Sherpas people have a higher concentration of haemoglobin in their blood than the general population. Since the 1920s, Sherpas have been acting as guides and porters for climbers. They are physically fit, resistant to hypoxia, hard-working and many have been trained to speak English and have mountaineering skills, and almost every climbing team includes Sherpas. They have achieved the "three most" at the cost of their lives: the highest number of successful climbs on Everest, the highest number of anaerobic ascents and the highest number of people killed on Everest (around 60). The Sherpas have made a significant contribution to the history of Everest climbing. Guiding and logistical services for climbing teams from all over the world have become one of the Sherpas' main sources of income.