Each year climbing deaths occur on Mont Blanc, and on the busiest weekends, normally around August, the local rescue service performs an average of 12 missions, mostly directed to aid people in trouble on one of the normal routes of the mountain. Some routes require knowledge of high-altitude mountaineering, a guide (or at least an experienced mountaineer), and all require proper equipment. All routes are long and arduous, involving delicate passages and the hazard of rock-fall or avalanche. Climbers may also suffer altitude sickness, occasionally life-threatening, particularly if they do not acclimatize to it.