If you're new to exercising, this is fairly normal. You may be "healthy", but are probably not fit and smoking is definitely not helping. No matter how slowly you start, it is still a new stress to the body and it has to adjust. First, make sure that you look ahead while walking, not down. That will cause problems with your equilibrium and induce dizziness after you're off the treadmill. Also, don't eat *immediately* before going to the gym. Give yourself 30 to 60 minutes so that you don't have a full stomach and have a chance to digest before exercising. Next, try to avoid smoking for a few hours before going ( or try quitting
) so that you are not causing further problems with oxygen uptake, which can also cause dizziness and nausea. The fact that your heart rate is in the 160 to 170 range at a walking pace might be cause for concern, however. That's pretty high for a minimum of intensity. If you have been sedentary up until now, that might help explain it, but you might try slowing it down a bit so that your heart rate isn't so high. If you stay consistent with it, it will become easier to do and take longer to get your heart rate up. If you have the means, you might consider checking with a doctor just to make sure there are no other underlying issues. Just get blood pressure, basic physical and basic blood work.