The pollution choking most Indian cities and causing premature deaths due to pulmonary and cardiac diseases can be hazardous in yet another way. Air pollution makes a person more susceptible to tuberculosis and public health experts say that high levels of microscopic particulate matter is probably adding to the large number of cases of TB in the country.

India already has a quarter of the world’s disease burden of tuberculosis. An estimated 3.8 million Indians are thought to suffer from the condition.

TB has been documented to have a high incidence among workers in particulate dust-generating occupations such as coal mining, gold mining, and stone. But now doctors day that exposure to air pollution reduces immunity to disease. Air is deemed to be polluted when it contains high levels of particulate matter smaller than 10 microns and 2.5 microns.

"PM 2.5 and PM10 cant be good for the lung,” said Dr Barry Bloom, from Harvard University. “It would be adding to the TB burden in the country."

A 2014 study published in the International Journal of Tuberculosis and Lung Disease showed that indoor air pollution was independently associated with TB. The study was conducted in Pune, Maharashtra by a collaboration of scientists from the city’s BJ Medical College, and from Johns Hopkins University and Maine Medical Centre in the United States.

A study published in March in the journal Cell on the mechanism of TB infection could explain how pollution is associated with the disease. While the study is primarily on the effects of smoking, the results of the study could be collated to effects of pollution as well, said Lalita Ramakrishan from the department of Medicine at the University of Cambridge in the United Kingdom. She led a study with the University of Washington, Seattle, studying the genetic variants that increase susceptibility to TB in zebrafish, a see-through animal model for studying the disease.

Macrophages on the surface of the lungs are the first line of defence when a bacteria enters the lung. Macrophage – or "big eater" in Greek – is a type of white blood cell that engulfs the bacteria and digests it and is often successful in stopping TB infection in the lungs.

Smoke-clogged macrophages of cigarette smokers are unable to move to engulf infecting TB bacteria. Credit: Kevin Takaki and drawn by Paul Margiotta

“The macrophages in the lungs are adept at killing bacteria without creating too much of an immune response,” said Ramakrishnan. “It won’t cause any major inflammation, for instance.” Ramakrishnan is part of the India TB Research and Development Corporation, an initiative of Indian Council for Medical Research.

These macrophages are very good a killing bacteria. However, when smoke fills lungs up, they clog the macrophages and reduce their scope to digest bacteria or even move around.

Ramakrishnan offered the following analogy in a press release about the research released in March. “Macrophages act a bit like vacuum cleaners, hoovering up debris and unwanted material within the body, including the billions of cells that die each day as part of natural turnover,” she said. “But the defective macrophages are unable to recycle this debris and get clogged up, growing bigger and fatter and less able to move around and clear up other material.”

The clogged macrophages, as Ramakrishnan describes them, are not able to kill the TB bacteria effectively, leaving them to multiply and let the infection turn into a full-blown disease. On the bright side, the researchers found out that reduced exposure to smoke reduces the risk of contracting TB. The macrophages that were damaged due to smoke, die away and are replaced by nimble cells that can fight the infection.