In this article, we propose a new scheme DeGas to determine the boundary area of the toxic gas with planarization algorithm. This detected boundary area will ensure safe area around the toxic gas and will provide a decision reference for evacuation and rescue of the first-line workers in the large-scale petrochemical plants. Understanding the implications of these observations enable to find out an optimal trade-off between the cost of a number of deployed sensor nodes and the accuracy of the estimated toxic gas boundary area size.