CELINE VINCENT, SEBASTIEN MIKSA

WENEOS, Creil, France

In the field of sunscreen products, consumers are increasingly demanding products that not only offer superior sun protection but also additional resistances such as water resistance, friction (rub) resistance, sweat resistance, wet skin application effectiveness, and sand resistance. This indicates the growing awareness and need for comprehensive protection against the multifaceted challenges posed by sun exposure. Indeed, it is now well-known that while UVB rays are primarily responsible for sunburn and redness, UVA radiation, with its deeper penetration, is a critical factor in DNA damage, and the increased risk of skin cancer. Moreover, chronic exposure to UVA rays accelerates the aging process of the skin, characterized by wrinkles, leathery texture, and hyperpigmentation (age spots). This is due to UVA's ability to penetrate deep into the dermis, damaging collagen and other structural components of the skin. Finally, UVA radiation can cause pigmentation disorders such as melanogenesis (melanocytes produce more melanin can result in uneven pigmentation and dark spots, especially in individuals with darker skin tones), melasma (UVA exposure can worsened brown to gray-brown patches on the face) and other forms of hyperpigmentation by causing oxidative stress and inflammation in the skin.

In this way, sun protection formulations have evolved to address sensorially, transparent film and broader spectrum of ultraviolet radiation, acknowledging that both UVB (290 to 320 nm) and UVA (320 to 400 nm) rays pose significant risks to skin health. Within the specific UVA radiation, it includes UVA2 (320‒340 nm also represented by UVA-II) and UVA1 rays (340‒400 nm also represented by UVA-I) which represent for more than 80% of solar UV received on the earth. In recent years, studies have proven the harmful effects of the UVA1 in skin damages [1]-[7] while other articles highlighted the lack of sunscreen products with ultra-long UVA protection (370-400 nm) which could be solved by new UV protection including chemical or natural ingredients [8]-[11].

Nevertheless, current standardized in vitro methods [12], [13] allow the determination of the UVA Protection Factor (UVAPF), Critical Wavelength (CW) or the UVA/UVB ratio but no specific method was developed to measure in detail the Long UVA protection only. Moreover, even if regulations are available for the UVB and UVA protection for claiming purpose, the Long UVA labeling is not regulated to have a relative scale to compare products between them for consumer guidance. Beyond this point, it was scientifically difficult to express the real Long UVA protection of a sunscreen product without numbered criteria. As evidence, in vitro measurements of sunscreen strictly quantify its physical ability to block or absorb UV radiation but do not assess the biological effects of UV exposure on the skin, such as DNA damage, oxidative stress, or inflammation. Thus, while these methods indicate the sunscreen's potential physical protection, they do not capture the full spectrum of biological protection that may be incorrectly used for labelling Long UVA protection.

Therefore, the aim of the present study is the assessment and comparison of the Long UVA protection (and the Ultra-Long UVA protection) of different sunscreen products by means of a UV spectral absorbance curve from an in vitro procedure based on the referenced International Standard ISO 24443:2021. Results of this measurement procedure can be used for calculation of UVA Protection Factor I (UVA-PF-I), Critical Wavelength (CW) and UVA-I/UV ratio. Finally, a pass/fail criterion is proposed from the global comparison of sunscreen products to effectively claim a Long UVA protection or Ultra-Long UVA protection.

• Sunscreen product: Sixty-three products with different properties and cosmetic presentations were selected (Table 1). These are different forms including mainly emulsion (tinted or not) but also stick, biphasic and alcoholic spray, with various levels of protection ranging from SPF 15 to 50+. Furthermore, the selected products include reference standards (S2, P2, P5 and P8), classical sunscreen products without the Ultra-Long UVA label on the packaging and sunscreen products from the market with the Ultra-Long UVA label on the packaging (a total of 6 products).

• Substrate: The importance of substrate roughness on the reproducibility of in vitro SPF tests has already been demonstrated (intensity and shape of the curve). For the described tests, substrates used were molded polymethylmethacrylate (PMMA) plates (Helioplate HD6, WENEOS ex HelioScreen) and sandblasted plates (Helioplate SB6, WENEOS ex HelioScreen), for spectrophotometric measurements from 290 nm to 400 nm. These plates are in total compliance with the specifications described in the ISO 24443:2021 for in vitro UVA-PF determination for transmittance ([290 nm: >60 %T]; [300 nm: >69 %T]; [320 nm: >81 %T]) and roughness (several topographic parameters).

• Application: Before testing, products were stored at a controlled temperature for 24 hours were shaken just before their application to ensure a good homogenization. To obtain the correct application rate of 1.3 mg/cm² (HD6) or 1.2 mg/cm² (SB6), at least nine drops of the tested sunscreen product were applied by an automatic pipette (Multipette E3, EPPENDORF) with Combitips Advanced 100 µl across the entire PMMA plate.

• Automated spreading: Immediately after weighing, the sunscreen was spread using an automated device (HD-SPREADMASTER, WENEOS ex HelioScreen) which is a robotic arm performing precise and repeatable circular and linear strokes with a control of the pressure (compliant with the ISO 24443:2021). First, the product is spread on the whole area of the plate, using circular movements with a minimum of four passages from the top to the bottom of the plate. At the end of the first passage, a turn of the plate has to be done (¼ turn) to alternate passages and repeat this movement three times at least (about 30 s). Then the sample is rubbed on the plate surface using alternating horizontal and vertical strokes repeated at least three times alternate passages (about 30 s).

• Drying time: After automated spreading, the sunscreen was allowed to dry and settle at 27°C ± 2°C for at least 30 minutes (i.e. in compliance with the ISO 24443:2021 indicating a drying for 30 min to 60 min in the dark between 27°C ± 2°C and 32°C ± 2°C). This step is necessary to consider the relaxation of tested products. During this drying process, the temperature was maintained and controlled by means of a device (HD-THERMASTER, WENEOS ex HelioScreen).

• Transmittance measurement: Once the drying step was completed, it is now necessary to measure the UV transmittance spectrum through the thin irregular layer of sunscreen product spread on the substrate. The concept of transmittance corresponds to the quantity of UV transmitted through the sunscreen product evaluated for a single wavelength. This notion is linked to the absorbance which corresponds to the quantity of UV absorbed by the product.

The evaluation of sunscreen product’s absorbance was performed using a spectrophotometer from 290 to 400 nm (UV-2000S, Labsphere Inc.). Before taking measurements, the transmittance analyzer was validated and controlled according to the ISO 24443:2021 method including the linearity/additivity by calibrated reference standard PMMA calibration plates (Helioplate HD0, WENEOS ex HelioScreen) and Holmium Oxide filter for wavelength accuracy. Moreover, a PMMA plate covered with a film of white petrolatum (15 mg applied) was used to obtain blank transmittance.

Four plate replicates were used for each tested product, and nine UV transmission spectra were recorded for each plate at the different application locations (i.e. a total of 27 UV spectra). These steps were repeated before and after irradiation.

• Adjustment of UV spectrum: Following the ISO 24443:2021 principle, the first adjustment consists in fitting the in vitro SPF calculated from the measured UV spectrum (Equation 1)with the in vivo SPF value. For this purpose, the initial absorbance curve was multiplied by a correction factor C until the in vitro calculated SPF is equal to the measured in vivo SPF according to an iterative calculation process (Equation 2). The choice between the molded PMMA plates HD6 and the sandblasted PMMA plates SB6 was determined by the C factor that was closer to 1.

Equation 1:

• E(λ) = Erythema action spectrum (CIE-1987).

  I(λ) = Spectral irradiance received from the UV source (UV-SSR for SPF testing).

  A_0i(λ) = Mean monochromatic absorbance of the test product layer before UV exposure.

  dλ = Wavelength step (1 nm).

Equation 2:

where:

E(λ), I(λ), A_0i(λ) and dλ previously determined in Equation 1.

C = Coefficient of adjustment.

• UV exposure: To consider a potential photodegradation of tested products, it is necessary to perform an irradiation step. For this purpose, a solar simulator compliant with ISO 24443 specification (Model LS1000-4S-009, Solar Light Company, Inc. or Suntest CPS, AMETEK+) was used to expose samples. Moreover, prior to each UV exposure, the intensity of the UV source used for exposure was checked using either a spectroradiometer or a calibrated radiometer with sensitivity in the UVA.

The in vitro UVAPF0i is firstly calculated (Equation 3) and then used for the pre-irradiation dose D required for the UV exposure (Equation 4).

Equation 3:

where:

P(λ) = PPD action spectrum.

I(λ) = Spectral irradiance received from the UVA source (UVA 320-400nm for PPD testing)

A0i(λ), dλ and C previously determined in Equation 1 and 2.

The pre-irradiation dose D required for UV exposure has been calculated with the (Equation 4) and should be limited at a maximum of 36 J/cm².

Equation 4:

• Calculation of the Long UVA factors: Based on the adjusted absorbance spectrum, several factors are calculated to represent the potential of Long UVA protection.

a. In vitro UVAPF-I and UVAPF-I/Labelled SPF ratio: The final UVAPF-Ii is calculated according to Equation 5 including the UV absorbance value after UV exposure (Ai(λ)) for the same plate and the UVAPF-I is equal to the arithmetic mean of all valid individual UVAPF-Ii values obtained from all tests performed and expressed as a whole number. Contrary to the initial wavelength range of 320 to 400 nm, it was decided to limit the range to 340 to 400 nm to specifically consider the long UVA rays only, thereby avoiding the over-consideration of short UVA protection in the UVA-PF value.

Equation 5:

where:

P(λ), I(λ) are defined in Equation 3 but limited from 340 to 400 nm.

Ai(λ) = Mean monochromatic absorbance of the test product layer after UV exposure.

dλ and C previously determined in Equation 1 and 2.

The UVAPF-I/Labelled SPF ratio is an indicator of UVAPF-I properties of a sunscreen product, compared to SPF properties according to Equation 6. To have a good UVA absorption, the UVAPF-I has to be at least a third (1/3) of the labelled SPF. The final UVAPF-I/Labelled SPF ratio is the in vitro UVAPF-I obtained by this method on the labelled SPF.

Equation 6:

where:

UVAPF-I has been obtained by the present in vitro method.

Labelled SPF has been determined following local regulation.

b. Critical Wavelength (CW): The individual CWi (λci) is defined as the wavelength at which the area under the absorbance curve (Ai(λ)) of the product from 290 nm is equal to 90% of the integral of the absorbance curve from 290 nm to 400 nm of the same plate after the UV exposure at the D dose according to Equation 7. The final CW (λc) value is the arithmetic mean of all valid individual CWi (λci) values obtained from all tests performed.

Equation 7:

where:

Ai(λ) = Mean monochromatic absorbance of the test product layer after UV exposure.

λci = Wavelength at which the area under the absorbance curve (A(λ)) of the product from 290 nm is equal to 90% of the integral of the absorbance curve from 290 nm to 400 nm.

c. UVA-I/UV ratio value : The UVA-I/UVi ratio (individual UVA-I/UV ratio) is an indicator of Long UVA absorption properties (area per wavelength unit in the UVA-I portion of the absorbance curve) of a sunscreen product, compared to UV absorption properties (area per wavelength unit in the UV portion of the absorbance curve) of the one and same plate after the UV exposure at the D dose, rather than a ratio of transmission or protection factors according to Equation 8. The final UVA-I/UV ratio is the arithmetic mean of all valid individual UVA-I/UVi ratio values obtained from all tests performed.

Equation 8:

The aim of the present study is the analysis of the Long UVA (Ultraviolet-A) rays protection of sixty-three sunscreen products by means of a UV spectral absorbance curve from an in vitro procedure based on the referenced International Standard ISO 24443:2021 principle. To reach this goal, it was necessary to define a pass/fail criterion to highlight sunscreen products with a significant Long UVA protection based on all results of UVA Protection Factor I (UVAPF-I), Critical Wavelength (CW) and UVA-I/UV ratio.

For the first criterion, based on the COMMISSION RECOMMENDATION of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto [14] from European Union, we decided to use the same minimum UVAPF/Label SPF ratio of 1/3 but extended to the long UVA (expressed by the UVAPF-I/Label SPF ratio).

For the second criterion, we decided to represent the UVAPF-I/Label SPF versus the Critical Wavelength (Figure 1) to detect the minimum Critical Wavelength necessary to respect the 1/3 criteria (UVAPF/Label SPF ratio) as requested above. Based on the Figure 1 and by using the linear trend line, it is possible to observe that a Critical Wavelength of at least 375 nm (rounding up value) is requested to respect a UVAPF-I/Label SPF criterion of 1/3.

For the third criterion, we decided to represent all UVA-I/UV ratio values in a box plot (Figure 2) to determine a minimum protection to be afforded by a sunscreen product to claim the “Long UVA” protection. Based on the Figure 2, it is possible to observe that a UVA-I/UV ratio of at least 0.85 represents the median of all products evaluated and this criterion was, therefore, retained to only select sunscreen products with the highest UVA-I protection.

Therefore, based on these different above steps, it is noted that a product can claim “Long UVA” protection if all these following criteria are passed:

• UVAPF-I/Labelled SPF ratio ≥ 1/3
• CW ≥ 375 nm
• UVA-I/UV ratio ≥ 0.85

Finally, by using these three criteria simultaneously with all tested sunscreen products in the present study, it is possible to observe that the selected selectivity is about 75% (15 products pass the criteria and forty-eight products fail the criteria), i.e., 1 product in 4 can claim the Long UVA protection. Moreover, it is interesting to observe that only about half of the sunscreen products from the market with the Ultra-Long UVA label on the packaging pass all the three criteria.

Furthermore, we observed that a specific brand, a specific galenic (between emulsion, biphasic, stick, etc.), the labelled SPF level, and a specificity (tinted or not) are not the critical factor influencing the pass/fail criteria (with a percentage about 25% between cases).

The illustrations of the requested absorbance curves necessary to have a Pass conclusion or a Fail conclusion are represented in the Figure 3. From these curves, it is possible to observe that beyond the UVA-II impact (short UVA protection), the Pass conclusion is possible by the integration of the UVA-I protection but also with the consideration of the ultra-long UVA protection (370 - 400 nm).

Currently, the harmful effects of an overexposure to the sun are worldwide recognized and are focused on the UVB and UVA damages. As the solar spectrum is continuous electromagnetic waves, previous studies expressed the need for a protection beyond the UV rays with a Full Spectrum protection and which can be divided in different ranges such as the Blue Light, Visible Light, and Infrared protection. In addition, recent studies helped to understand the need to extend the protection in the UV range and more especially in the Long UVA and Ultra-Long UVA spectrum.

Based on the comparison of 63 sunscreen products, this study presents the development and the results of a new in vitro method with specific criteria to assess the Long UVA protection or Ultra-Long UVA protection provided by sun care products. Indeed, according to different results, the three criteria proposed to highlight products with a significant highest Long UVA protection were (i) UVAPF-I/Labelled SPF ratio ≥ 1/3, (ii) CW ≥ 375 nm and (iii) UVA-I/UV ratio ≥ 0.85.

To conclude, from these criteria, it was interesting to observe that one product in four can be positively evaluated to the UVA long protection (also called Ultra-Long UVA protection) and could be claimed accordingly.