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São Tiago de Custoias, nestled within the Porto District of Portugal, occupies a geographically significant position at approximately 41.2069° N, -8.6485° W. The town’s location, roughly 25 kilometers southeast of Porto, places it within a transitional zone between the Douro River valley and the Serra do Marão mountain range. This terrain is characterized by rolling hills and valleys, with São Tiago de Custoias itself situated at an elevation of around 270 meters above sea level. The surrounding landscape is predominantly rural, featuring agricultural fields – particularly vineyards and olive groves – interspersed with pockets of woodland. The proximity to the Douro River, though not directly adjacent, influences regional weather patterns and contributes to a generally moderate climate. The urban–rural gradient is relatively sharp; São Tiago de Custoias retains a distinctly rural character, with a population of just over 18,000, and limited industrial presence. The nearest significant industrial areas are concentrated around Porto and its metropolitan area, meaning that while direct industrial emissions are minimal within the town itself, regional pollution events can still impact air quality. The Serra do Marão to the east creates a barrier that can sometimes trap pollutants, particularly during periods of low wind. The valley setting also means that cold air drainage can occur during winter months, potentially leading to localized temperature inversions and stagnant air conditions. The town’s geography, therefore, presents a complex interplay of factors influencing its air quality, balancing rural purity with regional industrial influences and topographical effects.

São Tiago de Custoias experiences a Mediterranean climate, resulting in distinct seasonal variations that significantly impact air quality. Spring (March-May) generally offers the most favourable conditions, with increasing temperatures and more frequent winds dispersing any accumulated pollutants. However, pollen levels rise during this period, posing a challenge for allergy sufferers. Summers (June-August) are typically hot and dry, with stagnant air conditions becoming more common, especially during heatwaves. The lack of rainfall and reduced wind speeds can trap pollutants, potentially leading to a build-up of particulate matter and ozone. Autumn (September-November) brings a gradual cooling and increased rainfall, which helps to cleanse the atmosphere. However, agricultural burning, a traditional practice in the region, can contribute to localized spikes in smoke and particulate matter during this season. Winter (December-February) is often characterized by colder temperatures and occasional fog, particularly in the valleys. Temperature inversions are more frequent during this time, trapping pollutants near the ground and exacerbating air quality issues. Sensitive groups, such as children, the elderly, and individuals with respiratory conditions, should be particularly cautious during the summer and winter months, limiting outdoor activity during periods of stagnant air or fog. While the town benefits from its rural setting, regional pollution events originating from Porto can still impact air quality throughout the year, highlighting the importance of monitoring and awareness.