Normal outside air is used for dry air.  This means that both temperature and humidity constantly change. An air purification system, also known as an 'air purification street', must be able to handle these variables. It contains all or most of the following components: a water grid with wire mesh, antifreeze protection, air filtration, cooler, silencer, control vent and centrifugal fan. Furthermore, there are air ducts and a heater (fire), in some cases including a final filter and air ducts, that are connected to the dry tower. Most clients request an air purification system where all components are tuned to one another and/or are assembled on a skid.