<Li> 50% is below 5.6 km (18,000 ft). </Li> <Li> 90% is below 16 km (52,000 ft). </Li> <Li> 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line . By international convention, this marks the beginning of space where human travelers are considered astronauts . </Li> <P> By comparison, the summit of Mt . Everest is at 8,848 m (29,029 ft); commercial airliners typically cruise between 10 km (33,000 ft) and 13 km (43,000 ft) where the thinner air improves fuel economy; weather balloons reach 30.4 km (100,000 ft) and above; and the highest X-15 flight in 1963 reached 108.0 km (354,300 ft). </P>

Soil air usually has a higher carbon dioxide content than the air in the atmosphere