Photostability is a critical factor when evaluating the effectiveness of sunscreen in protecting the skin barrier function.33 Many molecules lose their UV filtering ability due to photochemical reactions that occur during sun exposure; in addition to leaving the skin unprotected against damage caused by UV radiation, they generate by-products and intermediate compounds of these reactions, which are reactive oxygen species (ROS), or other cytotoxic molecules that can sensitize and further damage the skin. Photoinstability is common even in commercially available sunscreens and therefore, the need for standardized and robust photostability measurements is highlighted.32–34

For many years, butyl methoxydibenzoylmethane (BMDM) was the only organic filter to provide protection against UVA1 radiation allowed in Europe and the US. However, this active substance undergoes significant degradation when exposed to light and is often combined with UVB filters (Octocrylene, OCR; Benzophenone-3, BP-3) and inorganic filters to become photostable and extend the coverage spectrum.3,28,34

Due to the high relevance of photostability for the sunscreen to be effective, several strategies28 have been used: combination of UV filters that stabilize each other against photodegradation31; micro and nanoencapsulation of UV filters2,32; addition of antioxidants, such as vitamin E3,17 or caffeine35; and development of structurally photostable molecules, such as the cis/trans isomerization phenomenon that occurs in terephthalylidene dicamphor sulfonic acid (TDSA).18

Few inputs are available that cover photostability and protection efficacy criteria at wavelengths greater than 340 nm, where the skin permeation of UV radiation is maximized, with TDSA standing out, which has a well-balanced, photostable molecular structure, with a peak of absorption at 345 nm, at the limit between UVA2 and UVA1 wavelengths.18 As it is hydrophilic, it also has synergistic properties when combined with lipophilic organic filters. When it is combined with BMDM, there is an increase in UVA protection. In vivo studies have shown protection against photoaging and the development of photodermatoses.18,28 Pyrimidine dimer formation and p53 accumulation were significantly reduced by TDSA formulations, leading to greater efficacy against a large number of biological lesions induced by sun exposure.18

Another more recently introduced organic filter is drometrizole trisiloxane (DTS), not yet approved by the FDA. DTS absorbs both UVB and UVA2 radiation and is highly capable of UVA protection when combined with TDSA.21,28

A comparative study of fifteen SPF 20 sunscreens marketed in Europe evaluated the photostability of each product through the area under the absorption curve (AUC) for the total UV range and for each radiation range separately (UVB, UVA, UVA2, UVA1); and the residual efficacy of SPF and UVAPF, both in vitro, measured by changes in the absorbance profiles of sunscreens before and after exposure to sunlight.34 All fifteen sunscreens were photostable in the UVB region; seven products, all containing a combination of ethylhexyl methoxycinnamate (EHMC) and BMDM with other UV filters, exhibited photoinstability in the full UV spectrum (290–400 nm), justifying the importance of stabilization strategies for BMDM; and eight products lacked stability in the UVA1 range, confirming the difficulty of organic filters in not photodegrading in this wavelength range. Only five sunscreens showed excellent or very good UVA1 photostability (AUC-UVA1 index above 0.93), being mentioned in the publication as S8 (EHMC, ethylhexyl triazone (EHT) e methylene bis-benzotriazolyl tetramethyl butylphenol (MBBT); AUC-UVA1 of 1.00), S10 (TiO2 and ZnO; AUC-UVA1 index of 0.98), S4 (EHMC and bis-ethylhexyloxyphenol methoxyphenyl triazine (BEMT); AUC-UVA1 index of 0.97), S12 (EHMC, MBBT and TiO2; AUC-UVA1 index of 0.96) and S15 (OCR, EHT, BMDM, TDSA, DTS and TiO2; AUC-UVA1 index of 0.93).34 This result highlights the importance of photostability assessment to ensure the effectiveness of sunscreen32,34; however, this is not usual information on the product packaging, nor is it a regulatory requirement in Brazil or in other countries.3,7

In addition to possibly aiding in the photostabilization of UV filters, topical antioxidants reduce the production of ROS, cytokines, and type 1 matrix metalloproteinase expression following UV and visible light irradiation.9 Thus, they are not considered sunscreens, but they are interesting additives to stabilize and attenuate the oxidative effect of UV radiation and even pollution, protecting the skin from oxidative damage and helping to reduce the risk of non-melanoma skin cancer.1,9,36

Antioxidants at the concentrations normally used in sunscreens did not influence the SPF value, nor did they delay the erythematous response, according to an in vivo study that compared the protective efficacy against UVB radiation (ISO 24444:2010) of an SPF 30 sunscreen (control) with SPF 30 sunscreen formulations containing several antioxidant/active anti-inflammatory substances, such as vitamin E, glycyrrhetinic acid, panthenol or Glycyrrhiza inflata extract, although the effects of antioxidants on skin alterations resulting from sub-erythematous exposure to UV radiation have not been evaluated, nor the responses to other radiations (UVA, VL and IR).36 While sunscreens are designed to act on the skin surface as protective shields, antioxidants are designed to work both on the surface and in the layers of viable skin cells. The use of a topical antioxidant product as an adjuvant is expected to be a logical complement to sunscreens, particularly as they do not need to physically remain on the skin surface during sun exposure, as required for sunscreens.1,2,9

The inclusion of antioxidants and DNA repair enzymes in topical sunscreens has extended the approach for an active photoprotection, capable of enhancing the protective power of traditional sunscreens.9,28 Photolyases are enzymes that occur naturally in bacteria, plants, and animals that experience high exposure to UV rays.9 In the presence of flavonoids that absorb UV radiation and transfer excited electrons to CPDs, photolyase acts to repair DNA.9 Patients being treated with photodynamic therapy for actinic keratoses (AK) who used sunscreen containing topical photolyase had longer remission times.9

Although endogenous DNA repair mechanisms are relatively effective, some damage persists and can accumulate with chronic exposure. Increased DNA repair effectiveness and genomic stability of skin cells have been observed with nicotinamide, also called niacinamide.37 Nicotinamide is a precursor of nicotinamide adenine dinucleotide (NAD+), an essential coenzyme to increase the production of adenosine triphosphate (ATP) that prevents the cell energy crisis induced by UV radiation. This action not only enhances DNA repair but also reduces UVA1- and UVB-induced immunosuppression and photocarcinogenesis,37,38 and p53 gene expression has been shown to be up-regulated by nicotinamide in the presence of UV.38 One study showed that nicotinamide reduced CPDs and 8-oxo-2-deoxyguanosine (8oxoG) in an in vitro model with human keratinocytes and in an ex vivo human skin model, in two possible pathways for repair of UV-induced lesions.37 A randomized, double-blind, placebo-controlled pilot study was carried out in elderly patients to assess the effect of topical 1% nicotinamide on AK compared to vehicle, both applied twice a day. The patients treated with nicotinamide had a significant 22% reduction in AKs at 3 months, compared to a non-significant reduction with a vehicle, showing a faster rate of resolution of AK with nicotinamide than with vehicle in a group of individuals with skin that was very badly damaged by the sun.39 This and other studies support the potential of nicotinamide as an active substance that may be effective in preventing and treating actinic keratosis.37–39

The development of an appropriate vehicle for sunscreen is of utmost importance, as viscosity and emulsifying agents influence the stability, sensory properties and surface tension of sunscreens, modulating the distribution of UV filters and the formation of a uniform film of the product on the skin.40 Currently, sunscreens are available in the most varied forms, adapted to the climate and habits of each country, such as fluid, cream, gel and aerogel, lotion, mousse, spray, serums, dry-touch texture, among others, and the type of vehicle and application conditions affect film thickness, which ultimately influences the effectiveness of the SPF.40 Additionally, although protection effectiveness is a requirement, clinical practice demonstrates that textures with a more pleasant touch and spreadability have greater cosmetic acceptability and increase patient compliance with the use of sunscreens.2,41

Recent publications reinforce the importance of uniform application in adequate quantity and periodic reapplication (every 2 hours).2,4,30 No uncovered skin should be left without sunscreen protection when applying the product, and all exposed areas should be included.30 The ideal film on the skin must have continuity, constant thickness, and a fine and homogeneous distribution of UV filters, which is made difficult by the fact that the skin is not smooth and flat.30 The thicker the layer of sunscreen applied to the skin, the more regular film tends to be formed and the higher the SPF and UVAPF.3,40 Another important aspect affecting sunscreen effectiveness is the vehicle capacity to form a coherent and consistent enough film to prevent migration of the formula over the skin.3,28 Technologies based on semi-crystalline polymers, for example, stabilize the filters in the formulation without the aid of surfactants, resulting in a film with constant thickness, uniform coverage, and an even distribution of filters on the skin. Studies conducted with this technology (INCI C12-22 alkyl acrylate/hydroxyethylacrylate copolymer, L’Oréal, Clichy, France – NetLock Technology) demonstrated its ability to increase the sun protection factor and promote film resistance after performing daily activities. In this study, the UV absorption of two sunscreens was evaluated through photos taken under exposure to UV light (Fig. 5). After activities such as office work, treadmill running, going to lunch, and showering, the UV absorption of the classic emulsion was noticeably reduced, while the emulsion with the technology based on semi-crystalline polymers remained unchanged.31

Figure 5.

Illustration showing UV radiation absorption capacity of two sunscreens evaluated by photographs taken under UV light. (Source: Moyal et al.31).

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Some vehicles, mainly because of the amount in which they tend to be applied, require special advice from dermatologists.4,22 Sunscreens in cosmetic powders should not replace traditional topical sunscreens due to their lower protective efficacy, but they are great options to reinforce UV protection; improve the visual appearance, reduce skin shine, produce a uniform skin color and texture; or on reapplication throughout the day for maintining the SPF.2,24 Spray sunscreens have shown to be as effective as lotions when applied correctly,4 with increasing user acceptance, albeit with questionable practical results due to the difficulty to adequately cover all exposed parts, distribute the product evenly and use the recommended amount.2 In the US, the preference has shifted from lotions to sunscreens in clear sprays, mainly because of the quick and easy application of these products.42 The lack of specific guidelines for the use of spray sunscreens and concerns about safety and adequate application by the patient may explain the hesitation of some American dermatologists to recommend sprays, as seen in a recent survey, where 99% of 540 dermatologists recommended sunscreens to their patients, but only 69% recommended the spray formulations.42 At the time of reapplication, spray formulations can be helpful in enhancing the effect of another sunscreen.2

In Brazil, due to the frequent demand for sunscreens that reduce skin oiliness, there is a large number of oil-free preparations, or those with components that adsorb lipids from the skin, such as different silica structures, comprising a product line with controlled skin oiliness, even suitable for the treatment of adolescents with acne.

Recommendations on the prescription and use of a higher SPF have been considered a strategy, from the scientific point of view, to attain more effective sun protection and compensate for the inappropriate application of sunscreens.4,23 The evolution in sunscreen development regarding cosmetics4,28 and the current offer of textures adapted to different skin types made it possible to use a higher SPF, which reinforces this recommendation. Special attention, including a targeted approach, should be given when choosing the vehicle for specific populations, such as sportspeople who practice outdoor physical activities and outdoor workers.2,23

UV filters are intended to act on the surface of the skin, protecting it from the harmful effects of sun exposure, and systemic absorption could undermine its assessment of efficacy and safety.43 However, the stratum corneum seems to be an effective barrier against penetration of UV filters, and the risk of significant systemic absorption should not be a concern. The most recent UV filters, most not yet available in the US, tend to have a high molecular weight and, regardless of the formulation, do not penetrate the skin, increasing the level of safety in the formulations.23,29

Currently, there is a growing public concern about the harmful effects of chemicals in sunscreens.3,23 The most common adverse effect of using sunscreens is the risk of photoallergy, particularly with benzophenone class organic UV filters, which are considered allergenic UV filters.28,44,45

The literature on BP-3, the main representative of the benzophenone class, indicates that it is absorbed by the skin to a greater extent than previously understood and can lead to significant systemic availability, in addition to data showing the presence of BP-3 in human breast milk, amniotic fluid, urine, and blood plasma and some studies that have shown estrogenic and anti-androgenic effects; however, the clinical relevance of these findings remains uncertain. 4,9,23,45–47

In Europe, BP-3 has been largely replaced by other broad-spectrum UV filters, but this replacement cannot easily be done in the US because many of these filters have not yet been approved by the FDA.9 There are also rare occurrences, regarding the widespread use in sunscreens, of contact dermatitis and photoallergy, particularly with cinnamate derivatives such as EHMC and OCR.23

Based on the current safety data available, OCR used as a UV filter in cosmetic products at a concentration of 10% can be considered safe, as there has been no evidence of potential endocrinological imbalance, as experimental studies have shown no adverse effects on reproduction and development parameters; and thyroid effects reported in repeated dose toxicity studies performed in an animal model at very high doses are species-specific and not relevant considering the doses of octocrylene used in humans.48 Nevertheless, the concern of formulators in achieving the highest UVA protection with the lowest possible total amount of organic filters to avoid adverse reactions, such as skin irritations and allergies; potential environmental impact; and ensure acceptable texture for better application and use of sunscreens is still valid.4,9,41

Sunscreen developers must select not only safe active substances but also preservatives, surfactants, emollients, emulsifiers, and fragrances to compose an excipient or vehicle with the least possible potential to sensitize the skin. In this sense, a survey of allergens in 910 dermatological products available in the Brazilian market indicated fragrance (62.2%) as the most frequent one, representing 73.3% in the category of analyzed sunscreens (n​​ = 145).49

On the other hand, mineral UV filters are rarely allergenic2 and, although controversy has recently arisen regarding the systemic absorption of micronized particles from these inorganic filters, studies have not shown potential damage and reinforce the role of these active substances in protecting the skin against adverse effects induced by UV radiation, including DNA damage and skin cancer.3,23 A review article has gathered information that reinforces that titanium dioxide nanoparticles (nano-TiO2) are considered non-sensitizing and mildly or non-skin irritating.29 Furthermore, there is no evidence of carcinogenicity, mutagenicity or toxicity following dermal exposure to nano-TiO2; no cytotoxic effects were reported using a 3D human skin model; and nano-TiO2 does not penetrate the skin or reach the circulation after being applied to healthy or compromised skin, showing no health risks in concentrations of up to 25%.29 In Europe, the only restriction on the use of nano-TiO2 is in formulations that can lead to lung exposure through inhalation, i.e., spray and powder products.29

Concerns about the adverse effects of sunscreens stimulate dermatological research and investigations. Observational studies have reported a positive correlation between facial sunscreen use and risk of frontal fibrosing alopecia (FFA), but the review of the available literature did not provide sufficient evidence to establish a direct causal relationship between sunscreen and FFA.50 In a recent study conducted in Brazil with 451 women diagnosed with FFA, compared to women without FFA, no finding associated the use of sunscreen to FFA, although a possible relationship of FFA and hair straightening with formalin or even with nighttime facial moisturizer has been identified.51

To date, there is not enough data to support the fact that sunscreens cause toxicity in humans, but there are signs that indicate that further studies are necessary, particularly on the effect of certain UV filters on the environment.4,9,45,52

UV filters have been reported as a threat to coral reef coastal ecosystems.9,47 In real-world conditions, when assessing all threats to corals, sunscreen does not seem to be the main issue. Global climate changes, pollution and sewage are some of the main factors contributing to coral degradation.47,53

The symbiosis relationship between corals (hosts) and single-celled algae (dinoflagellates) is currently the object of much debate due to extensive records of bleaching of the world major reefs.45,53 Bleaching corresponds to the breakdown of coral-dinoflagellate symbiosis, the subsequent loss of symbionts that provided their hosts with photosynthetic products for their energy needs and gave them the characteristic color. When coral becomes stressed, it sheds algae, becomes whitish, and starves due to the loss of its main food source.52,53

To verify the photochemical response of a coral to sunscreen ingredients, a study evaluated the chronic effects of organic (TDSA, DTS, EHT, BMDM, and OCR) and mineral (ZnO) UV filters on the photosynthetic efficiency of symbionts associated with the coral Stylophora pistillata. The results indicated that TDSA, DTS and OCR, at relevant concentrations (higher than those reported in seawater) and taken individually for 5 weeks, did not induce bleaching, nor did they cause a significant decrease in the photosynthetic efficiency of the corals. EHT and BMDM showed a decrease in the concentration, suggesting that these products were adsorbed on the fish tank glass or were degraded by photoexposure.53

On the other hand, ZnO, which is classified as “hazardous to the aquatic environment” according to the Globally Harmonized System of Classification and Labeling of Chemicals (GHS) criteria, appeared as the most toxic compound. Other studies have shown that the marine impact of dissolved and nanoparticulate forms of ZnO should be carefully evaluated on endangered coral reefs.47,53

TiO2 caused minimal changes in symbiotic relationships and did not cause bleaching in corals of the Acropora genus, resulting in greater eco-compatibility.47 BP-3, found at detectable levels from 0.8 mcg/L to 1.4 mg/L in seawater, has been shown to cause coral bleaching in vitro,47,52 while another study evaluated the exposure of fish belonging to the species Poecilia reticulata to this filter, and the results showed that BP-3, at an environmentally relevant concentration, was genotoxic to freshwater fish, corroborating its environmental risk.54

The benefits of solar radiation are mainly related to vitamin D synthesis and the prevention of diseases such as osteoporosis, type 1 diabetes, cancer of the colon, breast, prostate, non-Hodgkin lymphoma, and autoimmune diseases.5 Most of the vitamin D is obtained by cutaneous synthesis through sun exposure, and the wavelength of solar energy that causes erythema considerably overlaps in the ultraviolet B region with that which synthesizes vitamin D in the skin. Theoretically, sunscreens that inhibit erythema should also inhibit vitamin D synthesis, which would be a possible adverse effect of sunscreen use, with public health implications. However, in practice, sunscreens can be used to prevent sunburns and still allow vitamin D synthesis.6,55

A quasi-experimental, randomized study carried out in the city of Rio de Janeiro (Brazil), during the winter, involving 95 healthy adults, evaluated the vitamin D synthesis induced by suberythematogenous sun exposure of a group under topical photoprotection (SPF 30), a group exposed to the sun without sunscreen and a group confined from sun exposure that received doses of 25-OH-vitamin D, before and 24 h after sun exposure (UVB = 20 mJ/cm2). Even after applying sunscreen on the face, neck and thorax, sun exposure induced a variation in vitamin D levels, comparable to that obtained without photoprotection.55 The use of sunscreen for daily and recreational photoprotection does not compromise vitamin D synthesis, even when applied under ideal conditions.56

One article presented the UV Index (UVI) measurements carried out in São Paulo (Brazil), in the period of 2005–2008, when 65% of the UVI measured 2 hours around the local midday during the summer showed very high levels (8 < UVI < 10) and extreme levels (UVI > 11), according to the WHO classification. During the winter, 40% of measurements around midday showed high or very high levels. UV measurements have also shown that in every month of the year, UV radiation levels are high enough to ensure vitamin D synthesis in the skin with minimal sun exposure for 10 minutes57; another study suggested sun exposure at noon twice a week for 5–30 minutes as sufficient for vitamin D production in white populations.2

Recently, the WHO recognized that sensible sun exposure offers health benefits, including vitamin D production.8 However, conscious exposure with the use of an efficient sunscreen is recommended.2 Regarding patients over 60 years of age, the suspension of daily use of sunscreen does not help in the synthesis of vitamin D, since it has been shown that the ability to produce vitamin D3 from the same UV exposure decreases with age,58 and can be reduced to about 50% of a child's capacity to fully synthesize vitamin D in the skin for these patients.58 Even with conscious and well-guided sun exposure, oral supplementation is indicated for patients at risk of developing hypovitaminosis D.2

The regulations for sunscreens vary between countries and, in a globalized world where patients have access to sunscreens from all over the world, and it is important that dermatologists know the main regulatory differences.

In Brazil, the National Health Surveillance Agency (Anvisa, Agência Nacional de Vigilância Sanitária) defines filters or sunscreens as cosmetic products in accordance with RDC N. 30 of June 1, 2012,7 which approves the Mercosur Technical Regulation on Sunscreens in Cosmetics and has a list of permitted UV filters containing 39 active ingredients in RDC N. 69 of March 23, 2016.27 Even though they aren’t medications, sunscreens must follow the technical requirements, labeling criteria, and efficacy assessment methods defined in the legislation to ensure its effectiveness and a high level of protection for the individual.7,27

North-American legislation classifies sunscreens as over-the-counter (OTC) drugs. In the US, only 16 UV filters have been approved by the FDA in the sunscreen monograph; an additional filter, ecamsule (TDSA), has been approved through a different process (New Drug Application) and has been used in US formulations. A new UV filter has not been approved in the US for over 10 years, leaving US manufacturers limited to offering broad-spectrum UV protection comparable to sunscreens from other parts of the world.4

In 2019, the FDA proposed new rules on sunscreens, and a series of preliminary studies were spurred to support data on the safety and efficacy of UV filters aiming to support the new classification of filtering actives into “generally recognized as safe and effective”.47 Consequently, studies on the risks of systemic absorption and detection of plasma concentrations of UV filters have been published, which do not imply toxicity or that they are harmful to health.59 In addition, the FDA has proposed increasing the maximum SPF value reported by manufacturers from 50+ to 60+ and adding a test to ensure broad and uniform protection against both UVB and UVA.60

In Europe, sunscreens are classified as cosmetics; however, quality requirements are equally high; the allowed UV filters are regulated by Annex VI of the Cosmetics Regulation (EC N. 1223/2009), and the parameters established for labeling are: SPF with a maximum value of 50+ and UVAPF with a value of at least one-third of the UVB protection.3,61

The importance of adequate protection from UVA radiation has increased and the United States and Europe have different regulatory standards for UVA protection in sunscreens. One study evaluated 20 US sunscreens (broad-spectrum; SPF 15 – 100+); nineteen of the 20 products (95%) had a critical wavelength >370 nm, an FDA requirement for registration of UVA protection. However, only eleven of the 20 (55%) products would meet the European standard for the definition of “UVA protection”, which is to have a UVAPF to SPF ratio of at least 1/3.62

The sun protection factor (SPF) is an international reference for expressing protection against UVB radiation,3 being in practice a primary measure of protection against UVB radiation and, to a lesser degree, against UVA2 radiation. Currently, sunscreens are manufactured in a wide range of very high sun protection factors, with SPF < 99 being allowed in Brazil. In parallel, the concern with the UVA protection of sunscreens has gained importance and the performance of each sunscreen in relation to UVB and UVA protection must be understood both in terms of quantity (level) and quality of protection.3,60

In Brazil, two methodologies are accepted for determining the SPF: ISO 24.444:2019 (also used in Europe, Canada, Australia and Japan) and FDA 1999, 2011 (used in the USA), both performed in vivo, and with the evaluation of the erythematous reaction, on the dorsal region, after application of 2.0 mg/cm2 of sunscreen. The SPF is the value obtained by the ratio between the minimal erythematous dose (MED) on the skin protected by a sunscreen and the MED on the unprotected skin, with MED being the minimum amount of ultraviolet radiation required to produce the first evident erythematous reaction, covering more than 50% of the exposure subsite and with clearly defined edges, observed between 16 and 24 hours after exposure to UV radiation, according to the adopted methodology,7 and, therefore, it is a measure of protection time in the laboratory, and not of sun protection intensity.

Although the assessment of SPF is carried out using standardized and established methods (ISO or FDA methodologies), it is possible to find variability in the results. This variability, inherent to the methodologies, demands a process with high-quality requirements to ensure that the result is reliable, reproducible and that guarantees protection to the consumer or patient. The criteria that impact the SPF oscillation include inter-individual variation, product application method, UV exposure, and the definition and reading of the minimal erythematous dose. Given the vulnerabilities of the standardized methodology, some companies develop quality policies, internally and externally, together with pre-qualified partner laboratories, for highly collaborative work, aiming to obtain a more authentic and reliable SPF and stimulate research and discussions on the evolution of methods to assess photoprotection with the ISO group and photoprotection experts.63

Given the complexity of measuring the SPF of a sunscreen, the formulation of individual sunscreens in compounding pharmacies practically makes it impossible to adequately determine the SPF, leading to an empirical establishment of the SPF value, without guaranteeing the photostability of the filtering system, since it is unfeasible to determine, in a robust way and with quality, the level of performance of the small quantities produced in different master formulations and vehicles.

According to the scientific community that studies the topic, the SPF is the only validated methodology to assess the effectiveness of sunscreens, as it correlates with the clinical result (erythema) of protected and unprotected skin. However, the SPF is not linear, and the same SPF does not mean that different sunscreens have the same biological effects.64 A mistake is to relate SPF to exposure time, which is only possible in the laboratory since the test conditions (amount of product, amount of radiation, etc.) are controlled. The erythematous response is directly related to the type of skin and, in real-life conditions, it depends on the amount of radiation, which can vary with the place and time of sun exposure and with the amount of product applied to the skin. SPF values could also be understood ​​as multiplicative ones, making one draw the conclusion that SPF 30 is twice as effective as SPF 15, for instance. However, an SPF 15 sunscreen absorbs 93.3% of the radiation that induces erythema; an SPF 30 sunscreen absorbs 96.7%; while an SPF 50 sunscreen absorbs about 98% of UVB radiation. It is more clinically and photobiologically relevant to measure the amount of electromagnetic radiation being transmitted to the skin, whether 6.7% for SPF 15; 3.3% for SPF 30 and 2% for SPF 50 (which means increasing the protection by more than three-fold when compared to SPF 15).2,4 In Brazil, where the radiation level is very high, the use of sunscreen with a higher SPF is important to reduce the amount of radiation that is passing through the filter and reaching the skin.9,23

Once the importance of broad protection (UVB and UVA) was understood, it was necessary to harmonize and standardize the measurement conditions aiming to transmit adequate information about UVA protection to the consumers. The studies by Moyal et al. (L'Oréal Recherche, France) demonstrated the relevance of persistent pigment darkening (PPD) as a biological outcome to assess UVA photoprotection, as well as the sensitivity and reproducibility of the in vivo methodology for the variety of UVA filters65 and, later, Moyal participated, together with COLIPA, in the development of a new in vitro method to measure UVA protection,66,67 which is correlated with the in vivo measurement. In the in vivo method, the UVA protection factor (UVAPF) is the value obtained by the ratio between the minimal pigmentary dose (MPD) on a skin protected by a sunscreen and the MPD on the same skin, when unprotected, with the MPD being the minimal dose of UVA radiation required to produce a persistent pigmentary darkening of the skin with clearly defined edges, observed between 2 and 4 hours after exposure to UVA radiation.7 In Brazil, a product can only be registered as a sunscreen (Fig. 6) if the protection against UVA radiation is at least one-third of the protection against UVB radiation (SPF) and the critical wavelength is at least 370 nm, to ensure protection against long UVA.4,7 The accepted methodologies for the determination of UVAPF are ISO 24442:2011 (in vivo) and ISO 24443:2012 (in vitro) and the results can be correlated.7 The ISO 24442:2011 methodology to establish UVA protection is similar to the ISO 24444:2010 used for the SPF, regarding the number of homogeneously selected volunteers, except that they are phototypes III to IV (Fitzpatrick), which will be irradiated to observe skin pigmentation.

Figure 6.

Sun protection factor (SPF) evaluation method.

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The critical wavelength (λc) is defined as the one for which the area under the integrated optical density curve, starting at 290 nanometers, is equal to 90% of the integrated area between 290 and 400 nm (Fig. 7).7,13

Figure 7.

Critical wavelength is a quality characteristic and allows for a statement about the scope of UV protection. A, Product with high UVB absorption and low UVA absorption. B, Product with uniform UVB and UVA absorption.

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The development of sunscreens with good UVB and UVA efficacy favors safer sun exposure; however, when there is no protection in the VL and IR spectral region, large amounts of free radicals are induced.15 The effectiveness of sunscreens can be extended to also cover the visible and infrared spectral region of solar radiation, adding pigments and antioxidants to the formulations capable of fighting the free radicals that are formed.4,15 Several validated methodologies are being presented in the literature, but there is no universally accepted standard as yet.68,69

A cross-sectional study was carried out to evaluate the in vitro transmittance to UVB, UVA and blue-violet light (400–500 nm) of 41 Brazilian commercial sunscreens, with SPF > 30, all opaque; and three more pigment-free sunscreens (control). The opaque filters had greater coverage than the controls, but only 63% (26/41) blocked >99.9% of VL, while some sunscreens that had high protection against VL let UVA radiation through, contradicting the hypothesis that all pigmented sunscreens protected from VL and UVA radiation.70

A VL assessment protocol to verify the capacity of a formula to prevent VL-induced hyperpigmentation showed that there is a correlation between the amount of iron oxide and the effectiveness of protection against visible light (VL); however, there are different studies and diverse parameters that demonstrate that it is possible to obtain good efficacy even with a reduction in the concentration of iron oxide. An iron oxide-free, titanium dioxide-only sunscreen offered 14% greater protection against VL than the unprotected skin in terms of reduced ITA value after 5 days.17,68

In Brazil, water resistance is regulated and must strictly follow the same methodology selected to determine the SPF (FDA or Colipa). In the US, the FDA prioritizes immersion in water and only then evaluates the SPF; while in the European guidelines (Colipa), water-resistance can only be claimed if more than 50% of the SPF is retained after the water immersion procedure: two 20-minute immersions for water-resistant products or after four 20-minute baths for very water-resistant products.3,7

The current performance measurements, SPF in vivo and UVA in vitro ISO 24443, conducted under standardized conditions, are partly responsible for the photostability of the final product.34 Photostability is not a regulatory requirement but can be assessed by comparing the in vitro absorbance of sunscreen before and after exposure to UV radiation.32,33

When looking at the packaging of sunscreen for dermatological use, the comparison between products tends to be impossible for the health professional and is even more misleading for the common consumer. The SPF is the information most commonly related to the effectiveness of sunscreens and the easiest to see on the sunscreen label (Fig. 8). However, its interpretation should not be based only on the numerical value itself, as it should also consider the adequate way to use the product in terms of applied quantity and regularity of reapplications. In Brazil, the SPF value can vary from 6 to 99 and, according to the pre-established category, sunscreens with an SPF between 30 and 50 are labeled as “High Protection” and products with an SPF between 50 and 99 are labeled as “Very High Protection”. These phrases refer to UVB protection only.

Figure 8.

Important characteristics for sunscreen indication.

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UVAPF can be represented on the packaging by a logo (

The ingredients are presented in all sunscreens, listed by the INCI nomenclature, in descending order of concentration in the formula, without highlighting each category of active substances or UV filters; that is, one needs to know the names and locate them in the list of ingredients.

The information on the label or packaging tends not to inform about the absorption peaks of the filtering system nor about its photostability.

A study to evaluate the effectiveness of protection against VL and IR radiation has not yet been agreed on, but by presenting studies in validated protocols, the product may exhibit the information “protects against visible light” and “protects against damage caused by infrared radiation”.

Finally, when choosing a sunscreen, it is also necessary to verify the information regarding water and/or perspiration resistance according to the consumer habits.

Cosmetic acceptability, particularly touch and visual appearance, should also be considered for better adherence to sunscreen use.30