Radiation: Effects of ultraviolet (UV) radiation on the skin, eyes and immune system

The best known acute effect of excessive UV exposure is erythema, the familiar skin reddening termed sunburn. In addition, most people will tan from the UV stimulation of melanin production, which occurs within a few days following exposure. A further, less obvious adaptive effect is the thickening of the outermost layers of the skin that attenuates UV penetration to the deeper layers of the skin. Both changes are a sign of damage to the skin.Susceptibility to skin damage depends on skin type; individuals with fairer skin will be more prone to sunburn or erythema, than people with darker skin Similarly, the ability to adapt to UV exposure (able to tan) also depends on skin type.

Chronic exposure to UV radiation also causes a number of degenerative changes in the cells, fibrous tissue and blood vessels of the skin. These include freckles, nevi and lentigines, which are pigmented areas of the skin, and diffuse brown pigmentation. UV radiation accelerates skin aging, and the gradual loss of the skin’s elasticity results in wrinkles and dry, coarse skin.

The eye is recessed within its orbit and shielded by the brow ridge, the eyebrows and the eyelashes. Bright light activates the constriction of the pupil and the squinting reflex to minimize the penetration of the sun’s rays into the eye. However, the effectiveness of these natural defences in protecting against the dangers of UV radiation is limited under extreme conditions such as sunbed use or strong ground reflection from sand, water and snow. Acute effects of UV radiation exposure include photokeratitis and photoconjunctivitis. These inflammatory reactions are comparable to a sunburn of the very sensitive skin-like tissues of the eyeball and eyelids, and usually appear within a few hours of exposure. Both can be very painful, but are reversible and do not result in any long-term damage to the eye or vision. Extreme forms of photokeratitis are 'arc-eye' and 'snow blindness'.

Cataracts are the leading cause of blindness in the world. Proteins in the eye’s lens unravel, tangle and accumulate pigments that cloud the lens and eventually lead to blindness. Even though cataracts appear to different degrees in most individuals as they age, sun exposure, in particular exposure to UVB, appears to be a major risk factor for cataract development.

The immune system is the body’s defence mechanism against infections and cancers, and is normally very effective at recognizing and responding to an invading micro-organism or the onset of a tumour. Although the data remain preliminary, there is increasing evidence for a systematic immunosuppressive effect of both acute and low-dose UV radiation exposure.

Animal experiments have demonstrated that UV radiation can modify the course and severity of skin tumours. Also, people treated with immunosuppressive drugs have a greater incidence of squamous cell carcinoma than the normal population. Consequently, beyond its role in the initiation of skin cancer, sun exposure may reduce the body’s defences that normally limit the progressive development of skin tumours.

Several studies have demonstrated that exposure to environmental levels of UV radiation alters the activity and distribution of some of the cells responsible for triggering immune responses in humans. Consequently, sun exposure may enhance the risk of infection with viral, bacterial, parasitic or fungal infections, which has been demonstrated in a variety of animal models. Furthermore, especially in countries of the developing world, high UV radiation levels may reduce the effectiveness of vaccines. Since many vaccine-preventable diseases are extremely infectious, any factor that results in even a small decrease in vaccine efficacy can have a major impact on public health.