In the late 1940s doctors noted a big rise in the number of deaths from lung cancer in the UK, Australia, Canada, US, Turkey and Japan since the end of World War One.
There were two potential culprits that had changed over the 20th Century - industrial pollution and smoking.
Richard Doll was the British statistician working for the Medical Research Council who published a ground-breaking paper in the British Medical Journal in 1950 that concluded there was "a real association between carcinoma of the lung and smoking".
He looked at the incidence of smoking and lung cancer in a large number of patients and compared their experience with people who had different cancers - what scientists call a control group.
His findings led him to give up smoking.
But despite many subsequent studies that have supported the connection, the tobacco industry has not accepted the research.