Definition of Pack-Years of Smoking and Link with Lung Cancer Risk

How to Calculate How Many Pack Years You Smoked

Senior woman smoking
What does the term pack years mean and how is this linked to the risk of lung cancer?. PhotoAlto/Eric Audras/Brand X Pictures/Getty Images

If you've heard the term pack-years, for example, in determining whether or not you meet the criteria for lung cancer screening, you may be wondering exactly what this means. Let's take a look at the definition, and why, in turn, this term is so important.

Definition: Pack-Years

The term "pack-years" is a measure of how much someone has smoked. Since lung cancer is directly related to the number of cigarettes smoked, using pack-years can help physicians identify which people have the greatest risk of developing lung cancer.

One pack-year of smoking would mean that someone had smoked one package of cigarettes (20 cigarettes) daily for one year.

Using pack-years helps physicians not only calculate the probable risk of lung cancer but the risk of many other conditions associated with smoking. The number of pack years is also very helpful as an objective measure of the number of cigarettes smoked when looking at studies of smoking and disease.

The number of pack-years smoked and risk of disease may have some limitations. For example, some studies suggest that women are more susceptible to the carcinogens in cigarettes because women appear to develop lung cancer after fewer pack-years of smoking than men. (Learn about how lung cancer is different in women.)

Calculating Pack-Years

Let's take a look at some examples so that you can calculate the number of pack-years you smoked.

 If N stands for the number of packages of cigarettes smoked daily, and T stands for the number of years of smoking, then PY equals the pack years smoked.

The equation looks like this:

N x T = PY

Now let's do a few calculations:

Jill smoked 1 pack of cigarettes daily for 20 years. She has a 20 pack-year history of smoking. Multiplying N (1 pack) times 20 (years smoked) equals 20 pack years.

Frank smoked 2 packs of cigarettes daily for 30 years. Multiplying N (2 packs) by N (30 years,) Frank has a 60 pack-year smoking history

Eleanor smoked 10 cigarettes (1/2 pack) per day for 30 years. Multiplying N (0.5 packs per day) by T (30 years,) Eleanor has a 15 pack-year history of smoking.

Pack-Years and Lung Cancer Risk

In general, the more pack-years you have smoked, the greater the chance of getting cancer. When the number of pack-years is put on a graph, there is almost a linear relationship between pack-years and cancer. The number of pack-years says more about your risk than the length of time you smoked.

That said, the relationship between pack-years of smoking and lung cancer is statistical, and individual people don't alway "follow the rules." Lung cancer occurs in never-smokers and in fact, lung cancer in never-smokers is the 7th leading cause of cancer deaths in the United States. On the other hand, most of us know of someone who was a lifelong heavy smoker and never got lung cancer

Pack-Years, Former Smokers, and Lung Cancer Risk

Using the calculation of pack-years is important for those who once smoked but have now quit. Unlike heart disease, the risk of lung cancer persists for a long period of time after a person quits smoking and never returns to normal.

Those who smoke continue to be at risk of lung cancer even decades after quitting.

In other words, if you have a 40 pack-year history of smoking, but quit 12 years ago, you are still at risk. You may be eligible for lung cancer screening and should make sure you are aware of the early symptoms of lung cancer.

Pack-Years and Heart Disease Risk

The number of pack-years someone has smoked is correlated not only with lung cancer but with heart disease as well. In fact, heart disease accounts for a large percent of deaths in people who smoke, and secondhand smoke is more likely to lead to heart disease than lung cancer.

Other Diseases and Pack-Years

If you smoked in the past, you may be interested in learning about the many cancers associated with smoking as well as the other medical conditions which are linked to smoking.

Lung Cancer Screening 

Recently, doctors have studied the number of pack-years of smoking to determine who should be screened for lung cancer. Studies suggest that people who have a 30 pack-year history of smoking, are between the ages of 55 and 80, and continue to smoke or have quit in the past 15 years, are candidates for CT lung cancer screening. Studies using these criteria have found that the mortality rate from lung cancer could be cut by 20 percent if people meeting these criteria undergo screening.

Pack Years and Disease Calculator

Formulas are now available in which pack-years are used to estimate the risk of developing conditions such as lung cancer and COPD. On a population level, these calculators can give us good information about risk, but there are several limitations when looking at the value for individual people. There are many risk factors for lung cancer, )or against lung cancer,) which are not considered in these calculations, and the risk of an individual may be much greater or much less than would be predicted using this measurement alone.

Limitations

While the number of pack-years a person has smoked is a useful tool in determining risk, it is not foolproof. There is some controversy that the duration of smoking may be an important factor to consider, especially in determining lung cancer risk. The age of onset of smoking may play an important role as well, for example, two people with the same calculated risk based on pack-years, the one who began smoking at an earlier age may be at greater risk.

A Word From Verywell

Since you are looking for the definition of pack-years, you may feel concerned about your smoking history (or that of a family member or friend.) We are here to let you know that even if you smoked in the past there are still things you can do to improve your wellness. If you smoked in the past, make sure to review the criteria for lung cancer screening. If you smoke, it's never too late to quit. And there are always things you can do to lower your lung cancer risk. For ideas on reducing your risk—and in a way that is fun rather than another list of things to avoid—check out these superfoods that may reduce lung cancer risk based on solid scientific research.

Sources:

Blackmon, S., and S. Feinglass. The United States Preventive Services Task Force recommendations for lung cancer screening. Thoracic Surgery Clinics. 2015. 25(2):199-203.

Fucito, L., Czabafy, S., Hendricks, P. et al. Pairing smoking-cessation services with lung cancer screening: A clinical guideline from the Association for the Treatment of Tobacco Use and Dependence and the Society for Research on Nicotine and Tobacco. Cancer. 2016. 122(8):1150-9.

Guaraldi, G. et al. Lung and Heart Diseases Are Better Predicted by Pack-Years than by Smoking Status or Duration of Smoking Cessation in HIV Patients. PLoS One. 2015. 10(12):e0143700.

Peto, J. That the effects of smoking should be measured in pack-years: misconceptions 4. British Journal of Cancer. 2012. 107(3):406-407.

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