<p>Chemical sensors have become essential tools for real-time detection of hazardous substances in complex environmental systems. This review synthesizes recent advances in sensor technologies, focusing on innovations in materials, architectures, and integrated platforms for detecting pollutants such as heavy metals, volatile organic compounds (VOCs), pesticides, and chemical warfare agents. Emerging sensor designs, ranging from electrochemical and optical systems to photonic crystal fibers, have achieved significant improvements in sensitivity, selectivity, and portability. The incorporation of advanced materials, including metal–organic frameworks (MOFs), carbon-based nanomaterials, and molecularly imprinted polymers, has expanded sensing capabilities across air, water, and soil. Applications increasingly rely on smart, networked systems powered by wireless communication, artificial intelligence, and Internet of Things (IoT) frameworks to enable autonomous, scalable environmental monitoring. Despite these advances, critical challenges remain in sensor stability, cross-sensitivity, reproducibility, environmental robustness, data interpretation, calibration, and large-scale deployment. Addressing these limitations is essential for realizing the full potential of chemical sensors in environmental governance, public health protection, and rapid hazard response.</p>