The study found that in India male smokers are about four times as likely to become ill with TB as non-smokers, and consequently four times as likely to die from the disease.
The study also showed the smokers had higher death rates from heart disease and from various types of cancer.
In total, about a quarter of the smokers studied were killed at ages 25-69 by their habit, those killed at these ages losing, on average, 20 years of life.
This is the first major study to be completed on how smoking causes death in India. It compared the smoking habits of 43,000 men who had died of various diseases in the late 1990s with the habits of 35,000 living men. More than 4000 of these deaths were from TB, but if the smokers had had the same low risks as non-smokers there would have been fewer than 2000 TB deaths. Few women in south India smoke, so the study was only of men.
Dr Vendhan Gajalakshmi of the Epidemiological Research Center in Chennai, India, led the research. She said: "Smoking causes half the male tuberculosis deaths in India. Almost 200,000 people a year in India die from tuberculosis because they smoked, and half the smokers killed by TB are still only in their thirties, forties or early fifties when they die."
Co-investigator, Sir Richard Peto, professor of medical statistics at the University of Oxford, UK, said: "In some parts of the world the main way smoking kills people is by damaging the lung's defences against chronic TB infection. Our study indicates that in rural India about 12% of smokers, but only 3% of non-smokers, die prematurely from TB."
Co-investigator Dr Prabhat Jha, of the University of Toronto, said: "Not only in Asia and Africa, but also throughout America and Europe, smoking will increase the number of people who develop clinical TB themselves and who can then infect others, unless properly treated and cured."
About a billion people worldwide are carrying live TB infection in their lungs, but if they do not smoke then most will never become seriously ill from it. Smoking increases the danger that any TB infection already in the lungs will get out of control and cause clinical TB, which can kill and can easily be spread to other people.
Tuberculosis still causes about 1.6 million deaths a year worldwide, including more than a million in Asia, 400,000 in Africa and 100,000 in the Americas and Europe, and in some countries it is now becoming more common.
The study is published 16 August in The Lancet medical journal. The full paper can be accessed at: http://www.ctsu.ox.ac.uk/projects/IndiaTB
Contacts:
Asia
Dr Vendhan Gajalakshmi
Epidemiological Research Center, Chennai
Office (tel/fax): 91-44-2644-6030
Cell: 91-98-4016-0050
Email: gajaerc@rediffmail.com
Europe
Professor Sir Richard Peto
Nuffield Department of Clinical Medicine
University of Oxford, UK
Office: 44-1865-404-801
Cell: 44-7771-960-329
Email: secretary@ctsu.ox.ac.uk
Press Office
Medical Research Council
Office: 44-20-7637-6011
E-mail: press.office@headoffice.mrc.ac.uk
North America
Dr Prabhat Jha
Centre for Global Health Research
University of Toronto, Canada
Office: 1-416-864-6042
Cell: 1-416-839-0332
Email: Prabhat.jha@utoronto.ca
Jessica Whiteside
University of Toronto Public Affairs
Office: 1-416-978-5948
Email: Jessica.whiteside@utoronto.ca
NOTES TO EDITORS: Lay Summary of Key Findings
- Mortality from TB is about four times as great among smokers as among non-smokers
- Risk of death from TB at ages 25-69, smoker versus non-smoker is 12% versus 3% in rural India, and 8% versus 2% in urban India (media can illustrate by bar diagram)
- Among men in India, smoking causes about half of all TB deaths and a quarter of all deaths from any cause in middle age
- About half of the deaths caused by smoking are from TB and other respiratory disease, about one third are from heart and other vascular disease, and smaller numbers are from mouth and lung cancer
- About one in four of all persistent smokers in India are killed by tobacco at ages 25-69, those killed at these ages losing about 20 years of life expectancy
- The risk is substantial both for cigarettes and for "bidis" (small Asian cigarettes used widely in India)
- In India smoking caused about 700,000 deaths in the year 2000, mostly among middle-aged men (35-69). Annual tobacco deaths in middle age in India are projected to reach 1 million by 2025
Journal
The Lancet