Toxic gases, such as ammonia (NH3), hydrogen sulphide (H2S) and hydrogen bromide (HBr) gases, even at very low gas concentrations, have harmful effects on human health. Accordingly, environmental monitoring, industrial plants, automotive, air quality assurance technology, and various low-concentration gases must be reliably detected. The effect of NH3, H2S and HBr gases on the sensitivity of carboxymethyl cellulose sodium (CMC) nanocomposites has been studied theoretically using density functional theory (DFT) at B3LYP/Lanl2dz basis set. Total dipole moment (TDM), electronic band gap energy (ΔE) and molecular electrostatic potentials (MESPs) for CMC, CMC/ZnO and CMC/ZnO/ different gases are calculated. The results showed that the sensitivity of CMC increased due to the interaction with ZnO molecules through both complex and adsorb interactions. As TDM of CMC increased to 35.1708 Debye and ΔE decreased to 0.2645 eV. Also, TDM increased to 28.7687, 31.5937 and 32.8856 Debye upon the exposure of the CMC/ ZnO surface to NH3, H2S and HBr gases respectively. Additionally, MESP maps confirm the results of the electronic properties where the sensitivity increased due to the increment in the dimer CMC electronegativity. So that, the proposed CMC/ ZnO structure could be used as a sensor for NH3, H2S and HBr gases.