<p>Exposure to fine particulate matter (PM_2.5) pollution, both outdoors and indoors poses a significant health burden in Africa, where concentrations are often high, but there are limited measurements. Two types of low-cost sensors were used during two distinct measurement phases conducted in Ibadan, Nigeria. In Phase I, indoor and outdoor PM_2.5 concentrations were measured for a two-week period in twelve households using a total of twenty-four Atmotube PRO sensors. Phase II consisted of a seven-month extended monitoring study conducted in two households (each equipped with one indoor and one outdoor sensor) and a school (1 sensor only) using five PurpleAir sensors. Across the twelve households in Phase I, daily median PM₂.₅ concentrations ranged from 12.0 to 18.0&#xa0;µgm⁻³ indoors, and from 12.2 to 20.0&#xa0;µgm⁻³ outdoors. The overall PM_2.5 indoor-outdoor (I/O) median ratio was 0.9 indicating that outdoor levels were typically slightly higher than indoors. In January (the dry harmattan season), daily median PM_2.5 concentrations were 98.0 µgm⁻³ indoors and 109.3 µgm⁻³ outdoors. In contrast, lower PM_2.5 concentrations of 21.4 µgm⁻³ indoors and 24.5 µgm⁻³ outdoors were recorded in May, a rainy season. In Phase II, we find that a substantial part (~ 90%) of PM_2.5 concentrations can be explained by variance in the outdoor concentrations. There was exceedance of WHO interim target IT-1 of 75 µgm⁻³ for PM_2.5 during the dry harmattan season. The findings highlight the need for continuous air quality monitoring infrastructure to track pollutant trends and offering insights for future research.</p> Graphical Abstract <p></p>