By choosing a right kind of material which senses gas rapidly by undergoing variation in one or the other physical property which can be quantified very well.

Try to maximise the surface area of the sensing material.

The gas sensing process is strongly related to surface reactions. The sensitivity of the gas sensors is influenced by many factors, including internal and external causes, such as natural properties of base materials, surface areas and micro-structure of sensing layers, surface additives, temperature and humidity, etc. The sensitivity (S) can be defined as Ra/Rg for reducing gases or Rg/Ra for oxidizing gases, where Ra stands for the resistance of gas sensors in the reference gas (usually the air) and Rg stands for the resistance in the reference gas containing target gases. Both Ra and Rg have a significant relationship with the surface reaction(s) taking place. By increasing more porosity, more roughness and reducing grain sizes of particles about 20 nm or less would be about 10 times more sensitive than that of using 25~40 nm particles in some metal oxides.

The most important factors affecting on the sensitivity of the gas sensors such as natural materials, surface areas and micro-structure of the sensing layers and temperature. Therefore, you can change one factore and fixed the others to get the optimum conditions which are required.