Montgolfier reasoned that it would be safe for humans to ascend to altitude. Over 200 years have passed since that first flight. This zone extends from sea level to 12,000 feet. When humans are exposed to altitudes above this zone, they are subjected to physiological hazards beyond their natural ability to adapt. One thing to keep in mind is that everything that occupies space and exerts weight is considered to be matter.
All matter is made up of atoms and molecules in varying densities.
These particles within the matter are kinetic and in constant motion. The slower the motion of the particles, the more dense the matter becomes. Also, as the particles are pushed closer together, the matter also becomes more dense. The best way to slow down kinetic molecules is to cool the matter. The best way to get them to move closer together is to add pressure to the matter.