Image Citation: [1]
Written by Grace Li ‘26
Edited by Yumiko Imai ‘26
Chronic skin conditions are long-lasting disorders that may persist for months, years, or even a lifetime. Among the most common is psoriasis, which causes rashes marked by itchy, scaly patches that appear on areas like the knees, elbows, trunk, and scalp [2]. The condition is thought to affect 3% of the world’s population and cycles through periods of flare-ups, causing pain, disrupting sleep, and impacting concentration [2]. While incurable, psoriasis is understood to be an immune-mediated condition that accelerates skin cell growth, causing the build-up of plaque- (1)like patches. The exact cause remains unknown, though it involves a hyperactive immune system, with immune cells mistakenly targeting healthy skin cells [2]. Researchers believe that both genetic predisposition and environmental factors play a role in this process [2].
Concerningly, the global prevalence of psoriasis has risen over the past few decades, indicating that environmental factors may have a greater impact on the development of this condition than previously thought. Population-based surveys in Northern Norway reported a doubling of self-reported lifetime prevalence, from 4.8% in 1979–1980 to 11.4% in 2007–2008 [3]. This significant increase raises questions about the factors driving this trend.
Sunlight exposure offers a biologically compelling explanation for how environmental factors can influence psoriasis prevalence. This effect can be understood through two primary mechanisms. The first involves reduced inflammatory control: vitamin D activation occurs when UVB radiation from sunlight stimulates the skin to produce vitamin D, which is then activated and plays a key role in regulating immune responses and reducing inflammation [3]. Insufficient sunlight in higher latitudes can result in lower vitamin D levels, potentially affecting immune regulation [3].
The second mechanism involves the direct immunomodulatory effects of UVA and UVB radiation on the skin. UV radiation has been shown to reduce antigen presentation, which is when specific immune cells display parts of foreign pathological substances – antigens – to signal T-cells to respond [4]. This process triggers T-cell activation, an important step in the body’s immune reaction. By reducing these signals, UV radiation lowers T-cell activation, leading to a lower immune response and helping to control the inflammation that contributes to psoriasis symptoms [4]. Additionally, UV exposure induces a type of regulatory T-cells (Tregs) with suppressive functions, which further control and reduce excessive immune responses [4]. These effects help explain how sunlight and phototherapy are effective in managing and clearing psoriasis.
The geographical variation in global psoriasis prevalence based on distance from the equator supports these mechanisms. For instance, populations closer to the equator, such as those in Egypt, Tanzania, Sri Lanka, and Taiwan, typically report lower rates of psoriasis due to greater sunlight exposure and its associated immune-modulating benefits [4]. In contrast, higher prevalence rates in regions like Europe and Australia may be attributed to reduced UV exposure [4]. In Australia, additional factors such as European migration and a mixed genetic background contribute to the complexity of psoriasis epidemiology in different regions [4].
Another way the environment impacts psoriasis is through climate and air quality. In a cohort study involving 474,055 participants, long-term exposure to air pollutants — such as fine particulate matter (PM 2.5 and PM10), nitrogen dioxide, and nitrogen oxides — was linked to an increased risk of developing psoriasis [5]. These air pollutants are thought to affect the skin by altering its natural microbial environment, triggering certain receptors in the skin called aromatic hydrocarbon receptors (AHRs), leading to inflammation and oxidative stress [5]. This disruption upsets the skin’s balance and leads to an immune response, contributing to the etiology of psoriasis [5].
The global rise of psoriasis underscores the intricate interplay between genetic predisposition and environmental influences on health – the increasing prevalence can be partly explained by environmental changes that come with modernization, such as reduced sun exposure and higher levels of air pollution. While treatments exist to manage symptoms, the complex nature of psoriasis calls for more extensive research to better understand how these environmental triggers and their mechanisms. Advancing our knowledge in this area is essential for developing more effective prevention strategies and improving the quality of life for those affected by this chronic condition.
References
Free Photo | Hands of patient suffering from psoriasis [Internet]. [cited 2024 Nov 12]. Available from: https://www.freepik.com/free-photo/hands-patient-suffering-from-psoriasis_23991834.htm
Psoriasis - Symptoms and causes - Mayo Clinic [Internet]. [cited 2024 Nov 12]. Available from: https://www.mayoclinic.org/diseases-conditions/psoriasis/symptoms-causes/syc-20355840
Danielsen K, Olsen A o., Wilsgaard T, Furberg AS. Is the prevalence of psoriasis increasing? A 30-year follow-up of a population-based cohort. British Journal of Dermatology. 2013;168(6):1303–10.
Enamandram M, Kimball AB. Psoriasis Epidemiology: The Interplay of Genes and the Environment. Journal of Investigative Dermatology. 2013 Feb 1;133(2):287–9.
Wu J, Ma Y, Yang J, Tian Y. Exposure to Air Pollution, Genetic Susceptibility, and Psoriasis Risk in the UK. JAMA Network Open. 2024 Jul 16;7(7):e2421665.
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