Pigmented protection: COSSMA

The various rays of the solar spectrum can damage the skin in both the short and long term. Artificial radiation is also not good for the skin in the long run. Mineral additives in the products of the daily routine can naturally increase protection.

Spending time outdoors, amongst nature, has been found to help with many mental health problems. Moderate sun exposure offers beneficial effects such as vitamin D synthesis, better immune response, improved cardiovascular health and contributions to well-being feelings. Now the “sun, sand & sea season” arrives, together with the chances of exposing ourselves excessively to solar radiation, which does not equal the benefits of a moderate exposure throughout the year.

The solar radiation reaching the Earth’s surface involves three components UV (ultraviolet), Vis (visible), and IR (infrared) light. Without an adequate sun protection, a proportion of the radiation reaches the skin and penetrates differently depending on its wavelength.

Effects of radiation

The UVB radiation (290 – 320nm) acts mainly on the epidermis, the most superficial layer. It causes erythema (sunburn) and partially penetrates the dermis. The UVA radiation (320 – 400nm) passes through the dermis and is responsible for photo-ageing and pigment darkening. Both UVB and UVA radiations induce delayed tanning and are associated with cellular damage, through direct or indirect DNA damage, and with potential risk of skin cancer. It is therefore essential to use creams that protect as much as possible from the two components of UV radiation.

The effects of visible light (from 380 – 700nm) and IR radiation (from 700nm – 1mm) are less studied and not so clear. Due to their wavelength, they can penetrate the skin far deeper that UV light, passing through the epidermis and dermis to the subcutis layer. It has been reported that visible light can induce pigmentation, especially in skin types IV–VI1, darker and more sustained than pigmentation induced by UVA light². The severity of photo-induced pigmentary disorders, such as melasma or post inflammatory hyperpigmentation may be related to the exposure to Vis radiation that also can induce reactive oxygen species (ROS) generation.

Included in the range of visible light is Blue light, BL, (380 – 500nm) also known as High Energy Visible, HEV. We are exposed to BL coming from solar light and to BL emitted from screens. It is considered to have a beneficial effect during daylight hours because it boosts attention, reaction times, and mood. However, at night, exposure to light suppresses the secretion of melatonin, sleep suffers, and it could contribute to the development of diabetes, obesity, or heart diseases³. Long exposure to HEV has been also related to DNA damage, cell and tissue injury, eye and skin barrier damage, and photoaeging⁴.

Yellow, red, and black iron oxides (FeOOH, Fe2O3 and Fe3O4) may block HEV light, separately or combined, and with this aim can be included in cosmetic formulations. It has been reported that Vis light induced pigmentation was suppressed by using tinted yellow iron oxide and patients with melasma prevented relapses better when using tinted sunscreen rather than untinted sunscreen^5,6.

Human skin is also exposed to infrared (IR) radiation from natural and artificial sources. IR radiation may be divided, according to its wavelength into IR-A (780 – 1,400nm), IR-B (1,400 – 3,000nm) and IR-C (3,000nm – 1mm). IR-A can penetrate the epidermal and dermal layers and reach subcutaneous tissues whereas IR-B and IR-C are absorbed mostly in the epidermal layers and increase skin temperature7. Due to its penetration ability, IR radiation may damage skin collagen by ROS radicals generation and by increasing the Matrix Metalloproteinases MMP-1 and MMP-9 activity that results in cleavage of fibrillar collagen and impairs the structural integrity of the dermis. IR and heat may induce premature skin ageing like UV radiation8.

In contrast, exposure to visible and IR-A radiation from natural sunlight can be beneficial to the skin depending on the right combination of wavelength, fluence, and irradiance. In early morning and late afternoon, conditions are favourable to prepare the skin for the deleterious effects of the mid-day UV radiation⁹.

graph 1: Absorbance spectra of mineral filters. graphic: AdP Cosmetics

Mineral protectors

On the other hand, the only mineral UV filters approved in Europe and included in US FDA sunscreen monograph are zinc oxide and titanium dioxide. These filters are typically commercialised as nanoparticles to minimise the whiteness on the skin. However, the more transparent the filter is on the skin (less visible), the less effective in protecting against the UVA, visible and IR radiation.

We are then in a scenario in which the development of cosmetics that protect from the different components of the solar spectrum, and not only from UV radiation is increasingly being sought. And we are aware that increasing the number of ingredients in the formula complicates the formulation work and the development time of a cosmetic product.

UV, Vis & IR protection and colour

In this context, we consider the evaluation of a range of mineral filters10 composed by titanium dioxide, an iron oxide (FeOOH or CI-77492, Fe2O3 or CI-77491, and Fe3O4 or CI-77499), with balanced size larger than 100nm, which had shown remarkable broad-spectrum ultraviolet radiation protection properties11.

The physico-chemical properties of these filters can be considered as candidates for protection against higher wavelength radiation. Thus, their absorbance properties in UV, Vis and near-IR regions were analysed with a Shimadzu UV-2600i UV-Vis spectrophotometer with integrating sphere. The absorbance curves at increasing wavelength (graph 1) confirm important UVB and UVA radiation protection (wavelength 290 – 320 and 320 –400nm respectively) for all of the filters. From the filter shown with dark blue line to the one shown with the red line, the iron oxides content increases and so does the area under the absorbance curve (the curve is at higher values in this region) and thus the protection properties of these filters from UVA radiation.

This trend continues in the visible region (from 380 – 700nm), where the absorption of blue light, BL (380 – 500nm), is especially remarkable and proportionally increases with the iron oxides content of the filter. With a higher content of iron oxides, a higher absorbance of BL (and Vis) radiation is explainable. Interestingly, all filters absorb in the near-IR region (from 700nm) with two main observations: the absorbance increases with iron oxides content and the curves tends to rise for all coloured filters except the one which does not contain black iron oxide CI-7499 in its composition.

Although the wavelength stopped at 1200nm, it is deduced that the absorbance will continue to rise for the filters that contain the three iron oxide pigments CI-7492, CI-7491 and CI-7499 in their composition thus protecting from IR radiation with higher wavelength.

On the other side, the absorbance curve of non-coloured titanium dioxide-based filters showed important UVB and UVA protection (up to 400nm). In contrast, the absorbance of visible radiation (380 – 700nm) and IR radiation (from 700nm) is much lower because there are not iron oxides in its composition. Although, there is a residual contribution to the protection in these areas (0.2 u.a. with BaSO4 as background) due to the titanium dioxide particles size (>100nm).

In summary, mineral filters have shown remarkable absorption of electromagnetic spectrum radiation up to wavelength of 1200nm which includes UVB, UVA, Vis and IR-A and potential absorption at longer wavelengths.

graph 2: Absorbance spectra of mineral pigments. graphic: AdP Cosmetics

Pigments to boost protection

There were also a range of mineral pigments evaluated, developed to provide colour and an increased protection against broad-spectrum UV radiation12. These pigments are composed by titanium dioxide and iron oxides, with balanced size larger than 100nm responsible of the UV protection boosting properties.

After the excellent results of the mineral filters evaluation, the physicochemical properties of the pigments led to consider them excellent candidates for protection against higher wavelength radiation. Thus, their absorbance properties in UV, Vis and near-IR regions were analysed with a Shimadzu UV-2600i UV-Vis spectrophotometer with integrating sphere. The absorbance curves at increasing wavelength (graph 2) confirm important UVB and UVA radiation protection properties (wavelength 290 – 320nm and 320 – 400nm respectively) for all the pigments, but especially important for colour pigments. The absorbance curve is kept at values higher than 0.8 – 0.9 (u.a.) for the yellow, red, and black pigments showing higher absorption of UVA radiation than the white pigment.

The absorbance profile from 380nm is different for each sample. Thus, the white pigment only shows residual absorbance in Vis and IR regions (0.2 u.a. with BaSO4 as background, wavelength 380 – 700nm and 700 – 1200nm, respectively) due to the titanium dioxide particles size (>100nm). The colour pigments showed important absorbance in Vis and IR regions (wavelength 380 – 700nm and 700 – 1200nm, respectively). More specifically, the red pigment filter showed remarkable blue light, BL (380-500nm), absorption while the black one showed the most important absorbance of Vis light of higher wavelength (570 – 700nm) and IR (from 700nm).

In general, absorbance UV-Vis profiles agreed with iron oxides profiles reported in literature6. Interestingly, the BL absorption properties of the red pigment are more remarkable than those of the black pigment. Besides, the absorbance of black pigment increases progressively after 400nm (in Vis and near IR regions) and it follows that it will remain after 1200nm. This was also observed with coloured filters that contain CI-7499 pigment in their composition.

References

1 Sklar LR, Almutawa F, Lim HW, Hamzavi I. Effects of ultraviolet radiation, visible light, and infrared radiation on erythema and pigmentation: a review. Photochem. Photobiol. Sci. 2013; 12:54-64. Available from: https://pubs.rsc.org/en/content/articlehtml/2013/pp/c2pp25152c

2 Mahmoud BH, Ruvolo E, Hexsel CL, Liu Y, Owen MR, Kollias N et al. Impact of long-wavelength UVA and visible light on melanocompetent skin. Journal of Investigative Dermatology. 2010; 130:2092-2097.

3 Blue light has a dark side. Harvard Health Publishing. Harvard medical School [cited 2021, July]. Available from: https://www.health.harvard.edu/staying-healthy/blue-light-has-a-dark-side

4 Coats JG, Maktabi B, Abou-Dahech MS, Baki G. Blue Light Protection, Part I-Effects of blue light on the skin. J Cosmet Dermatol. 2021 Mar;20(3):714-717. Available from: https://pubmed.ncbi.nlm.nih.gov/33247615/

5 Bernstein EF, Sarkas HW, Boland P. Iron oxides in novel skin care formulation attenuate blue light for enhanced protection against skin damage. J Cosmet Dermatol. 2021;20:532–537.

6 Lyons AB, Trullas C, Kohli I, Hamzavi IH, Lim HW. Photoprotection beyond ultraviolet radiation: A review of tinted sunscreens. J Am Acad Dermatol 2021;84:1393-1397. Available from: https://www.sciencedirect.com/science/article/abs/pii/S0190962220306940

7 Schieke SM, Schroeder P, Krutmann J. Photodermatology, Photoimmunology & Photomedicine. 2003 01 Oct; 19(5):228-234. Available from: https://europepmc.org/article/med/14535893

8 Cho S, Shin MH, Kim YK, Seo JE, Lee YM, Park CH, Chung JH. Effects of Infrared Radiation and Heat on Human Skin Aging in vivo. Journal of Investigative Dermatology Symposium Proceedings. 2009; 14 (1),15-19. Available from: https://www.sciencedirect.com/science/article/pii/S1087002415305049

9 Barolet D, Christiaens, Hamblin MR. Infrared and skin: Friend or foe. Journal of Photochemistry & Photobiology, B: Biology. 2016;155, 78–85.

10 enhanceU filters

11 Motos-Pérez B, Pérez R. Natural formulations with non-nano mineral filters. PersonalCareEurope. 2020 April; 119-122. Available from: https://content.yudu.com/web/1u0jl/0A1udfd/PCEApril2020/html/index.html?page=118&origin=reader

12 Motos-Pérez B, Sanchez A. Color sin límites, más protección. Industria Cosmética, 2018 Otoño, 58-61.