The sky, the stars, an image. The blonde, red, dark hair; the mirror, that photo, that moment, that face, the colors. A work of art, the glow of dawn and the colors of dusk. In reality, the setting of these images that surround us is composed of the emission, absorption, and reflection of light. But after all, what is light?
Light is the visible portion of electromagnetic radiation capable of interacting with matter. Light behaves like waves, and variations in its properties, such as its wavelength and frequency, translate into the different colors observed. These colors are part of a small spectrum visible to the human eye, whose wavelengths range from 400 to 700nm.
The spectral distribution of solar energy consists of about 3-7% UV (290-400 nm), 44% visible light (400-700 nm), and 53% infrared radiation (IR, 700nm-1mm). The contribution of the UV component on the skin has been well studied and applied in the technologies of products for sun protection, as it causes acute and severe damage after just a few hours of exposure. The UV component is formed by three wavelength regions: UVC (absorbed by the ozone layer in the atmosphere), UVB (reaches the keratinocytes, the cells of the outermost layer of the skin), and UVA (penetrates more deeply into the dermis). This is why sunscreens today provide protection against UVA and UVB rays, but not against IR or visible light. It was believed until then that the wavelengths of visible light and IR produced only heat after absorption, without causing any subsequent damage. Only recently have studies begun to explore the effects, including thermal effects, of non-UV radiation on skin physiology.
It is already known that the increase of enzymes that degrade collagen, the production of reactive oxygen species (ROS), and pro-inflammatory cytokines in the skin after UV radiation are important causes of damage to the skin itself, contributing to its photoaging.
But could the 44% of radiation coming from the sun, responsible for what we know as the visible spectrum to the human eye, also be capable of causing similar damage?
Researchers have shown that yes. But before we delve into the details, it is important to emphasize that visible light cannot be treated as the villain of the story, as, like everything in life, it should be used sparingly; it is necessary for physiological functions of the body, such as vitamin D production, and is also positively used in the treatment of diseases such as atopic dermatitis and acne. But let’s set aside treatments for now, as they will be discussed later, on another occasion.
Returning to the mechanisms of action, in human skin, the spectrum of visible light together with IR can increase the expression of enzymes called metalloproteinases (MMP-1 and MMP-9), decreasing the expression of procollagen, favoring the inflammatory infiltrate of macrophages (immune system cells) (Cho, et al, 2010). This, in turn, contributes to inflammation in the skin and premature aging damage. Additionally, in in vitro tests under the action of the visible light spectrum, an increase in ROS, pro-inflammatory cytokines (IL-1α), and MMP-1 was observed (Liebel, et al, 2012). In this last case, the use of antioxidant molecules combined with a sunscreen against UVA/UVB was able to minimize the formation of ROS caused by visible light, suggesting a possible beneficial effect of such molecules in controlling the early appearance of signs that contribute to aging. However, future studies that include a larger sample size, along with more in-depth data on possible mechanisms of action of this combination, are still needed to prove the clinical efficacy of these results.
Remembering that today we discussed a little about the portion of visible light, should we also rethink the care in therapeutic use, or its possible damage to the skin caused by IR radiation?
References
Cho S, Lee MJ, Kim MS, Lee S, Kim YK, Lee DH, Lee CW, Cho KH, Chung JH. Infrared plus visible light and heat from natural sunlight participate in the expression of MMPs and type I procollagen as well as infiltration of inflammatory cell in human skin in vivo. J Dermatol Sci. 2008 May;50(2):123-33. Epub 2008 Jan 14.
Liebel F, Kaur S, Ruvolo E, Kollias N, Southall MD. Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes. J Invest Dermatol. 2012 Jul;132(7):1901-7. doi: 10.1038/jid.2011.476. Epub 2012 Feb 9.
