Figure 1

Nocturnal cough frequency in 10 subjects during a trek to Mount Everest Base Camp, Nepal (5300 m), including 3 climbers in whom recordings were made at 7000 m on Mount Everest. Recordings were only possible in 3 subjects at between 4 and 5000 m due to logistical problems. The circled data point represents a climber in whom the recorder suffered battery failure after only 1 hour. Data plotted from [6].