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Usumatlán, nestled within the department of Zacapa in Guatemala, occupies a geographically significant position in the Motagua River valley. The city’s coordinates, 14.9489° N, -89.7767° W, place it amidst a landscape dominated by rolling hills and the fertile plains characteristic of this region. Elevation plays a crucial role; Usumatlán sits at approximately 180 meters above sea level, influencing atmospheric circulation and pollutant dispersion. The Motagua River, a major waterway, flows nearby, providing irrigation for surrounding agricultural lands but also potentially contributing to humidity and localised temperature variations. The urban character of Usumatlán is largely defined by its role as a regional market town, serving a predominantly rural hinterland. This creates a distinct urban-rural gradient, with agricultural activities – primarily subsistence farming and some larger-scale banana and palm oil plantations – dominating the surrounding areas. These agricultural practices, including the use of fertilizers and potential burning of agricultural waste, are key contributors to localised air pollution. The valley’s topography can trap pollutants, particularly during periods of calm weather, exacerbating air quality concerns. The lack of significant industrial belts within the immediate city limits means that transport and agricultural emissions are the primary drivers of air quality issues.

Usumatlán experiences a pronounced wet and dry season, dictating the annual air quality narrative. The dry season, typically from November to April, sees a significant increase in particulate matter. Reduced rainfall means less atmospheric cleansing, and agricultural burning for land preparation becomes more frequent, releasing substantial smoke and ash. Dust from unpaved roads also contributes to elevated levels of PM10 and PM2.5. January and February are generally the months with the poorest air quality. Conversely, the wet season, spanning May to October, brings relief. Frequent rainfall effectively washes pollutants from the atmosphere, leading to improved air quality. However, increased humidity can also foster the growth of molds and fungi, potentially triggering respiratory issues. September and October, during the peak of the rainy season, are generally the most favourable months for outdoor activity. Temperature inversions, common during the calmer mornings of the dry season, can trap pollutants near ground level, worsening conditions. Sensitive groups – children, the elderly, and those with pre-existing respiratory conditions – should limit strenuous outdoor activity during the dry season and monitor their health closely. Local winds, often influenced by the valley’s topography, play a role in pollutant dispersion, but are often insufficient to fully mitigate the effects of emissions.