Altitude sickness (also called Acute Mountain Sickness or AMS) is a sickness linked with the low oxygen in the air at high altitudes. The sickness is often brought on by mountain climbing or travel in un pressurized aircraft. Persons with altitude sickness may feel dizzy, crabby, breathless, blissful, or having a headache. Older people and those with lung or heart disorders may suffer breathing problems, heart failure, or fainting, needing emergency treatment. A long-term form of altitude sickness causes a higher red blood cell count, which results in blood that is thick and does not move easily through the blood vessels.