When I lived in Flagstaff, AZ at 7000" elev. I would occasionally go to Phoenix (1000') for medical exams. My blood work would always come back with too many red blood cells (RBC) per ml of blood. When I explained that I lived in Flagstaff I would get a pass on what would otherwise be considered an abnormal count. My RBC is still above average for a person in their mid 70"s even though I live at only 3400' elev. now. It may be genetics or it may be a residual of having lived at high altitude for six years; I suspect that it is genetics even though my mother suffered from altitude at anything over 4000'. I have worked at being above average fitness for my age most of my life.