**2. Etiology of lung cancer**

In addition to cigarette smoking, which is one of the well-known carcinogens, there are many other environmental as well as genetic factors that contribute to lung cancer, discussed as under:

#### **2.1 Smoking**

Smoking alone accounts for approximately 90% cases of lung cancer and the risk is directly proportional to the duration of smoking and the number of cigarettes smoked/day. The risk of lung cancer is the same with cigar and pipe smoking as it is with cigarette smoking. Smoking of low-tar or "light" cigarettes also increases

the risk of lung cancer as much as regular cigarettes. Menthol cigarette smoking increases the risk even more as menthol may allow people to inhale more deeply.

#### **2.2 Secondhand smoke**

With the passive smoking risk of lung cancer increases by 20 to 30%. More than 7000 deaths from lung cancer each year occur due to secondhand smoke.

#### **2.3 Exposure to radon**

Radon is a naturally occurring radioactive gas that is formed by the breakdown of uranium in soil and rocks. Uranium miners have small but significant risk of lung cancer due to exposure to radon. In Europe, 2% of all deaths from lung cancer in smokers were due to appreciable hazards from residential radon.

#### **2.4 Exposure to asbestos**

With exposure to asbestos, the risk of lung cancer is further increased and is seen more in smokers. The risk of lung cancer with asbestos exposure is directly proportional to the dose concentration and the type of asbestos fiber.

#### **2.5 Exposure to other cancer-causing agents in the workplace**

Metal exposure such as chromium, nickel, arsenic, and polycyclic aromatic hydrocarbons is also associated with lung cancer.

## **2.6 Previous radiation therapy to the lungs**

Radiation exposure for non-lung cancer treatment, like non-Hodgkin's lymphoma and breast cancer, also increases the risk of lung cancer. Certain disorders of the lung like idiopathic pulmonary fibrosis increase the risk of lung cancer independent of smoking.
