Classroom Air in Nigerian Schools Triggers Breathing Symptoms Even Below Chronic Risk Thresholds

Every school day, millions of children across sub-Saharan Africa sit through lessons in classrooms where the air they breathe may be quietly undermining their health. A new ten-week investigation in Benin City, Nigeria, has now provided one of the most detailed pictures yet of what students in tropical urban schools actually inhale, and the findings are striking. Researchers from the University of Benin monitored particulate matter, carbon monoxide, carbon dioxide and thermal comfort across six primary and secondary schools, then matched those measurements against detailed respiratory health questionnaires completed by the students themselves. Their results, published in Environmental Science and Pollution Research, reveal a troubling paradox: by the strict arithmetic of chronic health-risk assessment, the children appear safe, yet nearly six in ten report persistent coughing and a substantial share suffer chest pain, symptoms that the study links directly to the dust-laden air inside their classrooms.

The study, led by Aimuanmwosa Frank Eghomwanre and Michael Ovbare Akharame of the Department of Environmental Management and Toxicology, was designed to fill a conspicuous gap. While indoor air quality in European and North American schools has been extensively catalogued, comparable data from tropical African cities remain scarce, a deficit compounded by weak monitoring infrastructure and limited policy attention across the region. To address this, the team deployed a mixed-methods approach that combined objective environmental monitoring with subjective health assessment over ten weeks. The spatial and temporal dimensions mattered: measurements were taken across multiple schools and throughout the school day, allowing the researchers to track how pollutant levels shifted with occupancy, activity and weather, rather than relying on single snapshots that can miss the peaks that matter most for exposure.

The numbers they recorded are sobering. Concentrations of fine particulate matter, known as PM2.5 because the particles are 2.5 micrometres or smaller in diameter, ranged from 11.2 to 81.4 micrograms per cubic metre of air. Coarse particulate matter, PM10, spanned 20.1 to 138.6 micrograms per cubic metre. For context, the World Health Organization’s 2021 global air quality guidelines recommend annual mean PM2.5 levels of just 5 micrograms per cubic metre, with short-term limits far below the upper readings observed in these classrooms. At the top end of the measured range, students were breathing air carrying more than sixteen times the WHO’s annual guideline concentration of the finest, most penetrating particles. Carbon dioxide, a reliable proxy for ventilation adequacy in occupied rooms, peaked at an average of 729.3 parts per million with a standard deviation of 28.4 ppm, a level that signals crowding and insufficient fresh-air exchange even though it remained below some occupational thresholds.

Thermal conditions added a second layer of discomfort. The researchers calculated the temperature-humidity index, or THI, a composite measure that combines air temperature and relative humidity to gauge heat stress on the human body. Values across the classrooms ranged from 28.3 to 30.1, a band the study characterises as indicating severe heat discomfort. In tropical climates, high humidity compounds heat load by impairing the evaporation of sweat, the body’s primary cooling mechanism. Previous research has shown that overheated classrooms impair cognitive performance and concentration, and the Nigerian findings suggest that thermal stress and air pollution are not separate problems but intertwined features of the same poorly regulated built environment. Hot, still air also favours the resuspension of settled dust, meaning that physical discomfort and particulate exposure can reinforce one another.

To translate these exposures into health terms, the team applied the hazard quotient framework of the United States Environmental Protection Agency, a standard method for estimating non-carcinogenic risk from inhalation. The hazard quotient compares an estimated daily intake with a reference concentration considered safe over a lifetime; values below one are conventionally interpreted as indicating no immediate chronic risk. For both PM2.5 and PM10, every hazard quotient calculated in the study fell below unity. On paper, then, the students of Benin City are not on course for the chronic, cumulative respiratory damage that regulatory frameworks are designed to prevent. Yet the questionnaire data told a very different story, and it is this discrepancy that gives the study its urgency.

Using questionnaires adapted from the British Medical Research Council’s respiratory symptom instrument, the researchers found that 58.2 percent of students reported coughing, 32.5 percent reported phlegm production, and 26.7 percent reported chest pain. These are not trivial figures. Acute respiratory irritation, it turns out, frequently manifests at exposure levels well below the chronic thresholds embedded in the hazard quotient methodology. The explanation lies in the biology of the airways: fine particles deposit deep in the bronchial tree and trigger inflammation, mucus secretion and cough reflexes within hours or days of exposure, long before any cumulative dose approaches the levels associated with chronic disease. A hazard quotient built on long-term averaging can therefore coexist with a school population that is coughing through every lesson.

The multivariate analysis sharpened the picture considerably. Elevated particulate matter exposure was strongly associated with increased risk of coughing, an association that held at the p < 0.001 level, and significantly increased the odds of chest pain, with a crude odds ratio of 2.74 and a p-value of 0.001. In practical terms, students in the more heavily polluted classrooms faced nearly triple the odds of reporting chest pain compared with their peers in cleaner environments, before adjustment for other factors. Coughing, the most prevalent symptom, tracked particulate levels most tightly of all, consistent with the established physiology of PM2.5, which provokes airway irritation and reflex coughing as the lungs attempt to clear deposited particles.

Perhaps the most unexpected finding concerned sanitation. After adjusting for confounders including age, sex and socio-economic status, poor sanitation emerged as the strongest predictor of phlegm production, with an adjusted odds ratio of 5.54 and a 95 percent confidence interval of 1.35 to 22.80, significant at p = 0.018. Students in schools with inadequate sanitation faced more than five times the odds of producing phlegm compared with those in better-maintained institutions. This result echoes a growing body of evidence linking dampness, mould and poor building maintenance to respiratory symptoms in children, and it underscores that classroom air is shaped by more than outdoor pollution: the condition of the building itself, from its toilets to its ventilation openings, is a health determinant in its own right.

The broader context makes these findings more consequential. Sub-Saharan Africa’s urban populations are expanding rapidly, and school infrastructure has struggled to keep pace, with classrooms often crowded, naturally ventilated only through windows that may be closed against dust or traffic fumes, and sited near unpaved roads or open waste. Nigeria’s own national environmental air quality regulations exist on paper, but enforcement in school settings is minimal, and the country’s air quality monitoring network remains sparse. Studies from Addis Ababa and other African cities have similarly documented fine particulate levels exceeding WHO guidelines, suggesting that Benin City is not an outlier but a case study in a regional pattern. Children are particularly vulnerable because they breathe more air per unit of body weight than adults, their airways are still developing, and they have little control over the environments in which they spend their days.

What the Benin City study ultimately delivers is a methodological warning wrapped in a public health finding. Risk assessment tools built around chronic exposure thresholds, however rigorous, can systematically understate the lived experience of children in tropical schools, where acute symptoms flourish at concentrations regulators would deem acceptable on paper. The authors argue that particulate matter control and sanitation improvement should be treated as immediate priorities rather than long-term aspirations, and their data give school administrators and policymakers concrete targets: better ventilation to curb carbon dioxide buildup, dust suppression to cut particulate loads, and basic sanitation upgrades that could more than halve the burden of phlegm-producing respiratory illness. For the students of Benin City, and for millions like them across the tropics, the air in the classroom is not a background condition but an active determinant of health, and the evidence now shows that waiting for chronic thresholds to be crossed before acting would mean waiting far too long.

Subject of Research: Classroom air quality, thermal comfort and respiratory health risks among schoolchildren in urban Nigeria

Article Title: Spatio-temporal assessment of classroom air quality and thermal comfort as determinants of respiratory health risks in urban pre-tertiary institutions

Article References: Eghomwanre, A. F., & Akharame, M. O. (2026). Spatio-temporal assessment of classroom air quality and thermal comfort as determinants of respiratory health risks in urban pre-tertiary institutions. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38254-1

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38254-1

Keywords: indoor air quality, particulate matter, PM2.5, respiratory symptoms, thermal comfort, school health, Nigeria, carbon dioxide, sanitation, hazard quotient, children's health, tropical cities