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San Sebastián, nestled within the Retalhuleu Department of Guatemala, occupies a geographically significant position in the southern highlands. Located at approximately 14.5667° N, -91.6500° W, the city sits at an elevation of roughly 1,600 meters (5,249 feet) above sea level, placing it within a zone characterized by cooler temperatures compared to Guatemala’s coastal lowlands. The surrounding landscape is a tapestry of rolling hills and valleys, typical of the Guatemalan highlands, with agricultural lands dominating the immediate periphery. This includes significant coffee cultivation, a key economic driver for the region, and smaller plots dedicated to maize and other staple crops. The urban–rural gradient transitions relatively quickly, with small villages and rural settlements blending seamlessly into the city’s edges. While San Sebastián isn't directly adjacent to a major river, it benefits from several smaller streams and springs, contributing to the local water supply. The proximity to larger urban centers like Retalhuleu and Quetzaltenango influences its economic and social dynamics. Topographically, the city’s layout is dictated by the slopes, resulting in a somewhat dispersed urban form. The prevailing winds, generally from the east, play a crucial role in air circulation, but the valley setting can sometimes trap pollutants, particularly during periods of calm weather. The lack of extensive heavy industry within San Sebastián itself mitigates some pollution sources, but agricultural practices and vehicle emissions remain key factors influencing local air quality. The surrounding volcanic terrain also influences drainage patterns and potential for dust generation.

San Sebastián’s air quality experiences a distinct seasonal pattern dictated by Guatemala’s tropical climate, characterized by a wet and dry season. The dry season, typically spanning November through April, often sees a slight improvement in air quality due to increased wind speeds and reduced humidity. However, this period also witnesses increased agricultural burning as farmers clear land for planting, contributing to particulate matter pollution. The wet season, from May to October, brings frequent rainfall, which naturally helps to cleanse the atmosphere by washing away pollutants. However, persistent fog, particularly in the mornings, can trap emissions, leading to localized periods of reduced visibility and elevated concentrations of certain pollutants. Temperature inversions, common during the cooler months (December-January), can also exacerbate air quality issues by preventing vertical mixing of air. During these inversions, cooler air becomes trapped beneath a layer of warmer air, effectively creating a lid that prevents pollutants from dispersing. The agricultural cycle significantly impacts air quality; the planting season (often coinciding with the start of the wet season) sees increased fertilizer use and potential runoff, while harvesting periods can generate dust. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during agricultural burning periods and when fog is prevalent. Outdoor activities are generally more favorable during the wetter months, but vigilance is still required, especially during mornings with limited visibility. The interplay of rainfall, wind patterns, and agricultural practices creates a dynamic air quality profile throughout the year.