It barely touches the environment at all.
Stopping smoking makes *you* healthier (unless you're a caffeine addict, in which case, be very careful!) but the environment stays the same.
Just so you can compare, how much smoke does the family car give off in 5 minutes? Can you breathe it for 5 minutes? (No, don't try. People suicide by running their car in a closed garage for less than an hour.)
Now how much smoke does a smoker give off for a whole cigarette? Can s/he breathe that for 5 minutes (again, don't try, but find a smoker who will gladly demonstrate for you)? In fact, can this smoker do it for a whole hour? If s/he is a chain smoker than YES.
So there you have it. 60 minutes of cigarette smoking does hardly anything to the smoker (remember, s/he probably does more than 60 minutes of smoking EVERY DAY!), but breathing in 60 minutes of car exhaust will definitely kill you.
Taking 100 smokers off the street vs taking 100 cars off the road, I think you can see that taking 100 cars off the road will save us from a lot more pollution.