Enzymes activity can be affected by other molecules: decreased by inhibitors or increased by activators. Many drugs and poisons are enzyme inhibitors. Activity is also affected by temperature, pressure, chemical environment (e.g., pH), and the concentration of substrate. Some enzymes are used commercially, for example, in the synthesis of antibiotics. In addition, some household products use enzymes to speed up biochemical reactions (e.g., enzymes in biological washing powders break down protein or fat stains on clothes: enzymes in meat tendoue break down proteins into smaller molecules, making the meat easier to chew). The study of enzymes is called enzymology