Genetic mutations, both inherited and acquired, are one primary cause of high red blood cell counts, also known as polycythemia, says MedicineNet. Sleep apnea and tumors are secondary causes of polycythemia.
Other secondary causes of a high red blood cell count include smoking, kidney transplants, and living at higher altitudes where the air has less oxygen, according to Mayo Clinic. Anabolic steroids, dehydration and carbon monoxide poisoning are additional causes.
Polycythemia vera is a genetic mutation that can lead to high red blood cell counts, says MedicineNet. It is very rare and may be accompanied by an enlarged spleen and higher white blood cell and platelet counts. Mutations to the erythropoietin receptor gene also cause primary familial and congenital polycythemia.
Red blood cell counts sometimes increase to make up for decreased heart or lung function that has been providing lower oxygen levels, notes Mayo Clinic. Kidneys may also release too much erythropoietin, which ramps up red blood cell counts. Losing blood plasma frequently leads to increased levels of red blood cells.
For men, more than 5.72 million red blood cells per microliter is too high, states Mayo Clinic. For women, the figure is more than 5.03 million red blood cells per microliter. With children, the numbers depend on age and sex.