Aug. 2, 2011 -- The family of a 20-year-old British man who died as a result of a blood clot that formed after playing video games for up to 12 hours a day is speaking out about the health risks obsessive gaming can pose.

David Staniforth told The Sun that his son, Chris, spent most of his days playing the online game Halo and was accepted into a game design program at Leicester University.

"He lived for his Xbox. I never dreamed he was in any danger," Staniforth said.

The young man died in May from a deep vein thrombosis, the coroner told The Sun. The night before he died, his father told the BBC he was probably up all night on his computer.

"He had probably been on all night, on the computer at his desk, on Facebook or gaming -- one or the other," Staniforth said. After that, Staniforth said his son's friend said Chris felt a pounding in his chest but eventually fell asleep.

The next morning, Chris and his friend were going to apply for jobs and Chris collapsed outside the job center.

A deep vein thrombosis is a clot that forms in the deep veins in the legs. When the clot breaks off, it can move through the bloodstream and cause blockages. One the more common is a pulmonary embolism, which happens when there's a blockage of the main artery that carries oxygenated blood to the lungs. If it's not treated, up to 30 percent of people with pulmonary embolism die, according to the U.S. Centers for Disease Control and Prevention.

One of the biggest risk factors for developing blood clots that could eventually break off and travel to the lungs, heart or other vital organs is prolonged sedentary activity.

"Playing video games, long car rides and long plane rides predispose you to clots," said Dr. Phil Ragno, director of cardiovascular health and wellness at Winthrop University Hospital in Mineola, N.Y.

He warns that even though older, less active people are prone to blood clots because of more complicated medical histories, they can strike anyone of any age.

"We're seeing now that one of the biggest recreational activities is playing video games instead of being outside and being physically active," he said.

But tennis ace Serena Williams proved that even the most physically fit are still at risk. Back in March, Williams underwent emergency treatment for a blood clot that doctors say could have resulted from recovery from foot surgery and a long flight from N.Y. to L.A.

Other risk factors include obesity, smoking and hormone replacement therapy. Women who are on birth control are also at higher risk for blood clots, although the overall risk is still low. Pregnant women are also at higher risk for blood clots. Risk is elevated if women have a history of bleeding problems.

Anyone who is sitting or standing in the same position for a long time should take measures to move around, even if it's just for a few minutes.

"Try to stop every hour or so and just walk around for a few minutes to keep the blood circulating," said Ragno.

People should pay attention to symptoms of a blood clot that usually occur in the legs, including tenderness or pain in the calf, warmth, redness and swelling.

There are also serious signs of a pulmonary embolism to watch for, such as shortness of breath, rapid heart rate, a stabbing pain in the chest and an unexplained cough.

David Staniforth knows just how serious these medical problems can be, and he wants other young men and women to learn from his heartbreaking loss.

"Don't stop your child from playing the games. They love doing it, it's great for them," said Staniforth. "Just be aware. Enjoy it, but take a break."