Ultraviolet (UV) light means beyond violet, as violet is the colour of the shortest wavelengths of visible light, and thus UV light has a shorter wavelength than violet light. UV rays are invisible to humans but visible to a number of insects.

There are three types of UV radiation emitted by the sun which are classified according to their wavelength and which differ in their biological activity as well as the extent to which they penetrate the skin. Shorter wavelength UV radiation, being the most damaging type of UV radiation known as UVC, is filtered by the atmosphere and does not reach the surface of the earth. UVB is medium wavelength UV radiation which is very active biologically but not capable of penetrating beyond the superficial skin layers. UVA is a relatively long wavelength accounting for most of the UV radiation reaching the surface of the earth and it can penetrate into the deeper layers of the skin.

UVA rays

UVA rays are constantly present regardless of seasonal or weather changes and are powerful enough to penetrate clothing and glass. It used to be considered relatively safe, leading to the use of tanning beds which emits UVA rays, but now it is known to contribute to skin cancer.

UVA rays do not cause sunburn as easily as UVB rays but do penetrate the skin more deeply. It can travel through windows and clothing and cause cracking and shrinking in the collagen and elasticity in skin, making it responsible for signs of aging such as wrinkles, leathery and sagging skin as well as sun spots.

UVB rays

UVB exposure induces Vitamin D production in the skin and the majority of positive health effects are related to this vitamin such as calcium metabolism, immunity, insulin secretion, blood pressure and cell proliferation. However, overexposure to UVB radiation may lead to direct DNA damage, skin cancer and sunburn. Underexposure leads to a lack of Vitamin D.

High intensities of UVB light can be hazardous to the eyes as UV light is absorbed by molecules (chromophores) present in the eye cells and tissues. Too much UV light can also damage eye structures such as the cornea, lens and the retina.

UVB rays are the main culprit when it comes to sunburn and, unlike UVA rays, their strength differ according to the season and the time of day: they do more damage during the summer months and around midday.

How to protect your skin

All sunscreens protect against UVB rays, but it wasn’t until recent years that sunscreen started including UVA protection. Not all sunscreens do include UVA protection so it is advisable to look for one that specifically says UVA/UVB or “broad spectrum coverage” on the bottle.

Use a minimum of SPF 15 and reapply every hour or two. To see how long your sunscreen will last under perfect conditions, take the number of SPF and multiply it by 10. That is the length of time you’d be safe from the sun’s rays (that is, in perfect conditions where no water or sweating takes place). For example: SPF 20 x 10 = 200 minutes of sun protection. Protective eyewear is recommended when exposed to ultraviolet radiation and wearing a hat or using an umbrella also helps in protecting your skin from ultraviolet rays.

 

Sources
http://en.wikipedia.org/wiki/Ultraviolet
http://skincare.about.com/od/sunkissedglow/a/UVAandUVBRays.htm http://www.who.int/uv/faq/whatisuv/en/index2.html