# Soriasis and Seasonality: Exploring the Genetic and Epigenetic Interactions
## 4 Environmental Factors Affecting Psoriasis  
### Sunlight  
The Sun is a natural source of electromagnetic energy, from gamma radiation to radio waves. Due to the atmosphere filtering effect, primarily a range between 280 and 1000 nm reaches the surface of the Earth, consisting of 53% of infrared radiation, 43% of visible light, and 4% of ultraviolet radiation [77; 78]. Spectra of solar radiation are represented in Table 2.  
Approximately 95% of terrestrial UV is UV-A radiation, while the rest is UV-B radiation. The mechanisms of UV-R on the skin are represented in Figure 1. \begin{table}
\begin{tabular}{c c c} \hline
**Light Type** & **Abbreviation** & **Wavelength [nm]** \\ \hline \multirow{4}{*}{ultraviolet radiation} & UV-R & 100–400 \\  & UV-C & 100–280 \\  & UV-B & 280–315 \\  & UV-A & 315–400 \\ \hline visible light & VL & 400–760 \\ \hline infrared radiation & IR & 760–1000 \\ \hline \end{tabular} Abbreviations: UV-R—ultraviolet radiation; VL—visible light; IR—infrared radiation.  
\end{table} 
Table 2: Spectrum of solar radiation. Table adapted from [PERSON] et al. [77]. The significance of sunlight and its impact on psoriasis is well established. Phototherapy is one of the most common treatment options for children and adults [95; 96]. Exposure to solar radiation in skin influences several epidermis functions due to apoptosis, DNA damage responses and cell cycle control, innate and acquired immune regulation and inflammation, redox response and angiogenesis, circadian rhythmicity, and keratinocyte, and the immune regulation of the skin. The primary goal of this work is to study the effect of the immune regulation of the skin on the skin. Figure 1: Influence of UV-R on the skin. When UV electromagnetic waves reach the skin, a small part of them are reflected, while the majority penetrate the skin and are absorbed by different molecules [77]. UV-B penetrates the skin to a depth of about 0.1 mm and UV-A to a depth of about 0.8 mm [79]. While UV-B is absorbed by most of the molecules within the skin, UV-A is not absorbed by DNA and most proteins but can be absorbed by endogenous porphyrins, oxidized fatty acids, and B vitamins [77; 80]. The energy of absorbed UV is converted into heat, fluorescence, and chemical changes in the absorbing molecules, causing the formation of reactive oxygen species (ROS) such as superoxide anions, singlet oxygen, and hydroxyl radicals [77; 80]. ROS can damage DNA and promote T-cell apoptosis [77; 81; 82; 83; 84]. Damaged keratinocytes produce and release several immunomodulators, such as tumor necrosis factors (TNF), 6-formylindolo[3; 2; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36;  
differentiation [97]. In keratinocytes of psoriatic skin, UV-B of 311 nm wavelength exposure causes upregulation of pro-apoptotic genes, leading to the apoptosis and suppression of the keratinocyte differentiation. UV-B also causes increased levels of reactive oxygen species (ROS), which can result in oxidative damage to proteins, DNA, and lipids. Excessive production of ROS can disrupt redox homeostasis and cause DNA damage [97]. Due to the above mechanisms, phototherapy (especially UV-B of 311 nm) is one of the treatments for patients with PsV [97; 95; 79; 78]. Photochemotherapy with psolaren and UV-A irradiation (PUVA) used both locally (e.g., PUFA baths) and orally is another standard phototherapy used in psoriasis treatment [95; 77]. Lately, the spectra of visible light, the blue and red light, were used in psoriasis treatment; however, the results of the clinical studies are inconclusive [99; 100; 101; 102; 103; 104]. Heliotherapy is defined as medical therapy involving exposure to natural sunlight [105]. It was reported that heliotherapy may be an alternative to other treatments for psoriasis in low- and mid-latitude regions of the Earth, including cold and cloudy countries such as Poland [78; 106; 107]. In Italy [108] and Israel [109; 110], heliotherapy has been used in practice.