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This paper introduces ER-MAC, a hybrid MAC protocol for emergency response wireless sensor networks. ERMAC is designed as a hybrid of the TDMA and CSMA approaches, giving it the flexibility to adapt to traffic and topology changes. It adopts a TDMA approach to schedule collision-free slots. Nodes wake up for their scheduled slots, but otherwise switch into power-saving sleep mode. When an emergency occurs, nodes that participate in the emergency monitoring change their MAC behaviour by allowing contention in TDMA slots to achieve high delivery ratio and low latency. ER-MAC offers a synchronised and loose slot structure to allow nodes to join or leave the network. Simulations in ns-2 show that ERMAC outperforms Z-MAC with higher delivery ratio, lower latency, and lower energy consumption.