One of the most crucial additives are heat stabilizers. These agents minimize loss of HCl, a degradation process that starts above 70 °C. Once dehydrochlorination starts, it is autocatalytic. Many diverse agents have been used including, traditionally, derivatives of heavy metals (lead, cadmium). Metallic soaps (metal "salts" of fatty acids) are common in flexible PVC applications, species such as calcium stearate. Addition levels vary typically from 2% to 4%. Tin Mercaptides are widely used globally in rigid PVC applications due to their high efficiency and proven performance. Typical usage levels are 0.3 (pipe) to 2.5phr (foam) depending on the application. Tin stabilizers are the preferred stabilizers for high output PVC and CPVC extrusion. Tin stabilizers have been in use for over 50 years by companies such as PMC Organometallix and its predecessors. The choice of the best PVC stabilizer depends on its cost effectiveness in the end use application, performance specification requirements, processing technology and regulatory approvals.