: By how much does former smoking reduce one's lifespan? If someone in their late teens/early 20s smoked 20 cigarettes a day for 5-10 years, and then never smoked again, by approximately how much would that reduce their lifespan?

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This has to be a speculative answer. Putting a Number on Smoking’s Toll?
Important is this:

People who quit between 25 and 34 years of age gained about 10 years of life compared to those who continued to smoke.

Positive changes are not to be discounted. And stopping reduces risks.

Many current and former smokers want to know their risk of developing lung cancer in numbers. For example, some people want information such as "I have a 10% chance of developing the disease." Assigning a number to risk is very complicated and is often hard to interpret – while one person may think 10% is a high chance, another thinks that is a relatively low number. And for the person who is in that 10% and develops the cancer, the number is meaningless. Remember that statistics like these are numbers based on large groups of people. It can be difficult to translate what that means for any one individual. In other words, don't let the number convince you that it is okay to continue smoking.


5 years after quitting
Your risk of stroke is reduced to that of a non-smoker 2-5 years after quitting. The risk of cancer of the mouth, throat, esophagus and bladder is cut in half after 5 years. (US Surgeon General's Report, 2010)
10 years after quitting
The lung cancer death rate is about half that of a person who is still smoking. (US Surgeon General's Report, 2010)
15 years after quitting
The risk of coronary heart disease is that of a non-smoker's. (US Surgeon General's Report, 1990)

These are findings just about a subgroup of risks and they are presented to encourage people to stop smoking. Since reduction of smoking or smoking cessation seems always beneficial, this is much more motivating and indeed a positive outlook. The longer you live after you stop smoking, the stronger the reduction of risks related to smoking.

But that is not precisely what is asked for here. Calculating the reduced life expectancy by simply subtracting the risks or reductions mentioned above does not work very well. (Of course you can always do the math, but numbers can get meaningless.)

The calculation of expected negativity just criticised may be done as follows:
Time for a smoke? One cigarette reduces your life by 11 minutes:

10 years x 365 days x 20 cigarettes = 73000 cowboy moments

73000 cm x 11 rf = 803000 minutes lr

Exact calculations for the risks of an individual are impossible. Also keep in mind that the above calculation is based on just one paper, that only assigned and calculated numbers. That is quite different from measuring it. And applied to everyone the above formula is very imprecise, since there are many contributing factors, like age of smoking initiation, simply left out.

To look at the first quote from another perspective:

Life Expectancy


If a smoker quits before age 35, their life expectancy is the same as non-smokers.
If a smoker quits between the age of 35 and 65, add 5 years to their life expectancy as compared to others who continues to smoke.
If a smoker quits between the age of 65 and 74, add 1 year to their life expectancy as compared to someone who continues to smoke.

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I know a man who is now a healthy 84 years old and still going strong and still works at his small business store. He smoked for 30 years, I think up to 2 packs a day. When he quit, he was a health nut and continues to be a health nut -- lots of vegetables for dinner, no alcohol, no smoking, no drugs. His only vice now is sugar -- candy bars and ice cream.

I also know of a 85 year old man - a math professor - who actually still smokes everyday (in his office, too!) and is still kicking and going strong and teaching and working on a new book.

So, I'd have to say that genetics probably* plays a huge* role in determining whether you will land up with health issues / cancer from smoking. If you are predisposed to cancer / did not win the genetics lottery, you will suffer the consequences of smoking more so than others who may have "better" genetics than you.

Bottom line: look ahead, move forward in time. Don't beat yourself up over your past vices. Take steps toward living a healthier life.