We now know that exposure to certain chemicals, such as those found in cigarette smoke, can cause cancer. But at the start of the 20th century, scientists were not even sure that cancer-causing chemicals existed, and it was not until the 1920s and 1930s that research at The Institute of Cancer Research in London identified the first carcinogens in smoke and coal tar.
Scientists knew by then that workers in the paraffin, shale oil and coal tar industries had a high incidence of skin cancer, but the identity of the particular chemical within these tars that caused cancer remained a mystery. Finding the active cancer-causing agent became a mission of The Cancer Hospital Research Institute, which would become the ICR. In 1921, the institute’s director, Professor Archibald Leitch, recruited Ernest Kennaway– later to become Sir Ernest Kennaway - and charged him with investigating the carcinogenic properties of tars and oils.
Over the next 10 years, Kennaway and his team meticulously separated out the constituents of coal tar and tested each in turn to see whether they caused cancer when applied to the skin of mice. They found that the cancer-causing agents were a group of chemicals called polycyclic aromatic hydrocarbons (PAHs) and the first to be categorically identified was a compound called benzo(a)pyrene. It was the first pure compound to be shown to have cancer-causing properties, and the first carcinogen, of the many we now know about, to be found in cigarette smoke. Kennaway’s discovery was a major breakthrough and in 1941 he was awarded the Royal Society’s Royal Medal for his work.
Cook et al, (1932) The Production of Cancer by Pure Hydrocarbons.--Part I, Proc. R. Soc. Lond. B.,111, 773 455-484 doi:10.1098/rspb.1932.0068