Indoor air pollution and adverse microclimatic conditions are important environmental determinants of respiratory health, particularly in rapidly urbanising settings where housing conditions, ventilation and household energy use may increase exposure to airborne pollutants. Evidence combining objective measurements of residential indoor air quality with respiratory health outcomes remains limited in many Nigerian urban communities. This study assessed indoor concentrations of particulate matter, carbon monoxide, and carbon dioxide, as well as temperature and relative humidity. It examined their associations with respiratory morbidity among adult residents of Ibadan South-West Local Government Area, Nigeria. A descriptive cross-sectional study was conducted among adult residents of Ibadan South-West Local Government Area. Respiratory health information was collected using a semi-structured interviewer-administered questionnaire adapted from the European Community Respiratory Health Survey. Indoor particulate matter with aerodynamic diameters ≤2.5 µm (PM₂.₅) and ≤10 µm (PM₁₀), carbon monoxide (CO), carbon dioxide (CO₂), temperature, and relative humidity were measured in selected residences. Environmental measurements were obtained at three indoor locations during morning (09:00–11:00) and afternoon (13:00–15:00) periods, three days per week over a two-week monitoring period. Data were analysed using STATA MP/17. Descriptive statistics, tests of association and logistic regression were used to examine relationships between indoor environmental conditions and respiratory morbidity. Statistical significance was set at p<0.05. The analytical results comprised 215 adults, of whom 134 (62.3%) were classified as having respiratory issues. Mean indoor PM₂.₅ concentrations increased from 42.2 ± 11.5 µg/m³ in the morning to 55.1 ± 16.7 µg/m³ in the afternoon, while PM₁₀ increased from 137.9 ± 38.5 to 188.1 ± 48.4 µg/m³. Mean CO concentrations increased from 0.22 ± 1.1 ppm in the morning to 1.3 ± 4.3 ppm in the afternoon, and CO₂ increased from 520.2 ± 164.5 to 618 ± 341 ppm. Mean temperature increased from 24.4 ± 1.3°C to 28.0 ± 1.0°C, whereas relative humidity decreased from 64.4 ± 3.0% in the morning to 50.8 ± 9.2% in the afternoon. Above-optimum afternoon relative humidity was associated with increased odds of respiratory morbidity (OR=3.91; 95% CI: 1.65–9.27; p=0.002). Unexpected inverse associations were observed between respiratory morbidity and higher afternoon PM₁₀ concentrations (OR=0.33; 95% CI: 0.14–0.74; p=0.007) and between respiratory morbidity and normal versus low afternoon CO₂ concentrations (OR=0.31; 95% CI: 0.11–0.92; p=0.034). PM₂.₅, CO and most morning environmental indicators were not significantly associated with respiratory morbidity. Indoor environmental conditions varied between morning and afternoon monitoring periods, with higher mean concentrations of particulate matter, CO, and CO₂, and higher temperatures observed in the afternoon. Above-optimum afternoon relative humidity was positively associated with respiratory morbidity. The inverse associations observed for afternoon PM₁₀ and CO₂ should be interpreted cautiously given the cross-sectional design, categorical exposure classification and limited duration of environmental monitoring. Longer-term studies using continuous exposure measures and repeated-measures modelling are warranted.
