There is significantly less oxygen available in the high mountains so it is not surprising that traveling to high altitude often causes people to feel sick, but how this shortage of oxygen actually leads to altitude sickness is still not fully understood. Some scientists believe that it is due to swelling of the brain but the evidence for this hypothesis is not conclusive. The theory is that in susceptible persons, swelling could cause a small increase in the pressure inside the skull and lead to symptoms of acute mountain sickness. The swelling may be due to increased blood flow to the brain or leakiness of blood vessels.