Recent interest in visible light protection has given rise to a reemerging question: Is visible light harmful to skin? If one considers the basic tenets of toxicology, everything can be toxic at a given dose. So yes, visible light can be harmful. Perhaps the more poignant question would be: To whom is it harmful, and when? Or: Do the benefits of visible light outweigh its potential toxicity?
But before exploring these relevant considerations for skin care, it is important to step further back and ask: Upon its arrival to the earth’s surface, what happens to all the energy contained in visible light? And, for starters: What is visible light? This brief commentary considers the latter, to answer the former.
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Recent interest in visible light protection has given rise to a reemerging question: Is visible light harmful to skin? If one considers the basic tenets of toxicology, everything can be toxic at a given dose. So yes, visible light can be harmful. Perhaps the more poignant question would be: To whom is it harmful, and when? Or: Do the benefits of visible light outweigh its potential toxicity?
But before exploring these relevant considerations for skin care, it is important to step further back and ask: Upon its arrival to the earth’s surface, what happens to all the energy contained in visible light? And, for starters: What is visible light? This brief commentary considers the latter, to answer the former.
What is Visible Light?
What we call visible light is that which falls in the region of the electromagnetic spectrum between the wavelengths of 400-700 nm. This is detectable to the naked eye of most healthy human beings. Not all vertebrates detect and distinguish all the wavelengths in this region, and in some cases, the eyes of other animals detect electromagnetic wavelengths outside of this region. The description of the mechanisms of human vision is beyond the scope of this short review.
Visible light accounts for approximately 40-45% of the sun-emitted energy that reaches the earth. UV radiation accounts for just 3-5%. The remaining approximately 50% is infrared radiation. Ionizing radiation emitted by the sun does not reach the surface of the earth.
Visible Light and the Environment
When a beam of visible light hits a body, assuming the obvious meanings of these words, the photons can be reflected or absorbed by the molecules of the surface of the body. They can also penetrate the body and be absorbed or scattered by its molecular components; or continue on their path, undisturbed.
Scattering is a phenomenon analog to reflection, whereby photons are deviated from their course by obstacles in their path with the proviso that this deviation depends upon the wavelength of the photons and size of the scattering particles. On cloudless days, the sky appears blue because blue photons are more efficiently scattered by the molecules of the air and redirected to our eyes. At sunset on cloudless days, we see the sky as red because the blue photons have been scattered away and only red-yellow photons on the surface of the earth reach our eyes. Let us not forget that at sunset, the length of the path of the light across the atmosphere, from the sun to our eyes, is larger than it is at midday.
Besides the sky, objects around us have colors. These colors correspond with the wavelengths either reflected by the surface of those objects or backscattered by their components. The color of the object is complementary to the colors absorbed by the molecules constituting the object.
So, for instance, the blood appears red because it absorbs blue and green wavelengths, and grass appears green because it absorbs red and blue wavelengths.
Energy Dynamics
What happens to all the energy absorbed by the colored molecules on the surface of the earth? Since physics denotes that energy is conserved within a system, the absorbed energy is released from the absorbing (excited) molecules under other forms. For example, excited molecules can emit photons of different wavelengths; this refers to luminescence including fluorescence or phosphorescence.
Excited molecules also can undergo photochemical reactions and form new chemical compounds with neighboring molecules. They can transfer an electron, or even their energy, to oxygen molecules and generate reactive oxygen species (ROS).
The main way for excited molecules to release absorbed energy is via thermal decay; that is, by giving away their vibrational energy and increasing the kinetic energy of neighboring molecules. The net result will be a local increase of temperature.
This phenomenon is put to work in greenhouses built with glass walls that are transparent to visible light. Here, visible light is absorbed and its energy is released via thermal decay; this advantageously increases the temperature of the air contained in the greenhouse with the goal of supporting the growth of fruits and vegetables, even in the coldest of winters.
Do not confuse a greenhouse with the greenhouse effect, however. In a greenhouse, the glass walls keep the warmed air confined within the volume of the house and maintain a temperature appropriate for the growth of crops.
In contrast, the greenhouse effect is the heating of the atmosphere due to an excess of specific infrared-absorbing or infrared-scattering gas molecules—such as methane, carbon dioxide, water vapor, etc.—in the air. These molecules diffuse infrared radiation and send it back to the surface of the earth, thereby hindering the release of heat from the earth to outer space.
Foundations and lipsticks present aesthetically pleasing methods for protecting skin against the photochemical aggressions provoked by visible light.
Visible Light and Skin Appearance
When it comes to the skin and cosmetics, visible light can be a major player and must be taken into account. Visible light is partially absorbed by the melanin in the epidermis, and it also penetrates the skin. The papillary dermis contains subepidermal blood vessels that absorb blue and green light, and other photons are scattered around and can penetrate the skin down to the adipose tissue, where the fat absorbs part of the red photons.
In fact, the final perceived color of an individual’s skin, to the eye of the observer, is what is reflected and back-scattered—i.e, the total of the impinging light minus the photons that have been absorbed by the melanin, blood and fat tissue. Clearly, this explains why a dinner by candlelight could benefit individuals with aging and wrinkled skin.
In the region of the wrinkle, blood vessels are closer to the surface and absorb more blue and green photons than a wrinkle-free surface. The wrinkles are therefore clearly visible in daylight, but not so clear by candlelight, since it only contains red and yellow photons that are not absorbed by blood.
Reactivity of Visible Light
It is important to be aware, though, that visible light is not an inert entity; it can be a source of unwanted problems. Via photochemical reactions, for example, visible light can affect the stability or duration of a particular shade of hair dye.
Perhaps more concerning, though, are molecules in skin that are photosensitizers. These can either be endogenously produced—e.g., protoporphyrins and phaeomelanin—or taken up in the form of xenobiotics such as riboflavin. Upon the absorption of a visible photon, these molecules can transfer an electron or their energy to molecular oxygen, thus generating the free radical superoxide anion or very reactive singlet oxygen.
Oxygen-free radicals and reactive oxygen species (ROS) are known to provoke the peroxidative cascade of lipids—a chain of reactions that can alter the structure of the lipid bilayers in the cell membranes, therefore provoking cell death, necrosis and the consequent inflammatory reaction. It has been shown that visible light cast onto human epidermis equivalents induces the formation of hydrogen peroxide, the release of inteleukin 1-α and the induction of matrix metalloprotease-1 (MMP-1). Per unit energy, visible light is approximately 20-30 times less efficient than UV, although there is 10-15 times more visible light than UV in the solar spectrum. Therefore, the effects of visible light are, indeed, relevant.
Experiments performed in human volunteers also confirmed the effects of visible light in producing ROS, IL1-α and MMP-1.1 These experiments validated the phenomena described 20 years earlier by Rex Tyrrell, who observed that visible light generates singlet oxygen in cultured mammalian cells and induces the transcription of the heme oxygenase gene.2
The good news is the use of makeup composed of visible material hinders part of the harmful effects of visible light
on skin.
Visible Light and Aging
One could therefore consider visible light to be a potential factor in aging, insofar as it provokes inflammation and subsequent age-accelerating inflammatory phenomena. First of all, the recruitment into the dermis of circulating macrophages is facilitated by the release of hydrogen peroxide, which favors the crossing of blood vessel walls by circulating immune cells.
The second peroxidative step occurs during the chemotactic movement toward the damaged cells, with the release of singlet oxygen produced by the macrophages' myeloperoxidases. This is accompanied by the degradation of elastic fibers by MMP-1 and a loss in the organization of the extracellular matrix.
Finally, immune cells release hydrogen peroxide to digest the damaged cells. The debris is then engulfed by immune cells and eliminated through the lymphatic system. During these steps, "innocent" bystander cells can be damaged and the inflammatory reaction is therefore maintained.3
The good news is the use of foundations and makeup composed of visible material hinders part of the potentially harmful action of visible light on skin. More good news is that visible light has a bright side in that it also shows therapeutic effects, not the least on acne.
Porphyrins contained in the bacterium Propionibacterium acnes make it particularly sensitive to visible light. As such, when porphyrins absorb blue-violet light and catalyze the production of singlet oxygen, this behaves as an endogenous poison and kills the acne-associated microorganism, thus alleviating the inflammation and consequent redness and aesthetic discomfort;4 a review of light-based therapies for acne treatment was previously described.5
Another positive effect of visible light is in prematurely born babies whose systems are unable to remove bilirubin. Its accumulation carries the risk of brain- and liver-damaging jaundice. However, exposure to blue light makes it easy for newborns’ systems to remove bilirubin via excretion in bile and urine, thus avoiding these complications.
Conclusions
Visible light is a major environmental factor that facilitates the detection of environmental details and helps to maintain the appropriate temperature necessary for life on earth as we know it. However, visible light catalyzes photochemical phenomena, the consequences of which can be harmful to hair and skin. Thankfully, foundations and lipsticks developed by the cosmetics industry present aesthetically pleasing methods for protecting skin against the photochemical aggressions provoked by visible light.
References
- Liebel, F., Kaur, S., Ruvolo, E., Kollias, N., & Southall, M. D. (2012). Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes. J. Invest. Dermatol. 132, pp. 1901-1907.
- Basu-Modak, S., & Tyrrell, R. M. (1993). Singlet oxygen: A primary effector in the ultraviolet A/near-visible-light induction of the Heme-Oxygenase gene. Cancer Research 53, 4505-4510.
- Giacomoni, P. U., & Rein, G. (2017). Skin aging: A generalization of the micro-inflammatory hypothesis. In Textbook of Aging Skin (1289-1298). New York: Springer Verlag.
- Ashkenazi, H., Malik, Z., Harth, Y., & Nitzan, Y. (2003). Eradication of Propionibacterium acnes by its endogenic porphyrins after illumination with high intensity blue light. FEMS Immunol Med Microbiol 35, 17-24.
- Pei, S., Inamadar, A. C., Keshavmurthy, A. A., & Tsoukas, M. M. (2015). Light-based therapies in acne treatment. Indian Dermatol Online 6, 145-157.ction from UVA and UVB. Newer benefits, including anti-pollution, infrared and blue light protection, are being added.
