In modern days, as medical technology advances and the world's population grows, healthcare has been undergoing crucial changes. The common practice of clinical treatment is gradually being overhauled by ubiquitous healthcare systems. The wireless body area networks (WBANs) are becoming increasingly important for future healthcare applications. According to the human physiological information, WBAN not only sends the real-time notification to the user but also decreases the transmission delay of patient information to the physician for medical decision. When medical emergency information must be transmitted after a disaster occurrence, the number of WBANs and emergency traffic increase in the particular area. In the situations, a large number of WBANs may be close to each other, thus causing inter-WBAN interference and increasing the transmission delay. Therefore, in this paper, we propose a dynamic channel selection scheme (DCSS) in IEEE 802.15.6 WBANs. DCSS records the channel interference degree which is used to adaptively select the best transmission channel and to effectively transmit data. It mitigates the inter-WBAN interference to improve the QoS violation and reduce power consumption when there are WBANs coexisting.