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Castaños, Coahuila, nestled in northeastern Mexico, occupies a geographically significant position within the expansive Coahuila altiplano. Located at approximately 26.78° N, 101.42° W and an elevation of roughly 600 meters (1,969 feet) above sea level, the city’s terrain is characterized by rolling hills and plains, typical of the region's semi-arid climate. The surrounding landscape is dominated by scrubland and grasslands, transitioning into more rugged terrain towards the Sierra Madre Oriental mountain range to the east. This proximity to the mountains influences local wind patterns, often creating sheltered valleys where pollutants can accumulate. Castaños sits along the western edge of the Monterrey metropolitan area’s sphere of influence, though it retains a distinct rural character. The urban–rural gradient is noticeable, with agricultural zones, primarily focused on livestock and dryland farming, extending outwards from the city limits. While not directly adjacent to major rivers, the nearby Río San Juan provides a crucial water source and influences local humidity levels. The region’s industrial activity, largely centered around Monterrey, contributes to broader air quality concerns, with potential for transboundary pollution. The city’s relatively small population (27,314) means that local industrial emissions, primarily from smaller businesses and vehicle traffic, are the most immediate contributors to air quality challenges, exacerbated by the regional climate and topography. The prevailing winds, often light and variable, can trap pollutants, particularly during periods of atmospheric stability.

Castaños experiences a pronounced seasonal cycle that significantly impacts air quality. The dry season, typically spanning from October through April, is characterized by minimal rainfall and increased sunshine, leading to higher evaporation rates and lower humidity. This dryness, coupled with frequent temperature inversions – where a layer of warm air traps cooler air near the ground – creates conditions conducive to pollutant accumulation. Dust storms, common during this period, further degrade air quality, carrying particulate matter across the region. Spring (April-May) brings a slight increase in wind speeds, offering some respite from the stagnant air, but pollen levels also rise, impacting respiratory health. The onset of the monsoon season (June-September) introduces sporadic rainfall, which helps to cleanse the atmosphere and reduce particulate matter. However, intense storms can also stir up dust and temporarily worsen air quality. Summer months are generally hot, with high daytime temperatures and cooler nights. Fog, particularly in the early morning hours during the transition months (October and March), can trap pollutants close to the ground. Autumn (October-November) sees a return to drier conditions and the increased likelihood of temperature inversions. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor activity during periods of stagnant air, particularly in the mornings and evenings during the dry season. Staying indoors with air purifiers and monitoring local air quality reports are advisable during these times. Increased hydration and avoiding strenuous outdoor exercise are also recommended.