Studies have shown that smoking increases airway resistance. That means that the energy and oxygen cost of breathing increases, as much as
threefold, leaving less oxygen to be used for other activities. This
seems to be a combined effect of particles in the smoke and nicotine working on the nervous system. Yet another problem arises from a third
component of smoke (in addition to tar and nicotine), and that component
is carbon monoxide.
Inhaled carbon monoxide travels through the
bloodstream attached to hemoglobin, the same as oxygen. The problem
is the carbon monoxide gets there first, blocking out the oxygen, so there is less oxygen delivered to the working muscles. Thus, the
overall effect is that the smoker has less oxygen available for metabolism than would be available if he or she didn't smoke.
Fatigue sets in, endurance is lessened, and performance suffers. (By the way, carbon monoxide is the same stuff that comes out of the tailpipe of your car and, yes, it is lethal in large enough
doses. But so is nicotine.)
The good news is that these short term effects really are short term. Studies have shown that oxygen uptake and performance improves after as little as 24 hours abstinence from cigarettes. Any improvement an athlete feels he or she achieves through the
stimulating (or calming) effect of a cigarette is purely psychological.
There's no need to dwell on the long-term effects smoking has on performance.
You don't have to be a rocket scientist to figure out
that heart disease, lung cancer, or emphysema isn't going to help your
10K time.
HEALTHandFITNESS.com 
