These days people are spending a lot of time in the sun, strolling around the World Expo site, queuing for hours for pavilions and generally enjoying the outdoors.

Everybody knows about sunburn and the need for taking precautions with broad brims, umbrellas and sun block. But a more serious problem for some people is solar dermatitis - sun rash, sometimes called sun poisoning. (And don't forget skin cancer.)

Precautions are necessary not just on sunny days, but on cloudy days as well, because UVB radiation penetrates and burns and rashes are possible.

People with sun allergies (photosensitivity) should be especially careful because solar dermatitis can take many unpleasant forms, such as hives and blisters. These can be hot, itchy and painful, and they can get infected, ooze and lead to other problems.

Solar dermatitis varies from person to person and depends on skin condition and color, strength of light and length of exposure. People with pale skin are more allergic and susceptible than others.

In severe cases it can cause fatigue, fever, heart palpitations and vomiting, according to Dr Chen Jie, associate physician at Yueyang Hospital attached to Shanghai University of Traditional Chinese Medicine. Some severe cases can trigger the herpes simplex virus and have been linked to lupus sebaceous.

Some case may heal in two or three days, while others take longer.

"Sensitivity to sunshine differs for different people," says Dr Chen, "Most people won't get burned unless they are exposed to strong sunshine for more than an hour, but allergic people may break out in rashes in only 15 minutes."

Some foods and medications (such as sulfanilamide antibiotics) can make the skin more sensitive to the sun. Dr Chen advises people with light-sensitive skin to avoid foods such as mud snails, celery, Chinese milk vetch, amaranth and cabbage.