Catalyst poisons are substances that can block or permanently destroy the active surface of the catalyst. A distinction is made between reversible and irreversible deactivation. Irreversible damage is caused primarily by substances such as silicones (siloxanes), heavy metals (e.g. lead, zinc) or arsenic. Silicones are particularly critical in industry, because during oxidation they form a fine layer of silicon dioxide (SiO2) that “glazes” the catalyst and makes the pores inaccessible to pollutants. This effect cannot be reversed and requires the catalyst modules to be replaced. Reversible poisons, on the other hand, such as dusts, soot particles or certain greases, merely coat the surface physically. They can often be removed by thermal regeneration processes (controlled burn-off) or mechanical cleaning to restore full activity. A careful analysis of the exhaust gas composition is essential in order to detect the presence of such substances at an early stage. Appropriate pre-filtration stages or the use of specifically resistant catalyst coatings can secure the service life of the system even in demanding environments.