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Should blue light and conventional LED light sources raise concerns about retina damage? Are the current regulations sufficient? New studies from this year confirm the importance of LED light sources emitting the full spectrum with enough red.
LED technologies have dramatically changed the light we are exposed to on a daily basis. Most LED lights used indoors emit white light, which is produced by exciting a blue diode and covering it with a thin phosphor layer. The spectral power distribution (SPD) of the emitted light contains an intense peak in the blue spectral range with a maximum of around 450 nm, a peak in the yellow-green spectral range, and usually a very low emission in the red spectral range (see Fig. 1 C). However, this spectrum is very different from the spectra emitted by traditional incandescent bulbs (tungsten, halogen…) with a high content of long wavelengths (red) and a low content of short wavelengths (blue) (see Fig. 1 A).
The most common light sources in work and school environments today, are still fluorescent lamps. All these types of light sources differ significantly from the SPD of sunlight during the day, especially in the red and blue spectral ranges (see Fig. 1). The blue spectral range can be further divided into the so-called melanopic range, which is necessary for controlling our circadian rhythms (the cyan region), and the blue-violet spectral range within 400─455 nm, which is often referred to as the Harmful Blue Light (HBL) range. For this reason, the effect of blue light on organisms began to be investigated, leading to regulations and rules concerning its safety in terms of potential retina damage. High-intensity blue light exposure for a short time is referred to as Blue Light Hazard (BLH). But what about low-intensity and prolonged exposure to blue light? That question is answered by a new study from this year that examined how white light and selected spectral ranges affect albino rats and human retinal pigment epithelial (RPE) cells.
Fig. 1: Spectral Power Distribution (SPD) of different electric light sources compared to natural daylight. A) incandescent bulb with intense emission in the red spectral range but low in the blue spectral range, B) fluorescent lamp with dips in important spectral ranges (cyan and red), C) standard LED with intense emission in the harmful blue light range (HBL) with a dip in the cyan range and a lack of emission in the red spectral range, D) Spectrasol LED emitting full spectrum light with reduced radiation in the HBL range and plenty of radiation in the cyan and red spectral ranges, E) natural daylight at noon.
Photo-toxicity threshold and current regulations
Even though the eye structures are protected by a complex antioxidant system and other defence mechanisms, overexposure of the eyes to light, especially to light of short wavelengths, can cause irreversible damage to the retina. Light-induced retinal damage (photo-retinitis) leads to photoreceptors (rods and cones) death and damage to retinal cells, especially the retinal pigment epithelium (RPE), which is located under the photoreceptor layer and serves to recycle and degrade photo-pigments and isolates the retina from the bloodstream (see Fig. 2). Thus, RPE cells are very important for the survival of photoreceptors. Currently, only blue light with a maximum of 455 nm is used to evaluate the photo-toxicity of a light source. The threshold dose for the primate retina is 22 J/cm2 and for rodents 11 J/cm2. The values determined for primates were the basis for setting the current regulations and standards: IEC 62471 states that when the dose is less than 2.2 J/cm2 for 10 000 s (less than 3 hours), i.e. 1/10 of the phototoxic threshold for primates, the light source is considered as risk-free. Indeed, most light sources fall into this category and should not therefore pose a risk of retinal damage, but is this really the case?
Fig. 2: Retinal structure. The retina is composed of several layers and different types of cells. The retinal pigment epithelium (RPE) is composed of RPE cells that are connected to the choroid through Bruch’s membrane and help with the recycling and degradation of photoreceptor (rods and cones) photo-pigments. The image was adapted from S. Yang, J. Zhou, and D. Li, “Functions and Diseases of the Retinal Pigment Epithelium,” Frontiers in Pharmacology, vol. 12, no. 2021, doi: 10.3389/fphar.2021.727870.
Is blue light really the only cause of retinal damage? And can red light possibly help?
In the first study, albino rats were exposed to blue light (maximum 450 nm). Even at a dose of 0.2 J/cm2, immune cells infiltrated the retina which indicated an ongoing inflammatory reaction. Cellular DNA damage was also evaluated. At a dose of 0.5 J/cm2, the number of damaged cells was up to 100 times higher than in the control group. Among other things, there was a decrease in photoreceptors and changes in their morphology. These changes were observed even after a week’s interval after exposure to blue light with a dose of 0.5 J/cm2. However, exposure to white light with a CCT of 2700 K and a total dose of 2 J/cm2 with about 10 % of the blue spectral range induced 10 times more retinal cell damage than exposure to blue light with the same dose (the dose of blue light in white was identical to that of monochromatic blue light: 0.2 J/cm2). These results suggest that another spectral range must play a role in photochemical damage to the retina. This hypothesis was confirmed in another experiment where rats were exposed to green light at a wavelength of 507 nm. In comparison to earlier electric light sources, LED sources typically have a significantly reduced emission in the red spectral range, and therefore the effect of red light at 630 nm wavelength with a dose of 0.1 J/cm2, white light with a dose of 0.9 J/cm2 and the addition of red light to white light with a total dose of 1 J/cm2 was further evaluated. The results are also consistent with previous studies showing that red light has the potential to attenuate the harmful effects of blue light and thus partially protect against photoreceptor death, although according to statistical analysis, there is no significant difference (reduced number of damaged cells by a factor of about 3).
The second study used similar methodological procedures. Retinal pigment epithelial (RPE) cells derived from human induced pluripotent stem cells (hiPSCs) were exposed to white light with a CCT of 3300 K and a dose of 3.6 J/cm2. Although the photo-toxicity threshold was determined for blue light only, i.e. 22 J/cm2, the dose of polychromatic light is well below the threshold. Calculation of the proportion of blue light in white determined the dose of blue light to be 0.185 J/cm2, i.e. more than 100 times lower than the photo-toxicity threshold, and yet the RPE cells showed changes in overall cellular structure, DNA damage, activation of cellular stress and autophagy (a cellular process used to maintain cellular homeostasis, during which old or damaged cellular components and proteins are recycled, thus representing an emergency source of energy under stressful conditions). By analysing biochemical markers, it was found that blue light is most likely to be the main cause of DNA damage. While red light appears to induce modulation of DNA detection and/or repair mechanisms including inhibition of inflammatory processes.
In conclusion
The results of the studies suggest that the photo-toxicity threshold for both rodents and primates should be tightened, as even a significantly lower dose caused eye damage. It appears that not only blue light but also green light contributes significantly to retinal damage. Based on the same methodologies for determining the photo-toxicity threshold for rodents and primates, it can be assumed that the photo-toxicity threshold for humans is not low enough and that the full Spectral Power Distribution (SPD) should be considered, as it has been confirmed in both studies that red light can modulate the negative effects of short wavelength light. Therefore, as part of prevention, it is very important to expose oneself to daylight and to use sources with a balanced full spectrum and with sufficient emission in the red spectral range during the day, even at low intensities.
Author’s note
This article is not meant to scare readers. It aims to inform and draw attention to the importance of exposure to daylight that is balanced in its spectrum. However, given that we spend a truly enormous amount of time indoors, it is a question of what electric light sources we want to use during the day. To support and synchronize circadian rhythms, we need full-spectrum light sources with sufficient emission in the melanopic range (455─490 nm) during the day, and to modulate the potentially harmful effects of short wavelengths when using this light on a long-term and regular basis, we need sufficient emission in the red spectral range. Tungsten and halogen bulbs and LEDs with reduced blue content are suitable, but especially in the evening, they do not contain sufficient energy to stimulate activity and cognitive performance.
Mgr. Tereza Ulrichová, Spectrasol
Literature:
1) A. Françon, F. Behar-Cohen, and A. Torriglia, “The blue light hazard and its use on the evaluation of photochemical risk for domestic lighting. An in vivo study,” Environment International, vol. 184, p. 108471, Feb. 2024, doi: 10.1016/j.envint.2024.108471.
2) A. Françon et al., “Phototoxicity of low doses of light and influence of the spectral composition on human RPE cells,” Sci Rep, vol. 14, no. 1, p. 6839, Mar. 2024, doi: 10.1038/s41598-024-56980-9.
Figures:
Introduction figure: F. Behar-Cohen et al., „Light-emitting diodes (LED) for domestic lighting: any risks for the eye?”, Prog Retin Eye Res, vol. 30, No. 4, pp. 239-257, Jul. 2011, doi: 10.1016/j.preteyeres.2011.04.002.
Figure 2: S. Yang, J. Zhou, a D. Li, „Functions and Diseases of the Retinal Pigment Epithelium”, Frontiers in Pharmacology, vol. 12, Jul. 2021, doi: 10.3389/fphar.2021.727870.
