SCR systems are sensitive to contamination and plugging resulting from normal operation or abnormal events. Many SCRs are given a finite service life due to known amounts of contaminants in the untreated gas. The large majority of catalyst on the market has a porous structure and a geometry optimized for increasing its specific surface area. A clay planting pot is a good example of what SCR catalyst feels like. This porosity is what gives the catalyst the high surface area essential for reduction of NOx. However, the pores are easily plugged by fine particulates, ammonium sulfate, ammonium bisulfate (ABS), and silica compounds. Many of these contaminants can be removed while the unit is on line by ultrasonic horns or soot blowers. The unit can also be cleaned during a turnaround or by raising the exhaust temperature. Of more concern to SCR performance are poisons, which will degrade the catalyst and render it ineffective at NOx reduction, possibly resulting in the oxidation of ammonia which will "increase" NOx emissions. These poisons are halogens, alkali metals, alkaline earth metals, arsenic, phosphorus, antimony, chromium, lead, mercury, and copper.