Spain's Edurne Pasaban, the first woman to have climbed the world's 14 highest peaks, said Thursday she now wants to become the first female to climb Mount Everest without bottled oxygen. Pasaban, 37, became the first woman to scale the 14 mountains above 8,000 metres (26,245 feet) in May 2010 after she reached the summit of 8,027-metre (26,489-foot) Shisha Pangma, situated in China's Tibet region. But she told a Madrid news conference that "something was missing" because her team used supplementary oxygen when they scaled Mount Everest, the world's highest mountain, in May 2001. "This is why I have come out with this 'Challenge 14+1', which is Everest without oxygen," she said. Only 132 people have managed to scale the 8,848-metre Mount Everest without oxygen, all of them men, Pasaban said. "This is an ambitious project," she said.Pasaban said she will head to the Himalayas with the rest of her team on Monday and hopes to start her climb of Everest "after May 20, and if possible on May 23" -- on the anniversary of her first successful climb of the mountain a decade ago. About 50 expeditions, involving between 200 and 300 people, are expected to try to climb Everest from the southern, Nepalese side during this time. Pasaban said she was worried about congestion during the final stage of the climb, which includes the final ridge, a 13-metre rock known as the Hillary Step, because only a handful of people can pass at a time. During her climb of Everest in 2001, Pasaban said she had to wait around 45 minutes for a Chilean team to pass. "I had oxygen and there was no problem, but if we had to wait three quarters of an hour for someone to climb or descend the Hillary Step, we will run out of oxygen," she said. The region above 8,000 metres is known as the "Death Zone" because of the thin air and treacherous conditions. Italy's Reinhold Messner was the first climber to conquer Everest without supplementary oxygen in 1978. Messner was also the first to scale all 14 mountains above 8,000 metres.