Purpose: Oil exploration in the Niger Delta presents a paradigmatic case of economic prosperity existing alongside severe environmental degradation and public health neglect. This study quantifies the cumulative health risks from multi-media exposure to contaminants emanating from oil exploration activities in Rivers State, Nigeria. We employ the Health Hazard Index (HHI) framework to address critical gaps in prevailing singlepollutant risk assessments, which overlook synergistic effects and systematically underestimate the true public health burden. Methodology: A convergent parallel mixedmethods design integrated quantitative environmental sampling with geospatial analysis. A stratified random sampling approach collected 256 environmental samples (air, water, soil) across eight oil-producing communities. Analytical techniques included GC-MS for VOCs/PAHs and ICP-OES for heavy metals, adhering to stringent USEPA protocols. The HHI incorporated noncarcinogenic hazards via hazard quotients (HQ), carcinogenic risks (ILCR) using probabilistic Monte Carlo simulation, and child-specific exposure factors. Results: Findings reveal catastrophic pollution levels: benzene in air (28.7 μg/m³) exceeded WHO limits by 3.2×; lead in water (0.35 mg/L) surpassed standards by 35×; and soil PAHs (42.3 mg/kg) indicated Carcinogenic risks 1,150× above regulatory thresholds. The computed HHI reached a maximum of 8.5, signifying severe health hazards. Children exhibited 3.2× higher vulnerability than adults, and single-pollutant models were found to underestimate cumulative risk by a factor of 4.8. Conclusion: This study provides the first empirical evidence that Nigeria's prevailing regulatory approach is catastrophically inadequate. The findings necessitate an urgent paradigm shift in environmental governance towards a cumulative risk framework. We recommend the legislative adoption of HHI standards, targeted remediation technologies, and the establishment of community bio monitoring clinics. The methodology establishes a replicable model for assessing industrial pollution in low-resource settings globally, advancing both environmental science and justice.