Dr Paolo Boffetta, of the Unit of Environmental Cancer Epidemiology of the International Agency for Research on Cancer in Lyon, France, who worked on the study, said: "We do not really know if asthma as such causes the increased risk".

BBC: Women are at greater risk than men

Professor Valerie Beral, who is head of Cancer Research UK's Cancer Epidemiology Unit in Oxford, said this study was in line with previous research, although the figures differed.

BBC: Pill increases breast cancer risk

Epidemiology has a long list of accomplishments, including most famously identification of the association of smoking with risk of lung cancer, but also encompassing the identification of many other exposures, medications, and infectious agents as risk factors or protective factors for specific diseases.

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