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Wireless sensor networks (WSN) has a great application value and prospect in the fields of military, agriculture, environmental monitoring, medical health, industry, intelligent transportation, building monitoring, space exploration and so on. The operation of WSN is normal under lower traffic loads. But in case of an emergency situation, like fire break out large volumes of data have to be generated and delivered to the sink. Such kind of situations results in severe channel collision and thus decreases the communication throughput in medium access control (MAC) protocols. Hence data aggregation has been put forward as an essential paradigm for eliminating the redundancy. In this paper, we propose DA (Data Aggregation) MAC, a hybrid of TDMA (Time Division Multiple Access) and CSMA (Carrier Sense Multiple Access) MAC approaches which employs center at nearest source technique. Initially the nodes make use of TDMA for its slot assignment. Each node wakes up for its corresponding slot assignment and then goes to sleep mode thus saving energy. But during any emergency situation nodes participating in it change its MAC behavior by allowing contention in TDMA slots. Instead of data transmission by the one hop neighbors of the affected node, data aggregation is implemented for the node with the minimum hop distance for reducing the overhead. This data aggregation technique is called the centre at nearest source approach. The simulation is carried out by means of NS2 (Network Simulator). Analytical and experimental results show that the proposed hybrid MAC protocol outperforms in all its aspects.