UVA Radiation Effects

Until recent years UVA was considered to have very few, if any, deleterious effects and so was of little academic interest because UVA rays don’t cause the acute “sunburns” we remember as red, painful, and possibly with blisters, etc. They are caused by UVB rays. Dermatology and medical researchers have known for years of the familiar effects of UVB radiation (290-320 nm) on human skin. UVA radiation is not safe, and many of the harmful effects produced by UVB radiation are also seen with exposure to UVA radiation. As our knowledge has grown in “Skin Biology” over the past 50 years, solar radiation has now become a major environmental factor deleterious to our health. UV radiation from the Sun (290-400 nm) has been implicated in the induction of many acute and chronic harmful effects on human skin. About 52% of the solar radiation is in the UVA range, so UVA radiation (320-400 nm) is abundant in natural light. The UVA output by the Sun is significantly greater than the UVB throughout the year. The major portion of the UVA radiation does reach the surface because the atmosphere has no specific role in filtration or lessening the UVA radiation. A major source of UVA radiation from artificial sources is from low-pressure mercury arc fluorescent lamps use the lighting industry in tanning salons, tanning beds, and phototherapy units. Most of these lamps emit high intensity UVA radiation (13-20 mW/ cm²).The UVB flux from the atmosphere goes under significant seasonal variation. Because the quantum efficiency of UVB in terms of erythema is several orders of magnitude greater than that of UVA photons there's a greater need for protecting from acute sunburn from UVB rays. The contribution of UVA from sunlight and causing acute reactions is only about 18-20%, but UVA is present from sunup until sundown every day and does not appreciably change seasonally.Transmission of UVA into the dermis of the untanned fair-skinned individual is approximately 50% of the impinging flux. This UVA radiation can reach to the dermal papillary of hair in antigen face the UV effects can also occur to the vascular network in the deeper layers of the reticular dermis. Thus the radiation is very capable of inducing structural and functional changes in both keratinocytes as well as melanocytes, Langerhans cells and fibroblasts.The biological action of UVA radiation involves damage to DNA and other proteins. The damage is mediated through absorption of UVA by other molecules (flavins, nicotinamides, ubiquinone, tryptophan, porphyrins, and riboflavin) present in the skin. Photo exicitation of these from UVA radiation causes an energy interaction with oxygen molecules to produce reactive forms of oxygen referred to as free radicals. These reactive oxygen species are then able to induce significant damage in all proteins including DNA in the form of single-strand breaks and the induction of unstable dimers.

Numerous harmful effects result. Cell membrane lipids undergo lipid peroxidation that contribute to: inflammatory responses and the generation of inflammatory mediators, inactivation of enzymes, denaturation of proteins, and damage of cell organelles (involving mitochondria and impaired oxidative phosphorylation) that are essential for survival of cells. As the skin is profusely supplied with oxygenated blood, capillaries and small veins are easy targets for the reactive oxygen species produced by UVA radiation. The long-term effect from damaged small vessels is the weakening and dilation of the vessel to create prominent capillaries and enlarged veins.