<p>Over the past few years<b>,</b> Internet of Things (IoT) has surfaced as an effective technology which helps in interconnecting objects and computing devices together. Enormous potential of IoT has helped in providing more convenient and high-quality services for many day-to-day applications. Tangible objects can be connected to internet through IoT which enables rapid reception and transmission of data. Significant advancement of IoT technology further helps in connecting different smart objects via internet as well as facilitates in providing better methods for data interoperability. In many large-scale applications like healthcare monitoring, environment monitoring, smart agriculture, smart cities, smart grids, smart factories etc., IoT is playing a significant role. Machine learning, embedded systems, real-time analytics and sensors have influenced the concept of IoT. IoT sensors help in providing improved communication without the necessity for human-to-human and human-to-computer interface. This article critically analyzes various IoT sensor-based systems used in medical surveillance and in monitoring of air and water quality. The article reviews the state-of-the-art developments, real word application, and key technologies in these domains. The performance of various IoT sensor-driven systems used for monitoring in real time were also meticulously studied. </p>