To reduce CO₂ emissions from the manufacturing phase of building materials, sustainable structural design has been introduced. However, embodied CO₂ emission data in construction materials vary significantly depending on life cycle assessment (LCA) techniques, nations, and the time of data formation. In the embodied CO₂ emission data used in recent literature, the percentage differences in CO₂ emission data for concrete and steel as two typical building materials found to be 267 and 863%, respectively. These variations cause significantly different results in sustainable structural design. Thus, to avoid the dependency on the selection of embodied CO₂ emission data, a sustainable design model with a new design variable of CS-factor is proposed. It is found that CS-factor ranged from 0.0208 to 0.4545 for current embodied CO₂ emission data. It is revealed that the environmental impact for construction of the same beam structure can be increased by more than 9.65 times (from 191.7 to 1851.1) due to the variation in CO₂ emission data. Also, the total construction cost for the same beam can be increased by 152.4% due to the variation. For the future of the construction industry, it should be noted that building construction based on available embodied CO₂ emission data may not be a solution for green construction.