How do doctors discovered the link between cigarette smoke and lung cancer?

Researchers discovered over time that smokers had a huge increase in their deaths from lung cancer compared to non-smokers. The tar and other substances found in cigarettes were found in lungs of smokers as well. It wasn't too difficult to link it.

From epidemiological studies. Basically, the incidence of lung cancer in a sample of smokers is compared with the incidence in a sample of a similar size of non-smokers. However, it's not that simple. There are many factors that epidemiological studies have linked with lung cancer - radon gas in the home, asbestos, family history of cancer, spread of cancer from another part of the body, eating dairy produce, drinking alcohol, traffic fumes, taking antibiotics, sawdust, having a wood burning stove and even owning a colour TV!

Epidemiology is a far from exact science. If a carpenter tragically contracted lung cancer smoked, lived in a house with high radon levels, took antibiotics, whose father or mother died of cancer and owned a colour TV, what was the cause? And then it could be down to a factor that has yet to be identified. The HPV virus, which is believed to be a major factor for Cervical cancer, is currently being investigated.