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In an attempt to unravel the intricacies of outdoor air pollution in a traditional urban settlement, the study examined the extent, spatial variations and causes of declining air quality in Ogbomoso. The study area was delineated into core, transition and suburban zones and half of the identified residential precincts (54) from the three zones were selected. Data were obtained with gas samplers on the concentrations of Carbon II Oxide (CO), Sulphur IV Oxide (SO2) and Particulate Matters (PM1, PM2.5, PM4, PM7, PM10 and TSP) around 385 sampled buildings and questionnaire administered to the occupants. Concentration levels of the pollutants were also measured around seven specific land use areas. Analysis of Variance (ANOVA) was used to explain the inter-zonal variations in outdoor air quality. Findings revealed that the mean PM2.5 (46.3μg/m³) was higher than the WHO limit, though lower than the WHO IT-1 limit for maximum 1-hour average, while the mean PM10 (188.8μg/m³) was higher than both set limits. The mean TSP level (287.3μg/m³) was lower than the FEPA standard and mean outdoor CO levels were generally lower than set limits. The inter-zonal variations of air quality, using “Outdoor PM Aggregate” showed that outdoor air quality varied significantly with residential zone (p = 0.001). Land use sources of specific air toxics were also identified, with abattoirs and sawmills found to increase ambient levels of PM2.5 beyond WHO limit, while printing outfits increased ambient levels of SO2 and CO beyond the set limits. The study concludes that outdoor air quality in Ogbomoso is considerably low with respect to ambient levels of inhalable particles and recommended pragmatic land use planning measures which would help to ensure better air quality.

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