Cutting edge: Kenneth Baillie

Six Edinburgh University medical students were literally on top of the world when they researched why altitude affects humans.

"Breathtaking expedition. Inspired science. Brilliantly conceived." That is how Sir James Black, the Nobel prizewinning pharmacologist, described Apex Bolivia 2001, a medical research expedition to the Andes of Bolivia. Its six organisers, medical students at the University of Edinburgh, and its 20 experimental subjects were the subject of a Tomorrow's World special episode, "Mountain madness".

People die every year at high altitude because of fluid leaking out of blood vessels into the lungs and brain (high-altitude pulmonary oedema, Hape, and high-altitude cerebral oedema, Hace). These illnesses can kill within hours, but we do not fully understand what causes them. The only reliable treatment for them is immediate descent. A separate and bizarre phenomenon, high-altitude cough, can be severe enough to break climbers' ribs, but is also as yet unexplained.

What is happening when our bodies fail in the mountains? Why can only a week of acclimatisation enable a person to climb a mountain under his own steam, when if they had been dropped on the summit they would have died within minutes? Why are some people more susceptible to this illness than others? And what causes blood vessels to leak fluid into the lungs and brain?

During a holiday to South America, I met Chilean altitude physiologist Claus Behn. He showed me the Chacaltaya cosmic physics laboratory. At 5,200m, Chacaltaya is the highest laboratory of its kind in the world. This was an opportunity not to be missed. From March 2000, we worked hard to make our expedition happen.

Eager to generate as much research as possible from our labours, we studied the volunteers intensively, taking frequent blood samples and measurements of cognitive function, breathing and cough responses. In the blood, we looked for dangerously reactive molecules called free radicals that are released when cells lack oxygen. Free radicals are like bullets ricocheting around cells, causing damage at random. These bullets can damage the lining of blood vessels, allowing fluid to leak into the lungs and brain. This may have tragic consequences for the climber who develops cerebral oedema.

The body has a natural defence against free-radical damage in molecules called antioxidants. Antioxidants, including vitamins A, C and E, react with the free radical before it has a chance to do any damage - they could be thought of as a bulletproof vest for cells. These antioxidants may be a crucial defence against altitude illness. One of our most exciting results shows that this bulletproof vest became stronger with acclimatisation to the altitude. We found more antioxidants after five days at the extreme altitude of Chacaltaya. This could mean that the body is protecting itself from free radicals. If this is true, exactly how did the rise in antioxidants come about? The implications of this question may extend beyond altitude medicine. Antioxidants are important at sea level, preventing heart attacks and strokes.

We also moved one step closer to solving the mystery of high-altitude cough. The cough is not due to the cold, dry air. Mapping the propensity to cough over the expedition, we found that as the volunteers acclimatise their cough did not improve.

This was the first British medical expedition to Chacaltaya. It moved us closer to finding the causes of mountain illness, and it has raised new questions about human physiology. We hope that it will not be the last.

Kenneth Baillie led the Apex Bolivia expedition.