Smokers are twice as likely to develop active tuberculosis compared to people who have never smoked, a study in Taiwan has found, prompting calls for policymakers to be tougher on smoking.

The study tracked nearly 18,000 people in Taiwan representing a general population for more than three years.

"We found a two-fold increase in the risk of active TB in current smokers compared with never-smokers (those who have never smoked)," wrote the lead author Hsien-Ho Lin, a postdoctoral research fellow from Brigham and Women's Hospital in Boston.

One in three people in the world is infected with TB but 90 percent of these will remain latent infections. The remaining 10 percent will develop active TB and fall sick at some point in their lives because of weak immune systems. For example, many people who are infected with HIV/AIDS fall sick and die from TB.

Among the 17,699 participants in the Taiwan study, 3,893 were current smokers, 552 were former smokers and 13,254 had never smoked. There were 57 new cases of active TB by the end of the three-year study.

After factoring in considerations like sex, age, living in a crowded home, household income, marital status, alcohol use and employment, the researchers still found a higher risk of active TB among current smokers.

"Based on our analysis, 17 per cent of incident TB cases in this population were attributable to smoking," they wrote.

Smokers may have reduced ability to fight intruding viruses and bacteria, such as TB, in their lungs, the experts wrote.

"When these normal defence mechanisms are compromised, the development of TB might ensue upon exposure to the TB pathogen," they wrote in a paper published in the American Journal of Respiratory and Critical Care Medicine.

Addressing smoking would be key in any fight against TB.

"Based on results from ours and other studies, policymakers and public health personnel should consider addressing tobacco cessation as part of tuberculosis control," Lin wrote.

TB is still a leading cause of death in the world. There were 9.3 million new cases of TB in 2007 and 1.8 million deaths.

The World Health Organisation aims to bring the incidence of TB down to one case per million each year by 2050.

Dennis Yip, clinical assistant professor at the University of Hong Kong's department of community medicine, said the study was significant given its huge sample size and monitoring over a long period -- the gold standard of scientific studies.

"Previous studies have been much smaller. This is a Taiwan study but we have the same problem in China where smokers are getting younger. By the time they are 40, they would have smoked 25 years," said Yip, who was not involved in the study. – Reuters