The Sun emits ultraviolet radiation as UVA, UVB, and UVC rays. 98.7% of the ultraviolet radiation that reaches the earth’s surface is UVA since the ozone layer absorbs most, but not all, of the UVB and UVC rays. UVC rays are the highest energy and the most dangerous type of ultraviolet radiation, but very few of these rays reach the earth’s surface as they are filtered out by the ozone in the atmosphere. This may become a concern as more ozone is lost. A small amount of UVB reaches the earth’s surface but the vast majority of rays that reach us on the surface are UVA.
UVB radiation penetrates only the top layer of skin and is very important for the synthesis of Vitamin D. Overexposure to UVB rays is responsible for causing sunburn and for many basal cell and squamous cell cancers. UVB is almost completely filtered out by glass and, therefore, does not contribute to Vitamin D synthesis when you are indoors. At one time scientists believed that only UVB rays were responsible for the formation of skin cancer, they have since discovered that UVA rays also contribute.
UVA also contributes to skin cancer. It penetrates the skin more deeply than UVB does, weakens the skin’s immune system, and increases the risk of cancer – especially melanoma. UVA is not filtered by glass so just staying indoors will not protect you. UVA is even thought to break down Vitamin D in the skin, contributing to lower than normal Vitamin D levels in people that are overexposed to the sun. UVA causes the long-term, visible damage such as wrinkles, blotchiness, sagging, and discoloration by damaging the collagen fibers present in our skin.
Ordinary glass is partially transparent to UVA, but is opaque to the shorter wavelengths of UVB and UVC. For this reason, care is needed when spending long periods in the car. Tinted windows or specialized windows with “UV Blocker” will, however, block out the vast majority of UVA.
Sources of UV rays
Much of the damage to DNA in skin cells results from ultraviolet (UV) radiation found in sunlight and commercial tanning lamps and tanning beds. There are many other sources around, and some may even be dangerous. Artificial sources include black lights, curing lamps, germicidal lamps, mercury vapor lamps, halogen lights, high-intensity discharge lamps, fluorescent and incandescent sources, and some types of lasers. Arc-welding is another prevalent source of UV rays
Tanning beds deliver high doses of UVA, which makes them especially dangerous. What’s more, even occasional exposure to intense UVA puts you at greater risk of developing skin cancer than spending long hours in the sun does. An initial high dose of UV radiation will severely damage melanocytes, but not destroy them. When these damaged cells are subjected to further intense bouts of UVA light, they have little capacity to repair their DNA. This makes them more likely to become malignant.
Although UVC rays are filtered out by the atmosphere, they are found in sterilizing equipment such as pond sterilization units and can pose an exposure risk if the lamp is switched on outside of its enclosed pond sterilization unit.
Beneficial effects of UV rays
UVB is essential for the synthesis of Vitamin D in the skin (this important subject will be covered in another chapter). Ultraviolet radiation can have other medical applications such as in the treatment of skin conditions of psoriasis and vitiligo. UV rays are also used for the sterilization of materials such as food and water.
Harmful effects of UV rays
Overexposure to ultraviolet radiation can cause sunburn and some forms of skin cancer by damaging the DNA in our skin cells. The main damage caused by UV radiation is to the skin, but other parts of the body can also be affected.
High intensities of UVB light are hazardous to the eyes and exposure can cause welder’s flash (photokeratitis or arc eye) and may lead to cataracts, pterygium, and pinguecula formation; these are tissue growths that grow to cover the eye, and are probably the body’s mechanism of protecting the eye from further damage.