Over the past 20 years, tuberculosis rates have dramatically increased in 44 African countries, especially in areas of heavy mining, according to an American Journal of Public Health study.

“Rates of the illness have doubled in Africa over the past two decades, and have tripled in South Africa, which even in 1996 had the highest TB rates in the world,” The New York Times reported after the study’s publication.

Over 750,000 annual cases of TB can be traced back to mining work conditions.

“We have long known that the conditions on the mines—both above and below ground—are conducive to the spread of TB and HIV,” Brown University assistant professor of medical science, and researcher of the study, Mark Lurie said in a statement. “But this is the first time we have used statistical techniques to accurately quantify the contribution of mining to TB.”

According to the statement, “TB incidence in some mines is 10 times…higher than in the general population.”

Tuberculosis is caused by an inhaled bacteria, which multiplies in the lungs, causing a lung infection, or pneumonia.

“Miners are also known to spread tuberculosis to their families and communities, the researchers said, since nearly half of workers in large mining countries like South Africa are foreign and routinely travel across large distances,” Reuters reported.