My understanding is the same as Mary, IIRC a heavy smoker is effectively starting at a few thousand feet when at sea level!
From an Avmed article:-
Smoking. Smoking makes an individual more liable to suffer from hypoxia due to binding of haemoglobin with Carbon monoxide present in the smoke. A smoker who smokes prior to sortie has already compromised him-/herself to hypoxic insult, where s-/he is at an apparent altitude of 7,000, 14000, 22000 ft as compared to a non-smoker pilot at sea level, 10000 and 20000 ft, respectively. Be informed that if one smokes three cigarettes before a sortie, it is as if s-/he is already at an equivalent altitude of 8000 feet, with its implications due to compromised vision.