It's hard to believe that a marathon video game session could turn deadly, but that's exactly what it did to a 20-year-old man in the United Kingdom earlier this year.

Christopher Staniforth died this past May after an all-night gaming session. Like many nights before, he had been up most of the night playing his favourite game, Halo, against fellow players from around the world.

On the night he died, Chris complained of chest pains after his gaming session. He went to bed and the next morning, collapsed and died.

An autopsy found Chris had died from a pulmonary embolism. A blood clot had travelled to his lungs after forming in the veins of his leg.

"It was a complete shock," his father David Staniforth told CTV's Canada AM from Sheffield, England. He says his son had no pre-existing health issues.

Until his son died, Staniforth said he had only ever heard of blood clot in the legs – a condition called deep vein thrombosis – in the context of long airplane flights. He later learned that any bout of prolonged sitting puts one at risk of DVT.

"It really boiled down the fact the he was immobile in one place for any space of time. It didn't matter whether it was flying or playing on a game," he said.

Now, David Staniforth is launching a campaign to warn other parents and teens about the dangers of playing video games for too long. He's started a website called "Take Time Out" that urges gamers to step away from the fame once an hour to stretch and walk around.

Dr. Alan Bell with the Thrombosis Interest Group of Canada says while deep vein thrombosis doesn't get a whole lot of attention, it's actually a fairly common condition.

"It occurs in one or two per 1,000 per year – roughly the same rate as heart attack and stroke. However, it's always a shock when it occurs to a young healthy individual," he told Canada AM.

Older, less active, and overweight people are prone to blood clots because of their complicated medical histories, but the clots can strike anyone of any age.

Bell says most blood clots cause no symptoms, but occasionally there are a few to watch out for.

"If you develop DVT, you might have symptoms of swelling, pain and redness in the leg, which are very important symptoms that need to be treated immediately to prevent a pulmonary embolus," he said.

Bell says there are three things that put one at risk of deep vein thrombosis:

• Immobilization or stasis of the blood caused by sitting
• Trauma to blood vessels, such as during a break to the leg
• "Prothrombotic" conditions, such as inherited blood conditions, or pregnancy or cancer

Staniforth says Christopher was planning to go to university to learn to become a game developer. He knows his son loved gaming and wouldn't have wanted to give it up. His stepson too loves gaming, and now he offers him advice about how to avoid the condition too.

"Every hour, get up and move around. Get hydrated, take a drink. It's very much like the advice given on airlines: moving around, getting the blood circulating and keeping hydrated," he says.