What causes a high red blood cell count?

High red blood cell count may be caused by lifestyle factors or medical conditions such as:  

• Hypoxia or low levels of oxygen in the blood
• Congenital heart disease
• Lung diseases such as COPD, emphysema or pulmonary fibrosis
• Heart failure, which causes low blood oxygen levels
• Sleep apnea
• Exposure to carbon monoxide
• Smoking cigarettes
• Taking anabolic steroids or other performance-enhancing drugs
• Hepatocellular carcinoma, a type of liver cancer 
• Renal cell carcinoma, a type of kidney cancer 
• Polycythemia vera, a rare type of blood cancer in which the bone marrow produces excessive amounts of red blood cells