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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

One of the most common cancers, lung cancer usually occurs when a cancer-causing agent, or carcinogen, triggers the growth of abnormal cells in the lung. These cells multiply out of control and eventually form a tumor. As the tumor grows, it can block or narrow airways and make breathing difficult. Eventually, tumor cells can spread (metastasize) to nearby lymph nodes and other parts of the body. These include the

  • liver

  • bones

  • adrenal glands

  • brain.

In most cases, the carcinogens that trigger lung cancer are chemicals found in cigarette smoke. However, more and more lung cancers are being diagnosed in people who have never smoked.

Lung cancers are divided into two groups: non-small cell lung cancer (NSCLC) and small cell lung cancer. NSCLC accounts for about 85% of all lung cancers. These cancers are further divided into subgroups, based on how their cells look under a microscope:

  • Adenocarcinoma. This is the most common type of NSCLC. Although it is related to smoking, it is the most common type of lung cancer in nonsmokers. It is also the most common form of lung cancer in women and in people younger than 45. It usually develops near the edge of the lung. It can also involve the pleura, the membrane covering the lung.

  • Squamous cell carcinoma. This type of NSCLC tends to form a mass near the center of the lungs. As the mass gets larger, it can bulge into one of the larger air passages, or bronchi. In some cases, the tumor forms a cavity in the lungs.

  • Large cell carcinoma. Like adenocarcinoma, large cell carcinoma tends to develop at the edge of the lungs and spread to the pleura. Like squamous cell carcinoma, it can form a cavity in the lungs.

  • Adenosquamous carcinoma, undifferentiated carcinoma, and bronchioloalveolar carcinoma. These are relatively rare NSCLCs.


NSCLC is more likely than small cell cancer to be localized at the time of diagnosis. That means the cancer is limited to the lung or that it hasn't spread beyond the chest. As a result, it can usually be treated with surgery. It may not respond well to chemotherapy (anticancer drugs). However, sophisticated genetic tests can help predict which patients may show favorable responses to particular treatments, including chemotherapy.

Unfortunately, even when doctors think that the cancer is localized, it often comes back after surgery. This means cancer cells had started to spread before surgery, but they couldn't yet be detected.

Your risk of all types of lung cancer, including NSCLC, increases if you

  • smoke. Smoking cigarettes is by far the leading risk factor for lung cancer. In fact, cigarette smokers are 13 times more likely to develop lung cancer than nonsmokers. Cigar and pipe smoking are almost as likely to cause lung cancer as cigarette smoking.

  • breathe tobacco smoke. Nonsmokers who inhale fumes from cigarette, cigar, and pipe smoking have an increased risk of lung cancer.

  • are exposed to radon gas. Radon is a colorless, odorless radioactive gas formed in the ground. It seeps into the lower floors of homes and other buildings and can contaminate drinking water. Radon exposure is the second leading cause of lung cancer. It's not clear whether elevated radon levels contribute to lung cancer in nonsmokers. But radon exposure does contribute to lung cancer in smokers and in people who regularly breathe high amounts of the gas at work (miners, for example). You can test radon levels in your home with a radon testing kit.

  • are exposed to asbestos. Asbestos is a mineral used in insulation, fireproofing materials, floor and ceiling tiles, automobile brake linings, and other products. People exposed to asbestos on the job (miners, construction workers, shipyard workers, and some auto mechanics) have a higher-than-normal risk of lung cancer. People who live or work in buildings with asbestos-containing materials that are deteriorating also have an increased risk of lung cancer. The risk is even higher in people who also smoke. Asbestos exposure also increases the risk of developing mesothelioma, a relatively rare and usually fatal cancer. It usually starts in the chest and resembles lung cancer.

  • are exposed to other cancer-causing agents at work. These include uranium, arsenic, vinyl chloride, nickel chromates, coal products, mustard gas, chloromethyl ethers, gasoline, and diesel exhaust.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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