Smoking and Lung Cancer

How They're Linked and the Benefits of Quitting

According to the Centers for Disease Control and Prevention (CDC), smoking is linked to between 80% and 90% of lung cancer deaths in the United States. This makes it the number one risk factor for the disease. Although quitting dramatically reduces the risk, a history of smoking can stay with you and increase the likelihood of lung cancer even after 25 years of abstinence.

Even so, quitting offers benefits no matter how long you have smoked, reducing your risk of lung cancer by around 39% after five years—even if you are a heavy smoker.

Statistics

Lung cancer is the leading cause of cancer-related deaths in both males and females in the United States, according to the CDC. Overall, around 6.3% percent of men and women in the United States will be diagnosed with lung cancer at some point during their lifetime.

Men who smoke are 23 times more likely to develop lung cancer than those who don’t smoke, while women who smoke are 13 times more likely to develop the disease than their non-smoking counterparts.

Cigarette smoking can also cause other types of cancers, including throat, laryngeal, esophageal, stomach, kidney, pancreatic, liver, bladder, cervical, and colorectal cancer, as well as a type of leukemia called acute myeloid leukemia (AML). In fact, 40% of all cancers are linked to tobacco smoke in some way.

The lifetime risk of lung cancer is as high as 15% for a lifelong smoker. Overall, it is thought that such smokers have 10 fewer years of life and that roughly half of them will die from tobacco-related diseases, including chronic obstructive pulmonary disease (COPD).

It is important to note that never-smokers can and do develop lung cancer, though cigarette smoking remains the leading cause of the disease.

How Smoking Causes Lung Cancer

Lung cancer is a complex, multifactorial disease in which genetics, environment, and lifestyle all play a role. Cigarette smoking contributes by directly damaging DNA (the genetic coding) of lung cells and altering the way in which the lungs and immune system work.

Cigarette smoke contains a toxic mix of over 7,000 chemicals, around 70 of which are classified as carcinogenic (cancer-causing). These include arsenic, benzene, cadmium, chromium, formaldehyde, N-nitrosamines, nickel, and vinyl chloride.

When exposed to these and other chemicals in cigarette smoke, the cells of the lungs can begin to mutate and form cancerous tumors. There are a number of intersecting mechanisms that contribute to this:

  • Direct DNA damage: When exposed to carcinogens, the strands of DNA can begin to break apart. This not only causes cells to divide abnormally but prevents apoptosis, the programmed death of cells that allows them to be replaced with healthy ones. When this occurs, the cancer cells effectively become "immortal" and can replicate out of control.
  • Impaired DNA repair: Under normal circumstances, damaged DNA can be repaired. Tumor suppressor genes trigger apoptosis in damaged cells and instruct the body on how to make new, healthy ones. Chromium from cigarette smoke can bind to DNA and effectively silence those genes. Arsenic and nickel can do the same by triggering mutations in tumor suppressor genes.
  • Inflammation: When exposed to cigarette smoke, the body will respond by releasing pro-inflammatory compounds (such as interleukin-1β, prostaglandin E2, and transforming growth factor-β) in an attempt to minimize cell damage. Over time, the persistent inflammation can not only damage cellular DNA but alter the way that cells adhere to each other, allowing them to migrate freely and become invasive.
  • Damage to cilia: Cilia are tiny, hair-like structures lining the airways the brush debris out of the lungs. Certain toxins in tobacco smoke, like formaldehyde, can paralyze cilia and, over time, damage them irreparably. This allows harmful particles in cigarette smoke to remain in the lungs longer.
  • Impaired immune function: Although carcinogens in tobacco smoke are implicated in the formation of cancerous tumors, other chemicals can contribute by suppressing the immune system as a whole. Both nicotine and tar impair the body's innate (inborn) immune response and, by doing, hinder some of the mechanisms that might otherwise prevent cancer (such as apoptosis).

Effect of Pack-Years on Lung Cancer Risk

The risk of developing lung cancer is ultimately related to the number of pack-years a person has smoked. Pack-years are calculated by multiplying the number of packs of cigarettes smoked daily by the number of years of smoking. The greater the number of pack-years, the higher the risk of disease.

According to a 2018 study in BMC Cancer, the number of pack-years was directly associated with a person's risk of lung cancer compared to the risk seen in never-smokers:

Pack-Years Risk of Lung Cancer Increases
1-20 1.3-fold
21-40 3-fold
41-60 8.5-fold
61-80 12-fold
81-100 10-fold
Over 100 23-fold

Lung Cancer in Former Smokers

Around 40% of lung cancer diagnoses occur in former smokers. Even after smoking is stopped, the risk of lung cancer can persist and remains above that of a non-smoker for a lifetime.

Studies suggest that former smokers who develop lung cancer are diagnosed an average of 18 years after they quit cigarettes.

The age in which a person quits can roughly estimate the life-years lost to smoking. A 2013 study published in the New England Journal of Medicine broadly outlined the losses based on the age in which smoking was stopped:

Age Life-Years Lost
25 to 34 Nearly 0
35 to 44 1 life-year lost
45 to 54 4 life-years lost
55 to 64 6 life-years lost

Cancer Risk by Cigarette Type

In addition to pack-years, the type of cigarette smoked can contribute significantly. Different types of tobacco leaves, the presence or absence of filters, chemical additives, and the ambient conditions can all play a role in a cigarette's ability to induce cancer.

Japanese cigarettes, for instance, are manufactured so that fewer carcinogens are created during combustion. This, in addition to the use of activated charcoal filters, appears to reduce the overall risk of lung cancer among people in Japan, according to research from Pennsylvania State University.

This is evidenced by data released by the World Health Organization in which Japanese men have a lower lung cancer mortality rate compared to American men despite being more likely to smoke and more likely to do so at an early age. Moreover, lung cancer in Japanese men tends to occur between the ages of 60 and 64, no less than 10 years later than what is seen in American men.

A 2013 study in the journal Chest further reported that lifelong smokers who use filtered cigarettes are between 20% and 40% less likely to develop lung cancer compared to lifelong smokers who use non-filtered cigarettes.

Low-tar cigarettes appear to be just as dangerous as regular ones. In order to get the same amount of nicotine, users have to smoke more cigarettes and take more puffs, mitigating any benefit of the low tar content.

It is for this reason that the U.S. Food and Drug Administration (FDA) banned the use of the terms "light" or "ultralight" from cigarette labels and marketing under the Family Smoking Prevention and Tobacco Control Act (FSPTCA) of 2009.

Other Forms of Smoking

Cigarettes are not the only form of tobacco that raises cancer risk. Bidis and kreteks (clove cigarettes) imported from Asia can also increase the risk. Bidis and kreteks have higher concentrations of nicotine, tar, and carbon monoxide than conventional cigarettes sold in the United States.

Pipe and cigar smoking are almost as likely to cause lung cancer as cigarette smoking. In fact, the additional fermentation process used to cure these tobacco products can increase the concentration of carcinogenic N-nitrosamines compared to regular cigarettes.

It remains unclear what risks hookah smoking pose with regard to lung cancer, although a 2014 review in the International Archives of Medicine listed no less than 27 carcinogens identified in the vaporized fumes. Among these is the carcinogen benzene, which is found in higher concentrations in hookah smoke than conventional cigarettes.

By contrast, studies have been mixed as to whether marijuana smoking increases the risk of lung cancer. The same applies to e-cigarettes, which the American Cancer Society suggests may pose a risk of lung cancer.

Get Screened

If you are between the ages of 50 and 80, have at least a 20 pack-year history of smoking, and have quit within the past 15 years, you can get an annual lung cancer screening to track any changes in the lungs. Doing so can spot cancer early when it is still highly treatable and ensure that you live a longer, healthier life.

A Word From Verywell

The facts are simple: Smoking causes lung cancer, and even former smokers are at risk of the disease. With that said, it is never too late to quit. Many people who have kicked the habit find that they not only feel better but are motivated to improve their health in other ways as well. 

If you have smoked heavily for years, don't assume that the "damage is done" and that there is no point in quitting. Even if your risk of lung cancer remains elevated, there are safe and effective ways to monitor for cancer.

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