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Because air pollutants seriously affect human health, the government in Taiwan attempts to reduce the pollutants by passing many environmental laws and establishing various large-scale and expensive air quality monitoring systems. These monitoring systems are capable of large-scale monitoring, but they cannot immediately detect small-scale air quality changes. Wireless sensor networks are novel monitoring tools for small-scale monitoring. Through a wireless network, a large number of tiny sensor nodes can be deployed to a monitoring area. In this research, a reliable medium access control (MAC) protocol is presented for the urban air quality monitoring system based on the wireless sensor network. The proposed MAC protocol uses the concept of overhearing to reduce previously required control messages and data messages in the network. The reconfiguration and rejoining functions are also incorporated into the proposed MAC protocol for improving the stability of data transmission. Moreover, the proposed MAC protocol is designed for the sensor nodes built upon the TinyOS, aiming at air urban quality monitoring in Taipei, Taiwan. This urban monitoring system consists of 36 sensor nodes to detect carbon monoxide (CO) concentration, humidity, and temperature. The experimental data shows that the proposed MAC protocol is capable of stably working a long period of time, and it also indicates a significant difference in the pollutant concentration between the rush and non-rush hours.