Indonesia accounts for more than one third of the world’s tropical peatlands. Much of the peatland in Indonesia has been deforested and drained, meaning it is more susceptible to fires, especially during drought and El Niño events. Fires are most common in Riau (Sumatra) and Central Kalimantan (Borneo) and lead to poor regional air quality. Measurements of air pollutant concentrations are sparse in both regions contributing to large uncertainties in both fire emissions and air quality degradation. We deployed a network of 13 low-cost PM2.5 sensors across urban and rural locations in Central Kalimantan and measured indoor and outdoor PM2.5 concentrations during the onset of an El Niño dry season in 2023. During the dry season (September 1st to October 31st), mean outdoor PM2.5 concentrations were 136 µg m-3 , with fires contributing 90 µg m-3 to concentrations. Median indoor/outdoor (I/O) ratios were 1.01 in rural areas, considerably higher than those reported during wildfires in other regions of the world (e.g. USA), indicating housing stock in the region provides little protection from outdoor PM2.5. We combined WRF-Chem simulated PM2.5 concentrations with the median fire-derived I/O ratio and questionnaire results pertaining to participants’ time spent I/O to estimate 1.62 million people in Central Kalimantan were exposed to unhealthy, very unhealthy and dangerous air quality (>55.4 µg m-3 ) during the dry season. Our work provides new information on the exposure of people in Central Kalimantan to smoke from fires and highlights the need for action to help reduce peatland fires.