Lung cancer is a disease in which malignant (cancer) cells form in the tissues of the lung.

The lungs are a pair of cone-shaped breathing organs inside the chest. The lungs bring oxygen into the body when breathing in and send carbon dioxide out of the body when breathing out. Each lung has sections called lobes. The left lung has two lobes. The right lung, which is slightly larger, has three. A thin membrane called the pleura surrounds the lungs. Two tubes called bronchi lead from the trachea (windpipe) to the right and left lungs. The bronchi are sometimes involved in lung cancer. Small tubes called bronchioles and tiny air sacs called alveoli make up the inside of the lungs.

There are two types of lung cancer: small cell lung cancer and non-small cell lung cancer.

Lung cancer is the leading cause of cancer death in the United States.

Lung cancer is the leading cause of cancer death and the second most common nonskin cancer in the United States.

Tobacco smoking is the most important risk factor for lung cancer.

Anything that increases a person's chance of developing a disease is called a risk factor. The main cause of lung cancer is tobacco use, including smoking cigarettes, cigars, or pipes, now or in the past.

There are other risk factors for lung cancer, but even when taken together, their effect on lung cancer is very small compared to the effect of tobacco smoking. These include the following:

• Being exposed to second-hand smoke.
• Being exposed to asbestos, arsenic, chromium, nickel, or other workplace agents.
• Being exposed to radon, which can be found in the home as well as in the workplace.