Did you know that the oxygen concentration of air doesn’t change at altitude? It’s always 20.9 per cent on Earth, but as air becomes less dense, there are less oxygen molecules to breathe. Climbing Ben Nevis to 1,344m above sea level (as our writer Matt Ray did – p52) you wouldn’t really notice the difference. But above 2,000m you might. People have died from altitude sickness as low as 2,700m before. Above this altitude, you want to be cautious advancing more than 500m of vertical elevation per day – some experts recommend as little as 300m. On the 8,848m summit of Mount Everest there is only a third of the oxygen molecules as at sea level.