The vast majority of ultraviolet light is blocked, by the ozone layer, from penetrating through the atmosphere. The specific amounts of UV rays reaching the Earth vary, based on the kinds of rays and your altitude on the Earth. UV rays have far-ranging positive and negative effects, both on humans and on the environment.
Positive Effects on Humans
UV light is instrumental in triggering vitamin D production, which helps to strengthen the immune system, bones and muscles, reduces depression, prevents diabetes and obesity, fosters normal cell growth and helps to maintain hormonal balance. UV light helps some skin conditions, such as psoriasis, by reducing the itchy, scaly skin patches. Finally, UV light stimulates melatonin production, thus helping to reverse Seasonal Affective Disorder (SAD), regulate your moods and enable a regular sleep schedule.
Positive Effects on the Environment
UV light helps some animals' vision. Birds and bees, for instance, are better able to recognize certain markings on flowers, indicating that food is nearby, when they see the markings in UV light. Likewise, UV light from celestial objects often helps insects to navigate. Finally, UV light is useful in sterilization and disinfection. UV light from the sun, for instance, can kill microorganisms such as bacteria, by penetrating cell membranes and destroying the DNA.
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Negative Effects on Humans
UV radiation is the most prominent and universal carcinogen in the environment, causing up to 90 percent of the three main kinds of skin cancers (basil cell carcinoma, squamous cell carcinoma and melanoma). Furthermore, UV light has a suppressing effect on the immune system, by changing the distribution and function of disease-fighting white blood cells for up to 24 hours after UV exposure. Prolonged exposure to UV light damages eye tissue, damages skin and ages skin, by destroying the collagen and connective tissue beneath the top layer of skin.
Negative Environmental Effects
UV light not only damages skin, but it damages synthetic materials as well. UV light will weaken plastics used in consumer items, such as nylon and polystyrene, or cause them to lose strength due to UV light exposure. Furthermore, when many pigments absorb UV light, they change color. Thus, if you have fabrics or paintings that are exposed often to UV light, consult an expert as to how to obtain special protection for them, to avoid color losses or changes.