During the U.S. Open at Oakmont Country Club, people should be alert to skyward dangers, including golf balls. If someone yells "Fore," duck.

But Dr. John M. Kirkwood is concerned about an even greater danger in Oakmont skies, prompting him to utter a variation on the classic golf-ball warning: "Ten to 4!"

An expert in skin cancer, Dr. Kirkwood said people attending the tournament should consider the sun as the most life-threatening force overhead, golf balls included. Of concern are basal-cell and squamous-cell carcinomas, and deadly melanoma, all caused by exposure to ultraviolet radiation.

So he recommends avoiding direct sunlight between 10 a.m. and 4 p.m., when the sun is at a 45-degree angle or higher. When your shadow is equal to or shorter than your actual height, stay out of the sun.

"Between 10 a.m. and 4 p.m. is the window of time when the sun is most directly overhead and causes greater damage to the skin," Dr. Kirkwood said.

During those hours, don a hat, sleeves and trousers or remain in the shade. The body's need for vitamin D, which the skin manufactures from sun exposure, can be generated in minutes, he said.

Dr. Kirkwood, director of the University of Pittsburgh Cancer Institute's Melanoma and Skin Cancer Program, is an admitted heretic when it comes to sun worship. The incidence of melanoma is on the rise for all ages, with a worrisome trend among the young.

About 50,000 cases of melanoma are diagnosed each year, with 8,000 deaths for a 16 percent mortality rate. That does not include 50,000 to 70,000 Americans who have melanoma in situ, which is a mole gone bad but yet to spread into melanoma, he said.

Those statistics do not include less dangerous squamous-cell or basal-cell carcinomas that must be removed by a dermatologist to prevent problems.

While sunscreen reduces sunburn, caused by sun-damaged blood vessels, there is no evidence it prevents melanoma, which is caused by ultraviolet radiation penetrating deep into the skin, Dr. Kirkwood said.

Research shows why people worship the sun. Not only does it provide a tan, created when sunlight strikes pigment-producing cells known as melanocytes, but it also causes the release of endorphins -- compounds that reduce pain and create an opiate-like feel-good effect.

"When you sit in the sun, the endorphin response makes you feel better, and that's the attraction," he said. "They are getting an endorphin surge, a feel-good surge, that is part of the sun-tanning process."

Dr. Kirkwood questions whether endorphins are the major reason people lie in the sun or visit tanning salons. "That is why people are addicted to sports and sunbathing and turning their skin into shoe leather," he said.

Melanoma, which can strike at a young age, takes away more productive years of life than any other cancer, save for pediatric cancers, and is more common among women 25 to 35 years old than breast cancer, he said.

The good news? Melanoma, which can begin with a mole that turns ugly, is the easiest cancer to spot. People with moles should inspect them regularly, or have someone else make sure they do not change shape or color. New treatments are improving outcomes, he said.

But the incidence of melanoma continues rising in all ages, including the 45- to 50-year-old crowd that will attend the U.S. Open. Dr. Kirkwood said they should be particularly alert to the risks.

"We know that avoiding sun exposure is the best way to prevent melanoma: People with less exposure have a lower incidence," he said. "Make sure you have a hat, a parasol and sleeves and do what you can to block the sun, and if you have to be out there, sunscreen makes sense to prevent sun damage."

And avoid looking upward at Oakmont. Nothing there but sunshine and golf balls.

When you sit in the sun, the endorphin response makes you feel better, and that's the attraction.

-- Dr. John M. Kirkwood

"