Spatial Variation of PM10 and PM2.5 in Residential Indoor Environments in Municipalities Across Mexico City

Despite significant progress in controlling outdoor air pollution in Mexico City over the past three decades, indoor air pollution remains largely unaddressed. This is particularly concerning because health authorities advise people to stay indoors when outdoor pollution exceeds safe limits, yet indoor concentrations can be higher. Two optical particle counters were deployed simultaneously indoors and outdoors in 38 homes across all municipalities in Mexico City. The average indoor 24 h PM2.5 concentration was 24.5 µg m−3, while PM10 concentration averaged 78.6 µg m−3 compared to outdoor averages of 20.5 µg m−3 and 72.0 µg m−3. The PM2.5/PM10 ratio was 0.3 both indoors and outdoors. Only 20% of the homes exhibited maximum outdoor PM2.5 concentrations 3.6 times higher than indoor; in 18%, indoor and outdoor levels were similar (0.8–1.2); and 60% of homes recorded indoor maxima up to nine times the outdoor peaks. Elevated indoor PM2.5 was primarily linked with cooking and, to a lesser extent, cleaning activities. Peaks in PM2.5 persisted for 4–8 h before returning to baseline. Ensuring adequate indoor ventilation is critical to maintain indoor air quality below outdoor levels and comply with WHO guidelines, highlighting the need for targeted strategies to reduce indoor exposure in urban homes.