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Cases of photosensitivity can range from mild to severe - and milder cases are not always obvious.

"Developing redness of the skin is a normal response following excessive exposure to sunlight. Abnormal sensitivity to light or a sun allergy occurs when a person's response to sunlight falls outside the ranges for healthy subjects. This includes the development of redness at lower doses of sunlight than normal," explains Dr Daniel Glass from The Dermatology Clinic London.

What is polymorphic light eruption?

Whilst there are several different types of photosensitivity, polymorphic light eruption (PLE) is the most common, and it can present in different ways.

"Polymorphic means many forms, meaning that the rash can have many different appearances. The term 'light eruption' means the rash presents following exposure to sunlight. PLE can manifest as itchy small red bumps, larger red areas, and blisters," explains Glass.

As well as direct sun exposure, the rash may occur if a person is exposed to sunlight through a glass window and even sometimes fluorescent lighting. It usually appears within a few hours of exposure, and may occur in as little as 20 minutes.

A variant of polymorphic light eruption, called juvenile spring eruption, specifically affects the upper edges of the ears (the helices). It usually affects males in childhood or early adulthood and most often occurs in early spring. Classically, it's seen after strong sun exposure in relatively cold weather - later the same day, the helices become red and itchy, and a day or two later, blisters appear. Fortunately it tends to settle on its own within a couple of weeks. However, it's a good idea for people affected to wear a hat that covers their ears in spring and possibly summer for the next few years.

Who is affected?

Whilst photosensitivity may occur in people of any skin type, those with lighter skin are more commonly affected and especially a skin type known as: "Fitzpatrick skin phototype 1 - those people who have red hair, have freckles and burn easily in the sun," explains Glass. "It can affect anyone at any age, but is most common in women between the ages of 20 and 40 and tends to be more common at high altitude and in the spring."

Although the cause of the condition can sometimes be genetic, there are many other potential causes of photosensitivity.

"Certain drugs and chemicals can cause sensitivity while being applied to the skin or when taken by mouth," explains Glass. "The list of drugs and chemicals that can cause light sensitivity is long and includes various antibiotics, antifungals, cardiac drugs, and psychiatric drugs - to name a few." Topical anti-inflammatory creams and gels can also cause photosensitivity.

It is particularly worth bearing the risk of photosensitivity in mind if you're looking at antimalarial prevention tablets, which are almost exclusively needed in sunny parts of the world. Doxycycline, an antibiotic offered as an option for malaria prevention for some areas, can lead to photosensitivity.

What does the rash look like?

Whilst the rash may appear similar to sunburn, in the majority of cases it can be distinguished from burning by the initial presentation and progress of the rash.

"The most common rash is smooth-topped red bumps on the skin, which can coalesce into plaques. This is different to the classical flat red sunburn. The rash often persists for several days, clearing up without scarring if further sun is avoided. If the affected area is exposed to more sun the rash will get worse and spread," explains Glass.

What should I do?

If you're worried you or your child might be affected, it's best to talk things over with your GP, who will refer you to a dermatologist if necessary. Depending on the severity and your circumstances, a number of different treatments may be prescribed. Clearly, if you're taking any medication, it's important to speak with your pharmacist to see if that might be the cause. If it could be, see your GP - the solution might be as simple as stopping your treatment or changing to a different medicine.

"The rash can be treated with a corticosteroid cream or ointment, which may help it settle more quickly. Mild PLE may be controlled by sun avoidance and use of high-factor UVB and UVA sunscreen. More severe PLE may require oral steroids," explains Glass.

"In some cases, desensitisation treatment can be helpful. Desensitisation is a way of increasing the skin’s tolerance of sunlight by introducing slowly increasing doses of ultraviolet light in a special phototherapy cabinet. The treatment is started in early spring before the sun becomes strong enough to be problematic. The desensitisation needs to be repeated each year, supervised by a dermatologist."

Whilst photosensitivity can make enjoying the sun more difficult, taking precautions and getting the right treatment should prevent it from blighting your summer.