Public blockchains offer transparency and security guarantees, but suffer from limited throughput, high latency, and unpredictable transaction costs, making them unsuitable for the high-frequency data flows of the Internet of Things. In this work, we address these scalability challenges by introducing a hybrid architecture that leverages state channels to enable efficient cross-chain smart contract execution between a permissioned ledger, responsible for capturing pollutant measurements from IoT sensors, and a public blockchain, used for final settlement. Our approach enables the processing of sensor readings to be performed off-chain in a lightweight, low-cost environment, while preserving the integrity and non-repudiation guarantees of on-chain settlement through a state channel. We describe the protocol design in detail, and the use of our solution in a case study involving water quality sensors.