Smoking and Lung Cancer


By now, most people are aware of the connection between smoking and lung cancer. Yet we still hear the comments, "My uncle smoked for 60 years and never got lung cancer.” “My aunt never smoked but got lung cancer anyway." What are the facts about cigarette smoking and lung cancer, and what is the science behind these facts? Does it make a difference if you quit, and how much of a difference does it make? And since the majority of people who develop lung cancer are former—not current—smokers, what does everyone need to know?

The Statistics About Smoking and Lung Cancer

We know that smoking is a strong risk factor for lung cancer. The risk of developing lung cancer is directly related to the number of “pack-years" a person smoked. Pack-years are calculated by multiplying the number of packages of cigarettes smoked daily by the number of years of smoking. Lung cancer, in turn, is the leading cause of cancer-related deaths in both men and women in the United States.

It's important to note that never-smokers can and do develop lung cancer, though cigarette smoking remains the leading cause of the disease. Men who smoke are 23 times more likely to develop lung cancer than those who don’t smoke, and women smokers are 13 times more likely to develop the disease than their non-smoking counterparts. Overall, between 80 and 90 percent of lung cancers in the U.S. are considered to be caused by smoking.

It's also important to note that lung cancer is not the only scourge of smoking. Cigarette smoking causes many cancers and other diseases. Overall, it's thought that a lifelong smoker sacrifices 10 years of life to smoking and that roughly half of lifelong smokers will die from tobacco-related diseases.

What Percent of Smokers Will Develop Lung Cancer?

The lifetime risk of lung cancer in people who smoke is as high as 15 percent for a lifelong smoker. Quitting at any time reduces the risk, but a person who quits around the age of 50 still has roughly a 5 percent chance of dying from lung cancer.

In addition to the relationship between lung cancer risk and pack-years of smoking, an early age of smoking onset, and the presence of other risk factors may further raise this risk. For some risk factors, such as asbestos exposure, the increased risk is beyond what would be expected by simply adding the two risk factors together.

Former Smokers at Greatest Risk of Lung Cancer

The majority of lung cancers (over 50 percent) now occur in former smokers—people who once smoked but have quit. Unlike the risk of heart disease, which drops rapidly when someone quits smoking, the risk of lung cancer can linger and remains above that of a non-smoker for a lifetime.

If you are a former smoker and learning this for the first time, don't despair. Those who are ex-smokers can still lower their risk as well as increase their chance of surviving the disease should they develop it (see below).

Age at Quitting and Later Risk of Lung Cancer

The risk of lung cancer in former smokers is most affected by the age at which someone kicked the habit. The age of smoking cessation in relation to overall risk of death has been evaluated more closely than it's relationship to lung cancer alone.

As noted above, smoking takes roughly 10 years of life away from a lifelong non-smoker, with half of people dying from a tobacco-related disease. For those who quit between the ages of 25 and 34, risk returns almost to normal. Those who quiet between 35 and 44 years, can expect to regain nine of those 10 years. Quitting smoking between the ages of 45 and 54 reclaims six years, and quitting between 55 and 64 reclaims four years.

Time Since Smoking Cessation and Risk of Lung Cancer

How often does lung cancer occur years or even decades after quitting?

This number hasn't been quantified well, but a 2011 study looking at 600 people referred for lung cancer surgery can give us an idea. At the time of diagnosis, 77 percent of these people were former smokers and only 11 percent current smokers. The breakdown was as follows:

  • 14 percent had been smoke-free for less than a year
  • 27 percent were smoke-free for 1 to 10 years
  • 21 percent were smoke-free for 10 to 20 years
  • 16 percent were smoke-free for 20 to 30 years
  • 11 percent were smoke-free for 30 to 40 years
  • 10 percent were smoke-free for 40 to 50 years

It's apparent from this study that smokers may be at risk for a long period of time following cessation. In fact, the average time of smoking cessation prior to a diagnosis of lung cancer in this study was 18 years. Again, these numbers can be disconcerting if you are a former smoker, but there are still things you can do to lower your risk. Make sure to read on. It's also important to note that with the adoption of widespread lung cancer screening, these numbers may change.

You may have heard that the risk of lung cancer appears to increase between one and four years after smoking cessation. Instead of an increased risk in this period after quitting, it's thought instead that many people may quit due to early symptoms of lung cancer and that quitting is likely the result of lung cancer rather than the cause. After five years of abstaining there is a significant decrease in risk.

The History of Smoking and Lung Cancer

Following the 1964 Surgeon General’s report on Smoking and Health, the public became widely aware of the risk of smoking. In that report, it was estimated that smokers had a nine- to 10-fold increased risk of developing lung cancer compared to non-smokers, and smoking was declared to be the major cause of lung cancer in the United States. But we suspected a link between smoking and lung cancer long before that time. An article titled "Cancer by the Carton" graced the pages of Reader's Digest in 1952, and studies in Germany noted similar findings a few decades before that. A multitude of studies since that time have further defined the association.

Though lung cancer has always been with us, it was at one time fairly uncommon around the world. Up until 1492—when Europeans first came into contact with natives smoking tobacco—tobacco was found only in the Americas. The tired adage "the rest is history" speaks a biting truth, with smoking-induced lung cancer the number one cause of cancer-related deaths worldwide.

The Culprits in Tobacco Which Cause Lung Cancer

Prior to discussing the mechanisms by which tobacco may cause lung cancer, it's helpful to list some of the harmful chemicals in cigarettes which have been identified. Out of the several thousand chemicals present in tobacco smoke, there are roughly 70 carcinogens (chemicals thought to cause cancer). Some of these include:

  • arsenic (found in rat poison)
  • benzene (a component of crude oil often used to make other chemicals)
  • cadmium (found in batteries)
  • chromium
  • nickel
  • vinyl chloride (found in plastics and cigarette filters)
  • polycyclic aromatic hydrocarbons (PAHs)
  • N-nitrosamines
  • aromatic amines
  • formaldehyde (found in embalming fluid)
  • acetaldehyde
  • acrylonitrile
  • polonium-210 (a radioactive heavy metal)

There are many factors that may increase or decrease the carcinogenicity of tobacco. Different types of tobacco leaves, the presence or absence of filters, chemical additives, and the ambient conditions of smoking may all play a role in the ability of a cigarette to induce cancer. In addition, it may not be the specific chemicals in tobacco, but rather the mix of chemicals present.

The presence of fewer carcinogens in Japanese cigarettes has been hypothesized as one reason why Japanese men are less likely to develop lung cancer even though they smoke more—something referred to as the Japanese smoking and lung cancer paradox. The odds ration of smokers to non-smokers developing lung cancer in the United States is 40:1 in contrast to a 6.3:1 ratio in Japan. The use of activated charcoal in cigarette filters in Japan may also be a factor. Activated charcoal is best known for its use in binding poisons in the emergency room. Of course, factors such as diet and genetic make-up could be responsible for this paradox as well.

Low-Tar Cigarettes, Filters, and Lung Cancer

The addition of filters to cigarettes has changed the landscape of lung cancer to some degree. It's thought that people who smoke filtered cigarettes throughout life are 20 to 40 percent less likely to develop lung cancer than lifelong non-filtered cigarette smokers. Beyond the risk of cancer, however, the addition of filters appears to have changed the most common types of lung cancer, and consequently the most common symptoms of the disease (see below).

Along with the addition of filters, cigarettes became available with a lower cigarette tar content. Even though decreasing tar decreases exposure to this harmful chemical, cigarettes labeled as "light" or "ultralight" are just as dangerous as the regular varieties. In order to get the same amount of nicotine, those who smoke low-tar cigarettes often smoke more cigarettes and take more puffs, leading to a similar risk of lung cancer regardless of tar content.

How Does Smoking Cause Lung Cancer? The Science (Molecular Mechanisms) Behind the Facts

In order for a normal cell to become a cancer cell, a series of mutations must take place. In the nucleus of each of our cells lies our DNA—our genetic blueprint—which carries the instructions for each of the proteins made by the cell. Some of these proteins tell the cell to grow and multiply. Others aid in repairing DNA. Still others work to remove damaged cells so they cannot be propagated (in a process of programmed cell death called apoptosis). Smoking can result in these mutations in lung cancer cells by several different mechanisms, including:

Direct damage to DNA: Some of the carcinogens in cigarette smoke directly damage (cause mutations and other changes) the DNA of lung cells. In addition, some chemicals, such as chromium, help other carcinogens "stick" to the DNA of lung cells like glue, increasing the risk of damage.

Lack of DNA repair: Even if the DNA in our cells is damaged in some way, we have an elaborate system for repair of damaged DNA. Genes known as tumor suppressor genes code for proteins which repair damaged DNA or cause the death of abnormal cells. Arsenic and nickel both interfere with pathways for repairing damaged DNA.

An example of how this works has been noted with a type of tumor suppressor gene called the p53 gene. The p53 gene regulates cell division by keeping cells from dividing too fast or in an uncontrolled way. TP53 codes for the p53 protein that directs the reapir or elimination of cells with damaged or mutated DNA..One of the carcinogens in tobacco smoke, benzo(o)pyrene, has been found to specifically damage the p53 gene.

Inflammation: Whenever a cell divides, there is a chance that an "accident" in copying the genetic material of the cell will occur. When cells have to divide more often to replenish damaged cells, such as when the airways are damaged by tobacco smoke, there is a greater chance that one of these mistakes in cell division—a mutation—will take place. There are many compounds in tobacco smoke which cause inflammation.

Damage to cilia: Cilia are tiny hair-like appendages which line the airways. The cilia ordinarily capture toxins and propel them up and out of the airways like an upward brush stroke. Toxins in tobacco smoke, such as formaldehyde, damage the cilia so they are less effective at removing toxins. Other inhaled toxins may then "stay" longer in the airways to do their damage.

Immune function: Our immune cells are designed to detect and destroy abnormal cells such as cancer cells. When the immune system is not functioning properly, these early cancer cells may "escape." Some toxins in tobacco smoke may interfere with immune function.

Smoking, Filters, and Lung Cancer Types

The lung cancer types found in people who smoke often differ from those in non-smokers. Small cell lung cancers, which account for roughly 15 percent of lung cancers, occur almost always in individuals who smoke or have smoked. Non-small cell lung cancers (NSCLC), in contrast, although they mainly occur in people who have smoked, may also occur in non-smokers (especially the adenocarcinoma type).

Non-small cell lung cancer (responsible for 85 percent of lung cancers) is in turn broken down into lung adenocarcinoma (around 50 percent) squamous cell lung cancer (around 30 percent) and large cell lung cancer (around 10 percent.)

Historically, people who smoked were more likely to develop squamous cell lung cancer, and non-smokers, adenocarcinoma. With the switch from unfiltered to filtered cigarettes, adenocarcinomas have become more common in people who smoke as well.

Both small cell lung cancers and squamous cell lung cancers occur most often in the large airways—the bronchi. Prior to the use of filters in cigarettes, it's thought that most of the carcinogens lodged in these larger airways. With the addition of filters, it appears that carcinogens are inhaled deeper into the lungs—the location where most adenocarcinomas occur.

Genetics, Smoking, and Lung Cancer

Genetics may play a role in the connection between smoking and lung cancer in a few ways. It's far from clear what the exact association is, but it's thought that there may be a common genetic predisposition to becoming addicted to nicotine and the development of lung cancer.

From another angle, family history (genetics) may work together with smoking to increase risk. Many people are familiar with the BRCA2 gene mutations which have become known as one of the "breast cancer genes." We've learned that lung cancer is also linked with a BRCA2 mutation. Women who smoke and carry a BRCA2 gene mutation have double the risk of developing lung cancer.

Other Forms of Smoking and Lung Cancer

Cigarettes are not the only form of tobacco that raises cancer risk. Clove cigarettes, Kreteks, and Bidis also increase risk.

Both pipe and cigar smoking increase lung cancer risk. These forms of smoking have been linked most closely with small cell lung cancer and squamous cell carcinoma of the lungs. It's not certain how often pipe smoking leads to lung cancer, but cigar smokers are thought to have around five times the risk of developing lung cancer compared to non-cigar smokers.

In contrast, it's not certain whether or not marijuana increases lung cancer risk. Many of the carcinogens present in tobacco smoke are also present in marijuana smoke, but studies have been mixed—some showing an increase and others showing a decrease in lung cancer. It could be that there is more than one mechanism involved, as marijuana smoke may have anti-cancer effects as well, at least with regard to a type of brain tumor.

It's too soon to know if hookah smoking causes lung cancer, but there are significant concerns. A review of studies done between 1997 and 2014 found that hookah smoke contains 27 carcinogens. The levels of these chemicals vary, however, with some having higher concentrations and others lower levels than those in cigarette smoke. Benzene, for example, is one carcinogen which is found in higher concentrations in hookah smoke than smoke from cigarettes. Hookah also exposes people to a carcinogen not ordinarily present in cigarettes—the charcoal used to heat the tobacco in the pipe. Hookah smoke is inhaled more deeply in a larger volume than cigarette smoke.

It's been shown that e-cigarettes can damage lung cells, but as with hookah, we don't yet know what effect—if any—the use will have on lung cancer risk. When considering the effect of e-cigarettes and hookah, it's important to keep in mind the latency period with cancer. The latency period is defined as the time between exposure to a carcinogen and the later development of cancer. With smoking, the average population latency period is 30 years.

Nicotine and Lung Cancer Risk

What is the link between nicotine and cancer? With nicotine replacement therapies being used widely for those trying to quit smoking, the question about whether or not nicotine alone increases cancer risk is an important one.

While nicotine is clearly responsible for the addictive potential of cigarettes, and can be toxic, nicotine is not necessarily carcinogenic on its own. Studies suggest that rather than playing a role in the initiation of cancer, this chemical may work more often as a promotor—enhancing the development of cancer.

That's not to say that nicotine deserves a green light when it comes to cancer. For those already living with cancer, there are several ways in which nicotine may not be a good idea. It was found—in mice anyway—that nicotine contributed to the tumor growth and spread (metastasis) of non-small cell lung cancer cells. It's also thought that nicotine might enhance angiogenesis—the ability of a tumor to make blood vessels. In addition, nicotine may reduce the effectiveness of chemotherapy.

Secondhand Smoke and Lung Cancer

Secondhand smoke is a risk factor for lung cancer and is thought to cause around 7300 lung cancer deaths each year. A non-smoker living with a smoker (passive smoking) has a 20 to 30 percent greater chance of developing lung cancer. (Secondhand smoke is also thought to be responsible for around 34,000 heart-related deaths each year.)

Sidestream smoke, the smoke given off by the burning cigarette, accounts for around 80 percent of the smoke to which non-smokers are exposed, with mainstream smoke, the smoke exhaled by a smoker, accounting for the remaining 20 percent. We are still learning about how these differences may result in different types of lung cancer for smokers and exposed non-smokers.

Thirdhand smoke—the particles and gases left over after a cigarette is extinguished—may contain toxins, but we do not yet know if it has any effect on lung cancer risk.

Smoking After a Diagnosis of Lung Cancer (or Any Cancer)

Even if someone has been diagnosed with lung cancer, quitting smoking can make a difference. Quitting smoking with lung cancer can:

  • Improve the chance that you will survive. One study in patients with advanced lung cancer found that the median survival among those who quit at diagnosis was 28 months, in contrast to 18 months for those who continued to smoke.
  • Lower the risk of lung cancer recurrence.
  • Reduce the risk of complications with surgery. Smoking increases the risk of heart and respiratory complications following surgery. Those who smoke are also more likely to develop postoperative infections and have poorer wound healing.
  • Reduce the symptoms you experience with lung cancer. People who continue to smoke after a diagnosis of cancer experience more moderate to severe pain than those who are able to put cigarettes aside.
  • Improve your response to treatment. While lung cancer has not been evaluated specifically, people with head and neck cancer respond significantly better to radiation therapy if they quit smoking. In addition, smoking decreases the effectiveness of some chemotherapy drugs, and may decrease blood levels of the targeted therapy Tarceva (erlotinib) commonly used to treat lung cancer.
  • Decrease the risk of complications related to treatment. For example, people who smoke are more likely to develop radiation pneumonitis as a complication of radiation therapy than those who do not smoke.
  • Improve your quality of life. People who continue to smoke with cancer have lower energy levels, experience more shortness of breath, and have a reduced performance status relative to those who quit.
  • Reduce your risk of dying from conditions other than lung cancer.
  • Decrease your risk of developing a second primary cancer. Not only are those who already have cancer at an increased risk of developing a second unrelated cancer, but the treatments used to treat cancer such as chemotherapy and radiation therapy may increase risk as well.
  • Reduce your risk of exposing nearby non-smokers to secondhand smoke.

Lung Cancer Screening

As noted earlier, lung cancer is more common in former smokers than current smokers, but this is not a cause for panic. For those who smoked in the past, there is now a screening test available for the early detection of lung cancer. It's thought that if everyone who qualifies for screening was tested, the mortality rate from lung cancer could be reduced by 20 percent in the United States.

In the past, it was thought that performing yearly chest x-rays might help detect lung cancer at an early stage, but this is no longer recommended. Even though chest x-rays may find some lung cancers, it was found that screening for lung cancer with chest x-rays alone did not decrease the death rate from lung cancer; these tests failed to find lung cancer at an early enough stage.

In contrast, CT lung cancer screening has been found to find lung cancer at a stage at which treating the disease can improve survival.

CT lung cancer screening is recommended for:

  • People between the ages of 55 and 80.
  • Those who have a history of at least 30 pack-years of smoking (A pack year is calculated by multiplying the number of years of smoking times the number of packages of cigarettes smoked daily. For example, if someone smoked two packs per day for 15 years they would have a 30 pack year history of smoking.)
  • Those who continue to smoke or have quit in the past 15 years.
  • Those people who are in reasonable health such that they could undergo surgery if a cancer was found.

An unexpected positive finding is that people who undergo lung cancer screening are more likely to quit smoking.

The Stigma of Lung Cancer

Since smoking is associated with the majority of lung cancers, there is a stigma associated with lung cancer. A stigma that somehow individuals have caused their disease and "deserve" to have cancer. This stigma is damaging and unfair. We don’t confront people who are overweight or sedentary suggesting that they are responsible for illnesses they develop. Regardless of the cause of a cancer, or any condition for the matter, people who are struggling with a chronic illness need our unconditional caring and support.

'Smokers' vs. Non-Smokers' Lung Cancer

You may have heard someone comment in the past that they have "non-smokers' lung cancer." There are some important differences between lung cancer in non-smokers and lung cancer in people who smoke from a medical standpoint. Lung cancer in people who have smoked tends to have a poorer prognosis at each stage of the disease and is often less likely to have "targetable mutations" which can be treated with targeted therapies. That said, the immunotherapy medications may actually be more effective among those who have smoked than for non-smokers.

In contrast to these medical differences, however, making a distinction between smokers' and non-smokers' lung cancer only adds to the stigma of the disease. It's important that we advocate for people with lung cancer regardless of smoking status in order to raise awareness and increase the funding for research which can improve outcomes for anyone with the disease.

Resources for Quitting

Clearly, lung cancer increases the risk of smoking, and even after a diagnosis of the disease, smoking is detrimental. If you smoke and need help to quit, talk with your doctor. Take a moment to check out these 10 tips for managing nicotine withdrawal, as nicotine addiction is the most difficult aspect of quitting. And make sure to check out the following article which provides information ranging from motivational tips to resources for success:

Lowering Your Lung Cancer Risk as a Former (or even Current) Smoker

For those who once smoked, it can be devastating to realize you are still at risk. What can you do?

The first step is to talk with your doctor about CT screening. Do you meet the criteria for this test, or are there other reasons you should be screened? When lung cancers are found at an early stage they are much more treatable than those found at later stages.

In addition, consider your risk factors for lung cancer. You can't go back and quit smoking at a younger age, but there are things you can do. For example, since radon exposure in the home is the second leading cause of lung cancer, make sure to check the radon level in your home.

And keep in mind that reducing your risk doesn't necessarily mean following a long list of things to avoid. Reducing your risk can even be fun. Exercise as simple as gardening twice a week has been found to lower risk and adding some of these superfoods for lowering lung cancer risk to your diet can even be tasty.

A Word From Verywell

As noted above, it's clear that smoking causes lung cancer and that even former smokers are at risk. Yet it's never too late to quit smoking or to improve your lifestyle in other ways. In fact, many people who have kicked the habit find that they not only feel better, but feel motivated to improve their health in other ways as well. 

As a final note, if you know anyone with lung cancer, reducing the stigma of the disease can begin with each one of us. It doesn't matter if someone has smoked or not. People with lung cancer need our dedicated support. Treatments for the disease are getting better and the life expectancy is improving. The more we can dispel the stigma, the farther we can go in changing the outlook for anyone whoever has to hear those heart-wrenching words: "You have lung cancer."

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