Respiratory tract infections are among the most common reasons infants require medical attention worldwide, and acute otitis media, a painful middle-ear infection, remains a leading cause of antibiotic prescriptions and pediatric clinic visits in high-income countries. Because these infections tend to cluster in the earliest months of life, when the immune system is still learning to recognize and fight common pathogens, researchers have long suspected that the conditions in which a baby spends most of its time could either amplify or dampen that risk. The new study was designed to test this idea systematically, using prospectively collected prenatal data on housing conditions combined with detailed questionnaire-based interviews conducted when the children were approximately six and eighteen months old. By linking these responses to routinely collected Danish registry data, the team was able to construct a remarkably complete record of each child’s home environment and health outcomes.

The analytical approach relied on Poisson regression models with robust standard errors, a statistical framework well suited to estimating incidence rate ratios for binary outcomes such as whether a child had experienced pneumonia, bronchitis, frequent common colds, or acute otitis media by a given age. The researchers reported adjusted incidence rate ratios, abbreviated aIRRs, along with 95 percent confidence intervals, allowing them to quantify both the direction and the precision of each association. Crucially, the models adjusted for a wide range of potential confounders measured during pregnancy, including parental characteristics and socioeconomic indicators, which helps to reduce the risk that observed associations simply reflect differences in family background rather than genuine environmental effects.

Perhaps the most striking result concerned household crowding. Children who grew up in homes with less space per child were strongly and consistently more likely to develop pneumonia and acute otitis media by six months of age, and the association followed an exposure-dependent pattern, meaning the risk increased progressively as the amount of space available to each child decreased. This dose-response relationship is important from an epidemiological standpoint because it strengthens the argument that crowding itself, rather than some correlated but unmeasured factor, contributes to infection risk. The biological mechanism is intuitive: when infants share limited space with more people, particularly other children, they encounter a higher dose and greater variety of respiratory viruses and bacteria, increasing the likelihood that pathogens will colonize the airways or spread to the middle ear.

Yet the study’s deeper analysis revealed a subtlety that complicates the simple interpretation of the crowding effect. When the researchers disentangled the two components of the crowding measure, the available space per child and the total number of other children in the home, they found that the direct effect of space itself was likely less important than the direct effect of the number of children. In other words, a baby living with several siblings in a reasonably spacious apartment may face a similar infection risk to one in a smaller home with the same number of co-resident children. This distinction matters for public health because it suggests that interventions aimed purely at increasing living space may yield smaller benefits than expected, whereas reducing the number of close contacts, for example through daycare timing or hygiene practices, could be more effective.

Home ownership, often used in epidemiological studies as a marker of socioeconomic stability and housing quality, turned out to be largely irrelevant to infant infection risk. Across all four outcomes examined at both six and eighteen months, the only significant association was a modest increase in the risk of frequent common colds among children living in homes they did not own, with an adjusted incidence rate ratio of 1.07 and a confidence interval of 1.01 to 1.14. The authors interpreted this finding as evidence that ownership per se, as opposed to the conditions it might proxy, plays little role in shaping early-life respiratory health. This null result is notable because it suggests that within a country like Denmark, where housing standards are generally high and healthcare is universally accessible, the classic socioeconomic gradient in home quality may be less consequential for infant infections than exposure to other people.

The study also tested one of the most debated hypotheses in environmental epidemiology: the idea that early exposure to pets or livestock protects children against infections and allergic disease through immune-training effects. Contrary to expectations, the researchers observed no protective effect of dogs or cats on any of the respiratory outcomes at six months. Similarly, living in a farmhouse with livestock conferred no measurable benefit during the first half-year of life. These findings add nuance to a body of literature, much of it derived from smaller studies, that has suggested farm upbringing might shield infants from respiratory illness. Whatever protective signals exist in those earlier datasets, they do not appear clearly in this very large and carefully adjusted cohort analysis at the earliest ages.

However, the story changed by eighteen months. Children living in farmhouses with livestock showed small but statistically meaningful reductions in the risk of pneumonia, acute otitis media, and frequent common colds at that later age. Intriguingly, post hoc analyses suggested that any benefit might be attributable to characteristics associated with keeping cows, while any potential harm appeared to be linked to poultry. This divergence is biologically plausible: different animal species carry distinct microbial communities, produce different levels of dust and endotoxin, and are associated with different patterns of zoonotic exposure. The finding that cows and poultry pull in opposite directions may help explain why previous farm studies, which often lumped all livestock together, have produced inconsistent results. It also hints that the composition of farm exposure, not simply its presence, determines whether the net effect on infant respiratory health is protective or detrimental.

The authors conclude that crowding is associated with greater risks of respiratory tract infections in the first months of life, but that this association is largely driven by the number of children in the home rather than by space per se. Risks associated with farm living depend on the type of livestock present, while factors such as home ownership and pet ownership appear potentially unimportant. For parents and policymakers, the practical implications are measured rather than dramatic. In a high-income setting with good housing standards, the most modifiable risk factor identified is not the building itself but the density of young children sharing it. For researchers, the study underscores the value of very large prospective cohorts and of disaggregating broad exposure categories, such as farm living, into their component parts. As the children in the Danish National Birth Cohort continue to be followed, future analyses may reveal whether these early-life infection patterns leave lasting imprints on respiratory health, including the development of asthma and allergic disease later in childhood.

Subject of Research: The association between home characteristics such as household crowding, home ownership, pets, and livestock farming and the risk of respiratory tract infections and acute otitis media in infants.

Article Title: The association of home characteristics and risk of respiratory tract infections and acute otitis media in the first 18 months of life: a study of ~ 48 000 children in the Danish National Birth Cohort

Article References: Groot, J., Sigsgaard, T., de Moira, A. P., Keller, A., Bønnelykke, K., Loft, S., & Andersen, A.-M. N. (2026). The association of home characteristics and risk of respiratory tract infections and acute otitis media in the first 18 months of life: a study of ~ 48 000 children in the Danish National Birth Cohort. Environmental Health. https://doi.org/10.1186/s12940-026-01332-5

Image Credits: AI Generated

DOI: 10.1186/s12940-026-01332-5

Keywords: respiratory tract infections, acute otitis media, household crowding, Danish National Birth Cohort, infant health, farm living, livestock exposure, pets, home ownership, environmental epidemiology, pneumonia, pediatric public health