Poor ventilation in schools may spur TB spread

Poor ventilation in classrooms might contribute to tuberculosis spread among children, researchers in South Africa find. By asking students to wear air monitors, the scientists showed that the average student was breathing an unsafe level of others’ exhaled air three-fifths of the time.

Cape Town has a high rate of TB, which is spread when the mycobacterium goes airborne. High school students in Cape Town test positive for TB at a rate of more than 4 per 1,000. Since TB carriers in the early stages of infection often lack symptoms beyond a cough, school transmission is likely, says study coauthor Linda-Gail Bekker, a physician and scientist at the University of Cape Town.

While the fight against TB needs to center on identifying, isolating and treating infected people, says Paul Jensen, an environmental health engineer at the Centers for Disease Control and Prevention, “ventilation is important from a public health standpoint.” Carbon dioxide levels are a well-tested measure of exhaled air, he says, and serve as a proxy for what could happen if a person had undiagnosed TB. “Their conclusions are reasonable,” he says.

Improving ventilation could prevent transmission of the disease, Bekker says. TB cases have risen steadily in Cape Town since the mid-1990s, she says, where “people are sharing air for long periods of time” on public transport, in homes and at school.

In the study, researchers tracked classroom CO₂ levels with minute-by-minute precision. Bekker and her colleagues enlisted 64 students at a Cape Town school to carry portable CO₂ monitors during 91 days spread out over an academic year. The monitors took a reading every 60 seconds. The classrooms held an average of 31 students.

The scientists calculated that students spent about 60 percent of their time inhaling air containing more than 1,000 parts per million CO₂, a threshold for poor air hygiene that’s more than double the 350 to 400 ppm found outdoors. The CO₂ measurements indicated that classroom air was fully recycled only five or six times per hour on average, just half of the 12 air changes considered healthy, Bekker and her colleagues report in the May PLOS ONE.  

Bekker hopes the team can gather enough evidence to effect a change of school construction practices, specifically by adding more windows that open and opening existing windows. “A lot of building construction is not conducive to healthy living,” she says. Improving ventilation addresses TB transmission and goes beyond just finding TB cases and giving patients drugs, she says. “That’s not winning the war.”