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Castelo do Piauí, nestled in the southeastern region of Piauí state, Brazil, occupies a unique geographic position within the Cerrado biome. Its coordinates, -5.3222, -41.5525, place it at an elevation of approximately 380 meters above sea level, contributing to a relatively mild climate compared to lower-lying areas of the state. The surrounding landscape is characterized by undulating hills and plateaus typical of the Cerrado, interspersed with gallery forests along seasonal streams. This terrain influences local wind patterns, often channeling breezes through the town, which can both disperse and concentrate pollutants depending on atmospheric stability. The urban character of Castelo do Piauí is that of a small, historically significant town, largely reliant on agriculture and small-scale commerce. While not situated directly adjacent to a major river, the Rio Claro flows nearby, impacting local humidity and potentially influencing the formation of ground-level ozone under specific conditions. The urban–rural gradient is gradual, with agricultural lands – primarily cattle ranching and subsistence farming – extending outwards from the town limits. There are no major industrial belts immediately surrounding Castelo do Piauí; however, the region's agricultural practices, including burning for land clearing (though increasingly regulated), can contribute to particulate matter pollution, particularly during drier periods. The town’s location within a relatively isolated region means that long-range transport of pollutants from larger urban centers is less of a factor compared to cities closer to major highways or industrial hubs.

Castelo do Piauí experiences a tropical savanna climate, defined by distinct wet and dry seasons rather than the four traditional seasons. The dry season, typically spanning from May to September, presents the most significant air quality challenges. Reduced rainfall leads to lower humidity, creating a more stable atmospheric environment. This stability can trap pollutants near the ground, particularly particulate matter arising from agricultural activities like controlled burns, a traditional practice for pasture management. Temperature inversions, where cooler air is trapped beneath warmer air, are more frequent during this period, further exacerbating the issue. The wind patterns during the dry season are often weaker and more variable, limiting the natural dispersion of pollutants. The wet season, from October to April, brings increased rainfall and higher humidity, which naturally washes particulate matter from the air and promotes better ventilation. However, heavy rainfall can also lead to increased levels of biological particulate matter, such as pollen and fungal spores, impacting respiratory health. Months like August and September, coinciding with peak burning activity, should be approached with caution for individuals with respiratory sensitivities. During the wet season, outdoor activities are generally more favorable, but those with allergies should remain vigilant. Sensitive groups, including children, the elderly, and individuals with pre-existing respiratory conditions, should minimize exposure to smoke during the dry season and monitor local air quality reports when available. Increased humidity during the wet season can also promote mold growth, impacting indoor air quality.