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Itchy, swollen, red rashes when you’re outside?

Khoo Bee Khim

CNA – So, there you are, exploring one of the island’s green corridors, chilling on the beach, or maybe just doing a quick coffee run in the afternoon sun – and your skin starts to develop wheals or swollen,red rashes.

It’s just a slight sunburn, you think, although the textured patches are uncharacteristic of too much UV exposure. But soon, they begin to sting, itch and burn.

What is going on?

It could be solar urticaria or what is known as sun allergy. Occasionally, the wheals can blister and become crusty, said dermatologist at Eileen Tan Skin Clinic and Associates Dr Eileen Tan said. “They usually appear within minutes of sun exposure,” she said, adding that the disease usually manifests in those in their mid-30s and is more prominent among females.

The term “urticaria” refers to the hives or itchy, raised red areas on the skin caused by a reaction to certain foods, drugs, infections, stress – and in the situation of solar urticaria,
UV rays.

Solar urticaria is rare, according to senior consultant at the National Skin Centre Dr Eugene Tan. “It is estimated to comprise approximately 0.5 per cent of all cases of urticaria. On average, I see one patient with solar urticaria every two months.”

Dr Eileen Tan agreed. “In my experience, solar urticaria is uncommon. I see maybe three to five cases per year.”

Still, there is a chance you might have solar urticaria if you tend to break out in the sun.

You might have just ignored those episodes as they typically resolve “within a few hours when sun exposure is avoided”, said Dr Eugene Tan.

There’s also the possible likelihood that the hives are a reaction to Singapore’s infernal heat. When, then, do you break out the prickly heat powder or see a dermatologist for a sun allergy? Here’s what the dermatologists said.


One key distinction is, solar urticaria tends to occur on sun-exposed areas within a few minutes and even up to 30 minutes of sun exposure.

“It depends on the individual level of sensitivity to sunlight and the time of the day in which sunlight exposure occurs,” said Dr Eugene Tan. “Since the mid-day sun (between 10am and 4pm) is more intense, it will take a relatively shorter time of exposure to induce urticaria in the early morning or evening sun than when exposed to the mid-day sun.”

On the other hand, heat urticaria or heat rash is likely to appear in areas where sweat tends to accumulate such as the back, bra line and armpits – and is the result of temporarily blocked sweat glands, she added.

While both can cause itchiness, their physical appearances are different. Heat rash resembles small, inflamed blister-like bumps that can occasionally be filled with pus, said Dr Eileen Tan.

Moreover, heat urticaria can develop in a hot environment with or without sunlight; for example, taking a hot bath or being in a sauna, said Dr Eugene Tan.


The exact cause of solar urticaria is not known, said Dr Eugene Tan. “One hypothesis is that some individuals have a precursor molecule in their skin that is converted to a photoallergen or sun allergen when exposed to a specific wavelength of light,” he explained.

When the immune system detects these photoallergens, it mistakes them as dangerous invaders and sends out antibodies, in this case immunoglobulin-E or IgE, to bind to the photoallergens. This then triggers urticaria, said Dr Eugene Tan.

As for the radiation spectrum that causes solar urticaria, it varies from patient to patient, said Dr Eileen Tan. It can range from invisible ultraviolet B (or UVB) to visible light, or from 300 nanometres to 500 nanometres. For this reason, you could potentially develop solar urticaria even when indoors if you work next to an exposed window, said Dr Eugene Tan.

Some medications can also trigger the disease, added Dr Eileen Tan, including atorvastatin (statin medication for cardiovascular disease), chlorpromazine (antipsychotic medication), tetracycline (oral antibiotic) and oral contraceptives.


Sun protection is key to healthy skin but it is even more important if you suffer from solar urticaria, said both dermatologists.

“Patients are often able to avoid precipitating their urticaria because from experience, they know the amount of sun exposure that is needed to trigger their hives,” said Dr Eugene Tan.

Other than the use of a broad-spectrum sunblock, clothes and accessories that offer UV protection, the first line of medication is antihistamines. They can be taken around the clock for the first few days of a flare-up for symptom relief, said Dr Eileen Tan. “If needed, patients can take antihistamines 30 minutes before they go outdoors,” said Dr Eugene Tan.

Unlike eczema or other forms of skin allergy, oral corticosteroids are “rarely used” for solar urticaria, said Dr Eugene Tan, unless it is “a severe flare of acute urticaria”.

Other treatments that may be considered include injections of monoclonal antibodies that bind to and block IgE (omalizumab), oral immunosuppressant (ciclosporin or cyclosporine) and plasma exchange therapy.


According to Dr Eugene Tan, oral polypodium leucotomos (a fern extract) has been shown to confer UV-protective effects in patients with sun-sensitive skin disorders. A photo test may also be useful to patients to determine the provoking wavelength, he said. “For example, patients who are sensitive to UVA should look for broad-spectrum sunscreens with high UVA protection factor instead of only focussing on the SPF rating because SPF is a UVB protection factor,” he said.

In some cases, phototherapy to desensitise skin to UV rays may be used. “The gradual UV dose increment and increased tanning of the skin over time help to ‘harden’ the skin such that the patient will be able to tolerate a longer duration of sun exposure before urticaria is triggered,” said Dr Eugene Tan. “This can be achieved after two to three months of twice-weekly phototherapy.”

However, there are caveats to this method. “Phototherapy is more useful for patients living in temperate regions with seasonal climates,” he said. Added Dr Eileen Tan, “Phototherapy is often not a popular option as frequent, multiple visits are not feasible and convenient for working individuals.”

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