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: