Light and sun sensitivity of the skin, also known as photodermatosis, is the skin’s reaction to sunlight and primarily affects areas that are exposed to the sun. Usually it is seen as the formation of blisters (vesicles), reddish appearance of the skin and papules (little ‘bumps’ on the skin).

Ultraviolet (UV) rays from sunlight can causes degeneration of the elastic fibres of the dermis (deeper layers of the skin) and thickening of the epidermis (outer layer of the skin. This thickening of the epidermis is usually caused by an increase rate of cell division due to heat and light.

While exposure to sunlight causes the condition, hot environments, certain skin diseases and drugs may make the skin more sensitive to sunlight.

Causes of Sun & Light Sensitivity of the Skin

Many factors may result in skin sensitivity to sunlight. These include :

Genetic Disorders

This genetic disorder has a family history of sensitivity of the skin to sun exposure. These disorders include genetic diseases such as Bloom’s syndrome which has instability in the chromosomal structure and a defect in the repair of the DNA molecules. This causes a reaction in the skin when it is exposed to the light.

Porphyrias

Porphyria is a rare metabolic disorder that due to an enzyme deficiency which leads to an accumulation of breakdown products of metabolism in the blood, urine and faeces. An accumulation of these metabolites can cause toxicity to the nervous system resulting in the skin’s sensitivity to the sun felt as burning and itching.

Pellagra

Pellagra is a condition due to a deficiency of nicotinic acid in the diet. This can cause light sensitivity of the areas of the skin exposed to the sun and diarrhoea may also be noticed after sun exposure.

Polymorphous Light Eruptions

This condition is caused by the skin’s reaction to sunlight and lesions such as papules, plaques and erythema multiforme appear on the affected area. It is sometimes referred to as a ‘heat rash’.

Drug Induced Sun Sensitivity

A condition that causes the skin to react when exposed to the sun due to the intake of certain drugs. Many drugs such as ACE inhibitors, tetracycline, thiazides and phenothiazines can cause eruptions of blisters, eczema, pigmentation and excessive sunburn on the light exposed areas of the skin. These reactions are caused either by the medication dosage or an allergic mechanism to the drug.

Topical Chemicals

Chemicals that have contact on the skin such as coal tar and plant derived psoralen can cause sun sensitivity of the skin. Coal tar which is used in the treatment of the skin condition called psoriasis vulgaris leaves the skin sensitive to sunlight. The plant derived psoralens found in carrots, fennel, and parsnips results in the light sensitivity of the sun exposed areas of the skin although the psoralen drug is used in the treatment of psoriasis. In both cases the skin appears with reactions of redness and eruptions of blisters.

Photodermatosis should not be confused with general skin irritation caused by the sun. Within 20  minutes of direct exposure to strong  sunlight, most skin types will show signs of inflammation, like reddening of the skin accompanied by a burning sensation. Treatment for photodermatosis should be undertaken by a dermatologist after the cause of the light and sun sensitivity is identified.