The gases which may be lethal to humans through short-term exposure in laboratories or industrial settings may paralyze the olfactory sense and impose severe damage to the central nervous system and lungs. This study concerns gas sensors which allow individuals to avoid toxic gases that may be generated in spaces with residues of organic waste with a temperature at 50 °C or above. We investigated the response and selectivity of the sensors to kinds of toxic gases such as hydrogen sulfide, and acetaldehyde. We also tracked operating temperatures and catalysts. The thick-film semiconductor sensors that detect some toxic gases were fabricated using WO3/SnO2 and SnO2/WO3 and these were prepared via sol.gel and precipitation methods. The nano-sized SnO2 powder was mixed with various amounts of metal oxides (SnO2, WO3) and doped with transition metals (Pt, Ru, Pd, Ag, Au, and In). The metal-oxide thick films were prepared on an Al2O3 plate with a Pt electrode and a Ni.Cr heater via screen-printing method. Morphology, composition, particle sizes, specific surface areas, and phases of sensor materials were investigated by SEM/EDS, BET, and XRD analyses. The measured responses to various lethal gases is defined as the ratio (Ra/Rg) of the resistance of the sensor film in air to the resistance of the film in a toxic gas. The results indicated that the highest response and selectivity of the sensors for toxic gases occurred with doping with 1 wt% of various transition metals and 5-20 wt% WO3 to SnO2 at the optimum operating temperature range from 200 °C to 300 °C.