Photographer and climber Cory Richards was evacuated by helicopter from Everest Base Camp yesterday with what at first appeared to be altitude sickness but is now more likely to be pulmonary embolism, blockage in his lungs, likely from a blood clot.

Richards, who is on Everest shooting for National Geographic and The North Face on an expedition commemorating the 50th anniversary of the first American ascent, was at 23,000 feet on Saturday afternoon when he began experiencing shortness of breath. He retreated to Camp 2 at 21,200 feet, where doctors gave him oxygen. They called for a helicopter to pluck him from C2, but deteriorating weather conditions prevented the flight, so Richards continued to Base Camp at 17,600 feet mostly under his own power.

At Base Camp, doctors gave Richards blood thinner and received word that clouds would prevent a rescue flight there, too. But Italian climber Simone Moro, a friend who made the first winter ascent of Gasherbrum II with Richards, helped find a pilot who was willing to risk an attempt, and Richards was evacuated to Lukla. He was accompanied by National Geographic photo editor Sadie Quarrier, who jumped in the helicopter with him at the last minute.

“I hope it’s OK I left Base Camp,” wrote Quarrier from a borrowed laptop in Lukla. “I just went with my gut and thought someone he knows should be with him. I would have wanted the same. I hope we’ll both be getting back to Base Camp before too long. Day by day.”

This morning, Richards was moved to Kathmandu.

Richards became widely known after his climb of GII and the powerfully rendered film he shot, Cold, which chronicled the ascent, his doubts, and an avalanche that struck the team of three: Cory, Moro, and Denis Urubko. His health issues put the 50th anniversary expedition in limbo: His partner Conrad Anker won’t climb alone and it’s not clear who might accompany him.

“Conrad feels it’s very important to climb with someone you know well and trust,” said National Geographic editor Peter Miller said. “In short, we won’t know how this affects the climb until Conrad tells us.”

Photo by Corey Richards, portrait by Frank Kretschmann