Pack Years of Smoking and Health Risks

Pack years is a measure of how much you have smoked in your life. Despite what you might assume from the name, it is not the number of years you have smoked (although that is factored into the calculation).

One pack year of smoking is equivalent to smoking 20 cigarettes (one package) daily for one year. Someone who smokes a pack a day for, say, 10 years would not have the same number of pack years as someone who smokes two packs a day for the same duration.

Since lung cancer is directly related to the number of cigarettes smoked, physicians can use pack years to help assess someone's risk of the disease.

Pack years can also help doctors evaluate someone's risk of other diseases caused by smoking, including heart disease, other cancers, and more.

Researchers use pack years as an objective measure in studies on smoking and disease as well.

Lung cancer risk
Illustration by Joshua Seong. © Verywell, 2018. 

Calculating Pack Years

Two pieces of information are needed to determine someone's pack-year history:

  • The number of packages of cigarettes smoked daily (N)
  • The number of years of smoking (T)

Those numbers can be plugged into the following formula to determine pack years (PY):

N x T = PY

This table offers some examples of pack-year calculations.

 Smoking History  Formula Number of Pack Years 
 1 pack/day for 20 years  1 x 20  20
 2 packs/day for 30 years  2 x 30  60
Half a pack/day for 30 years  0.5 x 30   15

Lung Cancer Risk

In general, the more pack years you have smoked, the greater your risk of cancer. You would notice a virtually linear relationship between the two if you looked at a graph of related statistics.

In fact, number of pack years says more about your risk of cancer than the length of time you smoked.

Heart Disease Risk

The number of pack years someone has smoked is correlated not only with lung cancer but with heart disease as well.

Heart disease accounts for a large percentage of deaths in people who smoke, and secondhand smoke is more likely to lead to heart disease than lung cancer.

Pack Years and Former Smokers

If you quit smoking, you have lowered your risk of lung cancer compared to when you smoked. That is something worth acknowledging and celebrating. The effort to quit is always worthwhile.

That said, your risk of cancer has not been eliminated. Unlike heart disease, the risk of lung cancer persists for decades after you quit smoking and never returns to normal.

In an analysis published in the Journal of Medical Screening that looked at lung cancer in people with 30 pack years or more, risk was shown to reduce only gradually with each year of cessation. Researchers noted "no dramatic drop-off" after 15 years of quitting.

They went on to say that lung cancer risk in those with fewer than 30 pack years is also substantially elevated compared to never-smokers.

This all reinforces that pack years are important when considering the health of current smokers as well as former smokers.

You may be eligible for lung cancer screening and should make sure you are aware of the early symptoms of lung cancer.

Lung Cancer Screening

Pack years of smoking is an important factor in determining who should be screened for lung cancer.

Accordingly, the U.S. Preventive Services Task Force (USPSTF) recommends yearly screening using computerized tomography (CT) for people who:

  • Are 50 to 80 years old
  • Have a 20 pack-year history of smoking
  • Currently smoke or have quit within the past 15 years
  • Are physically healthy enough to benefit from treatment should lung cancer be detected

According to the American Cancer Society, people who are screened with CT are 20% less likely to die from lung cancer than those who have chest X-rays.

The USPSTF also advises that annual screenings should be stopped if:

  • A person has gone 15 years without smoking
  • They develop a health concern that dramatically limits their life expectancy
  • They are physically unable or unwilling to undergo lung surgery if if they're diagnosed with lung cancer

Limitations

While the number of pack years a person has smoked is a useful tool in determining risk, it is not foolproof.

For example, females appear to develop lung cancer after fewer pack years than males. Some studies suggest this is because women have an increased susceptibility to carcinogens in cigarettes—a factor pack years doesn't take into account.

There is also some controversy that the duration of smoking, in and of itself, may be an important factor to look at—especially in determining lung cancer risk.

The age someone starts smoking may play an important role as well.

Associating risk with pack years also ignores the fact that lung cancer also occurs in never-smokers. In fact, lung cancer in never-smokers is among the top ten causes of cancer deaths in the United States.

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