Bacteria in classroom dust linked to reduced lung function in schoolchildren, European study finds

Two common types of bacteria found in the dust settled on classroom floors are linked to measurably weaker lung performance in schoolchildren, according to a large European study presented Sunday at the European Respiratory Society (ERS) Congress in Barcelona. The findings, drawn from nearly 300 classrooms in 22 countries, suggest that what settles quietly on windowsills and floor tiles may be doing more to shape children's developing lungs than most school administrators or parents have considered.

The research, presented by Dr. Soutrik Banerjee of the Department of Environmental and Prevention Sciences at the University of Ferrara in Italy, focused on two bacterial groups routinely found in soil, water and household dust: Streptomyces and Mycobacterium species. Neither is typically classified as a dangerous pathogen. But when researchers matched dust samples from classroom floors to spirometry results from the children who sat in those rooms, they found a clear statistical relationship between bacterial load and reduced breathing capacity.

What the numbers show

Children in classrooms with higher concentrations of Streptomyces had, on average, 0.08 liters lower forced vital capacity (FVC) — the maximum amount of air a person can forcefully exhale after taking the deepest possible breath. Higher classroom levels of Mycobacterium species were linked to a 0.06-liter reduction in FEV₁, the volume of air exhaled in the first second of that breath, and a 0.13-liter-per-second drop in peak expiratory flow (PEF), a measure of how fast air can be pushed out of the lungs.

Those differences sound small in isolation, and researchers were careful to describe them as modest at the individual level. But the associations held up even after the team adjusted for a long list of confounding factors: each child's age, gender, body mass index, exposure to secondhand smoke, contact with pet allergens, family socioeconomic status, the age of the school building, geographic region and local outdoor air pollution levels. That the bacterial associations survived all of those controls is what gives the finding statistical weight, according to the research team.

Where the data came from

The bacterial and spirometry measurements were drawn from the SINPHONIE study, a project originally commissioned by the European Parliament and coordinated through the European Commission's Joint Research Centre to catalogue indoor air quality across schools continent-wide. SINPHONIE was one of the most ambitious efforts of its kind, sending trained teams into schools in dozens of European cities to vacuum-sample classroom floor dust, log building characteristics such as ventilation and dampness, and test the lung function of the children who spent their days there.

Banerjee's team returned to that dataset — nearly 300 classrooms across 22 countries — specifically to isolate the signal from two bacterial genera that earlier indoor-air research had flagged as common in dust but rarely studied for their effect on children's lungs specifically, rather than on asthma or allergy risk generally.

• Almost 300 classrooms sampled across 22 European countries
• Two bacterial groups measured: Streptomyces and Mycobacterium spp.
• Lung function assessed by spirometry, the standard clinical breathing test
• Statistical models adjusted for nine separate confounding factors

Why a fraction of a liter matters

A reduction of a tenth of a liter or less in a single child's lung capacity is not something a parent or teacher would notice day to day. The concern researchers raised is cumulative and population-level: children spend five days a week, roughly nine months a year, inside classrooms for more than a decade of their development, and lung function established in childhood tracks closely with respiratory health decades later. A modest reduction sustained across a school career, spread across millions of children, is the kind of exposure public health researchers watch closely even when no single case looks alarming.

"Although the reductions we found were modest for each child, they are statistically significant and could be important for the wider population," Banerjee said. "Children are exposed to school indoor environments every weekday and a mild to moderate reduction in lung function today may be a pre-cursor to preventable lung disease in later life."

Banerjee's team has not established that the bacteria themselves are the direct cause of the lung changes. Streptomyces and Mycobacterium species could plausibly irritate airways directly or trigger low-grade immune and inflammatory responses when inhaled repeatedly. But Banerjee also raised a second possibility: the bacteria could simply be markers of broader problems with a classroom's condition, such as chronic dampness, poor ventilation, infrequent cleaning or outdoor dust infiltrating the building through open windows or worn seals.

Reaction from outside the study

The research was presented as a conference abstract rather than a peer-reviewed journal paper, a status typical for findings unveiled at a live medical congress, meaning it has not yet gone through full external peer review. ERS convenes independent experts to comment on notable submissions, and the society's Paediatric Assembly weighed in on the classroom-dust findings.

"This large European study shows that children's lungs seem to suffer when two common types of bacteria are growing in dust in their classrooms. While not all bacteria are harmful, these findings suggest that the classroom environment is important in children's lung health and they support the idea that healthy school buildings matter. The practical message is not that schools should become sterile environments. Rather, schools should focus on good indoor air quality: adequate ventilation, control of dampness and mould, regular cleaning and proper building maintenance," said Professor Alexander Möller, head of the ERS Paediatric Assembly and professor of paediatric pulmonology at the University Children's Hospital Zurich, who was not involved in the study.

Möller, who reviewed the findings independently of Banerjee's team, said the practical takeaway was not to chase sterility but to invest in the basics of building maintenance. "Children spend a lot of their time in the classroom, so it's important that we understand how to keep this environment safe and healthy," he said. "This study suggests that investment in indoor air quality, building maintenance and monitoring of environmental conditions in classrooms is money well spent."

What happens next

Banerjee's group is presenting the findings this week at the ERS Congress, which runs through September 9 in Barcelona, and the abstract is expected to move toward full peer-reviewed publication following the meeting — the standard path for congress research that draws significant interest. The team has not yet identified which specific building conditions most reliably predict elevated Streptomyces or Mycobacterium levels, a question Banerjee said needs dedicated follow-up study, potentially including intervention trials that test whether improved ventilation or cleaning protocols measurably reduce bacterial loads and, in turn, protect children's lung function.

In the meantime, the study adds classroom microbiology to a growing list of indoor-environment factors — alongside dampness, mould, ventilation rates and outdoor pollution infiltration — that researchers say deserve routine monitoring in schools, particularly as many European school buildings age and tightening energy-efficiency renovations can inadvertently reduce natural air exchange.