonic acid salts (which were quite effective in both the UVB and UVA region) (1922), tannic acid (1925), benzyl salicylate (1931), para-aminobenzoic acid derivatives and 2-phenylimidazole derivatives (1942), anthranilic acid (1950), various cinnamates (1954), chloroquine (1962), benzophenones (1965) and many more since then [11]. A list containing the approved chemicals and inorganic filters that can be used in sunscreen formulations was published by the US Food and Drug Administration and the European Community [11].

Currently, general photoprotection measures include, among others, wearing protective clothing, sunglasses and a hat, seeking shade and avoiding sun exposure during peak sunlight hours, and the crucial relevance of using sunscreen [1]. Although no sunscreen is effective in reducing total UVR exposure, they are of paramount importance to minimize solar erythema, cutaneous immunosuppression, carcinogenesis, and skin aging [12]. The choice of the right sunscreen can be challenging and confusing for consumers due to the awareness of the product’s origin (either natural or synthetic) and whether they are eco-friendly and eco-sustainable. Over recent years, evidence suggesting that synthetic UVR filters may cause damage to the marine environment has emerged, eventually leading to the adoption of restrictive measures by some countries, namely to ban the sale and distribution of sunscreens containing those ingredients in certain locations (Hawaii, Key West, U.S. Virgin Islands, Palau, parts of Mexico, and the Caribbean islands) [2]. While oxybenzone has been shown to confer ecotoxicities that lead to coral reef bleaching [13,14], other UV chemical filters have been found in diverse marine organisms [14,15]. Hence, the research and development of an eco-friendly alternative is essential and might eventually lead to the reduction of the consumer’s concerns, increasing the use of sunscreens.

A recent demand for alternatives of natural origin by industries towards new consumer-oriented cosmetic formulations has led to a deeper investigation of natural sources for sunscreen application [16]. Studies reported that the addition of natural ingredients to sunscreens can increase their photoprotective properties through their antioxidant effects and the regulation of UV-induced skin inflammation, barrier impairment, and aging [16,17]. Since oxidative stress is induced by UVA through ROS, skin exposure to this radiation leads to oxidative DNA lesions [17]. The use of topical and systemic antioxidants has been explored as a means to deal with UVR-induced oxidative stress and UVA, in particular, reducing the damage caused by ROS, impeding or lessening tissue damage, and promoting repair after UVR exposure [17]. Some of these ingredients are used as extracts or come from plant extracts (tea extracts, lutein, flavonoids, fern extract, pycnogenol, and lycopene) and have been reported to protect skin against various UVR-induced damage endpoints [17].

Apart from the natural sources from terrestrial organisms, marine biodiversity represents an underexploited source of a wide range of naturally occurring UVR screening compounds, which can be used for cosmeceutical applications as eco-friendly and safer alternatives to synthetic UV filters [2,16,18,19]. Examples reporting algae-containing photoprotective substances (mycosporine-like amino acids (MAAs), scytonemin, sulfated polysaccharides, carotenoids, and polyphenols) [18,19,20,21,22,23] are undoubtedly the most common; however, photoprotective properties have also been described for other marine organisms like microorganisms [24], artemia [25,26,27,28,29], and plankton [30,31,32,33,34,35,36,37,38].

The aim of this work is, therefore, to analyze the use of natural ingredients in sunscreens marketed in 2021 in Europe (represented in this work by the Portuguese pharmacy market), corresponding exclusively to multinational brands.