The intense heat of summer and the glare of the sun can cause problems for people who experience cold and canker sores.

The same ultraviolet rays that cause sunburn can trigger an outbreak of cold sores. Scientists are not yet sure how UV rays promote cold sores, but they believe the rays are responsible for lowering the immune response of exposed skin cells.

A recent study by the National Institute of Health may shed some light on UV rays and canker sores. In the study, the lips of 38 people with a history of canker sores were exposed to 80 minutes of UV light on two separate occasions. At one session, a sunscreen was applied to the participants' lips; in the second session, a placebo ointment was used.

Without sunscreen, 71 percent of participants developed a cold sore within one week; with sunscreen, not one subject experienced an outbreak of cold sores.

"Dentists have long advised patients to protect their lips by using protective lip balms, not just in the winter, but in the summer too," says Vicki Grandinetti, D.D.S., a general dentist who practices on Chicago's Northwest Side. "People who are very active during the summer should apply a sunscreen of at least 15 SPF or higher and they should reapply it frequently."

Triclosan, an antimicrobial agent added to some toothpastes and mouth rinses, has been shown to have anti-inflammatory and analgesic effects that help ease the symptoms of canker sores. Researchers believe that when triclosan dissolves in the oral cavity, it stabilizes and protects cells from irritating agents and bacteria that may cause canker sores.

Canker sores are small -- usually 1-2 mm -- and often characterized by white, painful, noncontagious, inflammatory lesions. They appear on the soft tissues and folds of the inner cheeks and lips, or the floor of the mouth. Though their cause is unknown, canker sores normally clear up on their own within 10-14 days.