Lung cancer is the most commonly fatal of all cancers, killing more than half a million people worldwide every year. A majority of lung cancers are linked to tobacco use. As this habit becomes more prevalent, lung cancer numbers rise. It is the leading fatal cancer among adult males and is rapidly increasing in women, as more and more take up smoking.

Lung cancer develops when lung cells change into abnormal cancer cells after being continually irritated by something in their environment, known as a carcinogen. The most common lung carcinogens are the toxins in cigarette smoke, to which both smokers and those breathing other people's secondhand smoke are exposed. The Cancer Society of New Zealand reports that second-hand smoke is the third largest killer in the country, after active smoking and alcohol use. Industrial chemicals and dusts can also act as carcinogens and contribute to the development of lung cancer.

Cancer cells are cells able to divide very rapidly and do not follow the regular patterns of normal healthy cells. They form colonies or tumours of tissue that do not help the lungs to function normally. Some cancer cells break off and travel in the blood to different parts of the body to form new cancer colonies known as "metastases." When these cancer colonies grow so large that they interfere with normal lung function or the healthy functioning of other parts of the body, symptoms like pain, bleeding, loss of function (i.e. difficulty breathing), and /or weight loss may appear.

Types of Lung Cancer
Lung cancers are grouped according to the type of cells they contain. Because each cancer cell type behaves differently, selected treatment will also be different.

	Squamous cell carcinoma is the most common type of lung cancer (30 percent of cases) and is almost always associated with tobacco smoking. It develops in the lung cells lining the main bronchi (the largest air passages). This tumour often grows out into the airway and blocks it, leading to symptoms like breathlessness, chronic cough, and bloodstained (rusty) sputum. It usually spreads into the chest wall and local lymph nodes before metastasizing (sending cancerous cells to distant parts of the body). When this happens, it may be discovered and diagnosed early enough for effective surgical removal.
	Adenocarcinoma is the form of lung cancer most commonly found in non-smokers and accounts for 29 percent of all lung cancers. There is a high rate of this kind of cancer among non-smoking Hong Kong women. It develops in the glands supplying the lungs with mucous of the airways and is often found in the outer areas of the lung.
	Small cell carcinoma, another common cancer, (17 percent of lung cancer cases), is also strongly linked to cigarette smoking. This type is sometimes referred to as oat cell carcinoma because the cancer cells look oat-shaped. Small cell carcinoma can cause a rapid early death because it spreads or metastasizes quickly and is "silent," meaning it does not have early warning symptoms.
	Large cell carcinoma is responsible for 15 percent of lung cancer cases. It has large, rounded cells and grows in the airways eventually making it difficult to breathe.
	Bronchioloalveolar cell carcinoma is a rare tumour found in less than two percent of cases.
	Unknown cell types: Nine percent of tumours contain cancer cells which pathologists cannot identify or classify either because the cells are "mixed" (containing a range of different types of cells), undifferentiated (do not have characteristics belonging to any particular cell line) or come from another part of the body.
	Mesothelioma is not always classified as lung cancer because it develops in the membranes (pleura) covering the lung and the inside of the chest walls. A rare tumour, mesothelioma is strongly linked with exposure to asbestos. This is the reason asbestos is now considered to be a very dangerous material.