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Kimilili, nestled within Bungoma County in western Kenya, occupies a geographically complex position that significantly influences its air quality. Located at coordinates 0.7877, 34.7156, the town sits at an elevation of approximately 1,400 meters above sea level, contributing to cooler temperatures than surrounding lowland areas. The terrain is characterized by rolling hills and valleys, typical of the western Kenyan highlands, formed by ancient volcanic activity and subsequent erosion. Kimilili is not directly adjacent to a major body of water, though the Nzoia River flows nearby, impacting local humidity and potential for localized fog formation. The surrounding landscape is predominantly agricultural, dominated by smallholder farms cultivating maize, sugarcane, and beans. This agricultural activity, while vital to the local economy, contributes to particulate matter pollution through land preparation, harvesting, and the burning of agricultural residue. The urban-rural gradient is relatively sharp; Kimilili itself is a growing market town, but quickly transitions to rural farmland. Limited industrial activity exists within Kimilili itself, but regional transport routes pass through, potentially introducing emissions from vehicle traffic. The town’s position within a valley can exacerbate pollution events by trapping air masses and hindering dispersion.

Kimilili experiences a bimodal rainfall pattern, defining its ‘seasons’ as wet and dry rather than the traditional four. The long rains, typically from March to June, offer a period of relatively improved air quality. Rainfall effectively washes particulate matter from the atmosphere, and increased humidity suppresses dust. However, this period also sees increased fungal spore counts, potentially aggravating respiratory conditions. The drier season, July to October, witnesses a significant deterioration in air quality. Reduced rainfall allows dust and agricultural residue burning to dominate, leading to higher levels of particulate matter. November to February constitutes a shorter dry spell, often with hotter temperatures and increased dust from unpaved roads. Temperature inversions, common during the cooler nights of the dry seasons, trap pollutants near ground level. January and February are generally the worst months for outdoor activity for those with respiratory sensitivities. During the wet seasons, while overall pollution is lower, increased humidity can exacerbate asthma. Sensitive groups – children, the elderly, and those with pre-existing respiratory conditions – should limit strenuous outdoor activity during peak pollution periods and consider using masks when dust levels are high.