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. That's just one part of the math used to count pack years. The other part is how much you've smoked each day.

Since lung cancer is directly related to the number of cigarettes smoked, doctors can use pack years to help assess someone's risk of the disease. Pack years can also help doctors evaluate the risk of other diseases caused by smoking, including heart disease, other cancers, and more.

Researchers also use pack years as a standard way to measure data in studies on smoking and disease. This article looks at how to count pack years, and what the answers may mean for you.

Lung cancer risk

Verywell / Joshua Seong

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. if you looked at a graph of the related statistics between the two, you would see them rise together on the chart.

The length of time you've smoked, in and of itself, may be an important factor in determining lung cancer risk. It is part of the equation, though the number of pack years—which includes how heavily you smoke—is more often used by health professionals to determine your risk of cancer.

Heart Disease Risk

In addition to lung cancer, the number of pack years someone has smoked is linked to heart disease as well.

Heart disease accounts for a large percentage of deaths in people who smoke, while 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 celebrating. The effort to quit is always worthwhile.

That said, your risk of cancer won't go away. Unlike heart disease, the risk of lung cancer lasts for decades after you quit smoking and never returns to normal.

One research paper looked at lung cancer in people with 30 pack years or more, and found the risk was reduced only gradually for each year they were a former smoker. There was no "dramatic drop-off" after 15 years of quitting.

The same researchers went on to say that lung cancer risk in those with fewer than 30 pack years is also much higher when compared to never-smokers. It all means that pack years matter when considering the health of both current and former smokers.

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

Recap

"Pack years" is determined by the years you've smoked, times the number of cigarettes per day. It is a standard way of counting how much you've smoked and how that smoking affects your lung cancer risk. Pack years are one factor to help your doctor decide if you need annual lung cancer screenings.

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 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, female smokers 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.

A 2018 study found that for people with chronic obstructive pulmonary disease (COPD), pack years might not be the best way to measure. How long they've smoked, rather than adding in how much, was more closely linked with COPD disease than the "cigarettes per day in pack-years" estimates.

The same study suggests the age someone starts smoking may play an important role as well.

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

Summary

Doctors often use pack years to estimate the risk of getting lung cancer. The formula for pack years is based on how much you smoke (cigarettes per day) times the number of years you've actively smoked. While it will always help to quit, that won't entirely erase the risk caused by the smoking.

Pack years is a useful tool for assessing lung cancer risk, and it gives doctors a standard way to describe that risk in basic terms. Yet it also misses other factors, such as differences between male and female smokers, or how early in life the smoking started. Check with your doctor to understand what it means for your own risk and whether you should begin lung cancer screenings.

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