How UV Irradiation Causes Skin Damages and Diseases

What is Ultraviolet (UV) Radiation?

Ultraviolet radiation or UV rays are a form of electromagnetic radiation that emitted mainly by the sun and some man-made sources like electric arcs, mercury-vapour lights, tanning lights, black lights, etc. It was discovered by a German physicist Johann Wilhelm Ritter in 1801. Its wavelength ranges from 10-400 nm with corresponding frequencies which ranges from 30 PHz-750 THz. Due to the lack of photoreceptors to perceive ultraviolet and since the human eye-lens blocks the wavelength range of 300-400 nm, most humans are unable to see ultraviolet rays. However, under certain conditions, some humans can see near-ultraviolet rays which are in the range of 300-310 nm.

Types of UV radiation

UV radiation can be detected by Spectrometers and radiometers. Based on the range of the wavelength of UV radiation can be categorized into three major subtypes as Ultraviolet A, Ultraviolet B, and Ultraviolet C. Ultraviolet A and B have long wavelengths which allow them to reach earth’s surface while Ultraviolet C has the shortest wavelength preventing it from reaching the earth’s surface as it cannot penetrate through the ozone layer. Ultraviolet B radiation can penetrate the human skin epidermis and Ultraviolet A radiation penetrates the dermis of the skin. UV radiation has mixed effects causing both beneficial and harmful effects on humans.

Benefits of UV radiation

UV radiation is known to have a beneficial effect on the natural Vitamin D synthesis process and the production of endorphins in the human body. Vitamin D is an important factor to absorb calcium from the gastrointestinal tract and to store calcium in the bones to maintain strong bones in a normal healthy body. This is achieved through a series of steps by converting 7-dehydrocholesterol in the epidermis of the skin to cholecalciferol (Vitamin D). This series of reactions are mediated in the epidermis by Ultraviolet B rays. Continuing the process, succeeding steps take place in the liver and kidney stimulating the intestinal calcium absorption by 1, 25 -dihydroxyvitamin D3 which is the activated form of Vitamin D.

Therefore, sun exposure is necessary to maintain the sufficient amount of vitamin D levels in the human body. Lack of adequate vitamin D levels may lead to diseases like Rickets, osteomalacia, osteoporosis, etc. Apart from that UV radiation helps treat sarcoidosis, psoriasis, etc.

Health effects/Skin damages caused by UV exposure

Despite the benefits, health effects caused by the UV radiation outweighs with exposure to chronic UV radiation. This may cause numerous health effects such as atrophy, inflammation, pigmentary changes, premature aging/wrinkling of the skin, and skin cancers. Human skin gets exposed to UV radiation mainly through chronic sun exposure and from artificial sources like tanning beds. The amount and type of melanin is the main factor which determines the skin complexion and UV sensitivity. Skin melanin can be categorized into two types as eumelanin and pheomelanin. Individuals with dark complexion have eumelanin and they have a low risk of UV damage since they have less UV sensitivity whereas, the individuals with fair complexion have pheomelanin and they have a high risk of skin damage due to high UV sensitivity. This factor determines the risk of skin cancer.

Actinic Keratosis (AK)

Actinic keratosis or solar keratosis is caused by prolonged exposure of the skin to the sun. These are rough scaly patches that can be seen in the face, ears, hands, forearms, lips, and neck. This is more common in people with fair complexion and the main symptoms include scaly skin plaques, redness, hyperpigmentation, deep wrinkles, and ecchymosis.

UV radiation acts as a carcinogen to induce mutations in keratinocytes of the skin epidermis causing mutations in p53 at the molecular level. P53 is a tumor suppressor gene that mediates cell cycle arrest preventing the replication of cells with DNA damage in normal healthy cells. Therefore, mutations in the p53 cause dysregulation of the p53 pathway resulting in unchecked replication of dysplastic keratinocytes in the skin epidermis causing Actinic Keratoses. Actinic Kerostoses has a high risk of progressing into squamous cell carcinoma (SCC) if left untreated.

Photoaging

Photoaging is also known as premature aging of the skin due to chronic exposure to UV radiation. This also depends on the level of skin pigment or melanin in the skin. UV radiation induces Reactive-Oxygen Species (ROS) which involves age-related loss of elasticity of the skin. This is mediated by Ultraviolet A radiation resulting in oxidative stress triggering defective cellular signaling and cellular senescence.

Moreover, UV irradiation stimulates the secretion of Macrophage migration inhibitory factor (MIF) by keratinocytes and fibroblasts present in the dermis of the skin. MIF activates matrix metalloproteinases (MMPs) which cause alterations in the extracellular matrix of the connective tissue of the skin resulting in impaired integrity and degradation of collagen fibers which are responsible for the maintenance of the elasticity of the skin. The elasticity of the skin is important to maintain healthy skin and loss of elasticity plays a major role in photoaging and skin wrinkling.

Skin Cancers

Skin cancer is the most pernicious effect of UV irradiation which has a high incidence rate accounting for 40% of all cancers globally. The three most common types of skin cancers caused by UV radiation are basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and malignant melanoma. Chronic exposure of the skin to UV radiation activates various oncogenes while inactivating tumor suppressor genes resulting in activation of complex pathways leading to carcinogenesis. More specifically Ultraviolet B rays enter the skin generating mutagenic damages in DNA such as the formation of cyclopyrimidine dimers and causing mutations in the P53 tumor suppressor gene which involves DNA damage repair and elimination of mutated cells. Mutations in the tumor suppressors allow the mutated keratinocytes to proliferate achieving carcinogenic potential.

Moreover, Ultraviolet A rays facilitate the effects of Ultraviolet B by generating free radicals and UV radiation has immunosuppression action on the skin inhibiting antigen presentation, anti-tumor surveillance activity of dendritic cells and triggers the release of immunosuppressive cytokines leading to photocarcinogenesis.

How to avoid/reduce UV exposure?

• Avoid chronic exposure to the sun.
• Using sun protection (using sunscreen, wearing sun-protective shades and clothing such as long-sleeved clothes, hats, etc.).
• Avoid indoor tanning.