How best to breathe up a mountain?
This week marks the 60th anniversary of the first complete ascent of Mount Everest by Edmund Hillary and Tensing Norgay in 1953. But few people know that had it not been for the failure of one of two competing designs of breathing equipment - a completely different pair of climbers could have made it to the top first.
Here's your Quickfire Science from Hannah Critchlow and Kate Lamble.
The summit of Mount Everest is 8848 meters or 29,029 feet high, that's higher than some aeroplanes fly. At this altitude there is only a third of the oxygen available at sea level.
At higher altitudes, the same percentage of oxygen is found in the air, but the drop in overall atmospheric pressure means there is in effect, less air overall.
Above 8000 meters is the so-called 'death zone' where the amount of oxygen is not sufficient to sustain human life, even at rest.
Before the first ascent of Everest mountaineers typically used an open breathing system to deliver extra oxygen and make the climb easier. This system works by adding oxygen to the air the climber is breathing in, and their exhaled breath is released around them - like in scuba diving.
However one British Scientist Tom Bourdillon, thought that the system was wasteful as we absorb less than a fifth of the oxygen we breathe in. In an open system this oxygen we breathe out is lost.
In 1952 he developed a closed 'rebreathing' system which recycled the expelled climbers breath and passed it over soda lime to remove the carbon dioxide.
More oxygen was then added to the air, multiplying the amount the climber could absorb before it was breathed in again.
On the 1953 Everest expedition several groups of climbers made an attempt on the summit simultaneously. This included Tom Bourdillon and his climbing partner Charles Evans as well as Edmund Hillary and Tenzing Norgay.
Using the new re-breathing system, Bourdillon and Evans ascended Everest at record rate - climbing 1,500ft in 90 minutes. The same stretch took Hillary and Tenzing more than two hours.
Eventually they came within 90m of the summit of Everest but a problem with their new oxygen system forced them to turn back. No-ones sure what made the equipment fail but it could have been an air leak, or frozen valve
Three days later Edmund Hillary and Tenzing Norgay reached the summit using their open breathing system and became household names.
Since Bourdillon and Evans failed in their ascent, open systems have continued to be favoured for mountaineering but scientists are now using Bourdillon's design to develop medical devices which could help patients with obstructive lung diseases.