Although any device that promotes contact between air and water strips some volatile compounds, air strippers are usually packed towers or tray towers operated with countercurrent flow of water and air. The countercurrent flow removes particles from the water and into the air. This process is known as volatization or air stripping. Water is deposited into the system through the top and air is ventilated in through the bottom. Water that reaches the bottom of the system is typically considered treated, but additional testing may be done to determine if it is safe for consumption. Since many of the compounds stripped are hazardous air pollutants, the air exiting a stripper may require emissions control. Carbon adsorption is often used and catalytic oxidation is another option. There are mainly two different types of air strippers: Packed tower systems and sieve tray systems. Different types of air strippers are used depending on the type and amount of contaminants found in the water source being extracted.