Answer:
High red blood cell count may be caused by low oxygen levels, kidney disease or other problems.
Low oxygen levels
Your body may increase red blood cell production to compensate for any condition that results in low oxygen levels, including:
Heart disease (such as congenital heart disease in adults)
Heart failure
A condition present at birth that reduces the oxygen-carrying capacity of red blood cells (hemoglobinopathy)
High altitudes
COPD (chronic obstructive pulmonary disease) worsening of symptoms
Pulmonary fibrosis (scarred and damaged lungs)
Other lung diseases
Sleep apnea
Nicotine dependence (smoking