The ECD Urban Pollution Study is a community-based longitudinal cohort study conducted among 522 preschool children enrolled across 62 preschools (public Anganwadi centres and low-cost private preschools) in four neighborhoods of Patna, Bihar. Children were recruited at 3.5–5.5 years of age and followed prospectively with repeated assessments of environmental exposures and developmental outcomes. The study employed standardized protocols for measuring blood lead concentrations, anthropometry, haemoglobin, executive function, and school readiness using the International Development and Early Learning Assessment (IDELA). Five field investigators underwent intensive training and certification in child assessments, biological sample collection, anthropometric measurements, and electronic data capture to ensure high-quality and standardized data collection. Data were collected using digital platforms with built-in quality assurance checks, enabling secure data management and real-time monitoring. Community engagement was central to the project, with partnerships established with preschools, caregivers, local health workers, and community leaders to support participant recruitment, retention, and dissemination of study findings. Research activities included longitudinal follow-up visits, environmental and biological exposure assessments, developmental testing, and advanced statistical analyses to identify modifiable risk factors. Findings are intended to inform evidence-based interventions and policies that promote healthier environments and improve developmental outcomes for children living in resource-constrained urban settings.