If a flammable gas, for exemple methane, burns on the surface of a pellistore, this reaction will cause water and carbon dioxide. They distances from the sensor and they won't cause any damage. Many flammable gases have the same behavior when they burns. But not all flammable gases behave in this way: some of them produce solid substances which sediment on the sensitive element and they cause its gradual covering, while others produce chemical substances which are very corrosive and they damage the sensor. The permanent damage that happens in this case is called " poisoning".
Other gases cause a sensor damage which can be reactivate by exposing it to fresh air: this phenomenon is called "inhibition". Many of the most common cases of industrial poisoning are caused by silicones. When silicon burns it create silicons, whose poisoning originate a strong zero drift and a strong reduction of sensitivity. The most common substances which cause pellistore sensor poisoning and/or inhibition are:
Unsaturated Hydrocarbons (which contain C=C doble or triple bond)
These last have the effect to polymerize on the sensor, stopping its working. We want to point out that you don't need high concentrations to damage the sensors: they can be poisoned even with a few ppm (part per million). The concentration only determines the damage velocity: the higher is the substance poisoning concentration, the quicker will be the damage to the sensitive element. Since this deterioration is caused by chemical aggressions or by other reasons which always derive from the combustion process, it is better to use, with sensors which detect fammable gases with optical instruments, infrared sensors. They have limits too, but if the gases to be detected belong to their working field and the conditions of the environment are compatible, this is the only solution worth to be adopted.