A new study has found a higher incidence of tuberculosis among coal miners in sub-Saharan Africa, the result of close living quarters, infections with HIV-AIDS and the inhalation of mineral dust inside mines.

The study by U.S. and British researchers at Oxford University found that as many as 750,000 new cases of tuberculosis in sub-Saharan Africa every year may be due to coal mining.

Researchers examining data from nearly 50 countries found the potentially fatal lung disease is highest in 44 countries where coal mining is a common occupation, according to Mark Lurie, a professor of community health at Brown University in Rhode Island and co-author of the study.

Lurie says the normally cramped conditions in mines -- as well as the tight quarters of many simple homes --are conducive to the spread of tuberculosis, a highly contagious bacterial disease that is transmitted through the air when an infected individual coughs or sneezes.

Lurie says the disease can be spread further by infected migrants who travel from one country to another to work in the mines. "And every once in a while those men go home.  Now they've been exposed to a very high risk of becoming infected with TB.  And they'll then therefore increase the opportunities for them to spread TB back to rural areas when they return home," he said.

Experts say the incidence of TB has been on the rise for the past 20 years in the 44 African countries, from 173 cases per 100-thousand people to 351 cases currently.  They say the TB infection rate is ten times higher among mine workers than in the general population.

It's long been assumed that the increase in TB cases was due to the exceptionally high rate of HIV infection in Africa. More than two-thirds of all the people in the world currently living with HIV - the virus that causes AIDS - live in Africa.  And HIV-positive Africans account for nearly three-quarters of all AIDS-related deaths worldwide.

But Lurie points out that when researchers factored out the impact of HIV infections on the development of tuberculosis, their analysis showed that mining is also a major factor.

He says the data also show that exposure to airborne silica, a mineral found in mine dust, appears to put the subterranean workers at higher risk of contracting tuberculosis.

Lurie says tuberculosis is declining in ten African countries that were studied, a trend that suggests that improved mining conditions and aggressive TB treatment programs are working.  He says similar measures are needed in countries with the highest rates of tuberculosis.

In particular, Lurie says, efforts should be made to ensure that migrant miners carrying TB can continue to receive treatment for their disease wherever they travel next.  "You can figure out very quickly that if those people are taking TB treatment or HIV treatment when they are living on the mines, and then they returning home either maybe because they are sick or because their contract expires, then there may be a discontinuity of their medicines which could cause all kinds of problems that we would really very much like to avoid," he said.

The study on tuberculosis among miners living in Africa is published in the American Journal of Public Health.