You can use a carbon filter in several places. It can be used for point-of-entry (POE) to filter the whole house or point-of-use (POU) to clean water before you drink or cook with it. Some shower heads include carbon filtration to prevent you from inhaling chlorine gas in the shower. Carbon filters are also part of a reverse osmosis system or an ultrafiltration (UF) system. A carbon filter added to a UF system provides organic and chemical particulate reduction along with lead reduction. If you use a water softener to soften water treated by a municipal plant, then you should install a carbon filter before the water softener. If chlorine is removed prior to softening, then the softener resin lasts longer. Chloramines can cause O-rings, gaskets, and rubber seals to deteriorate, so removing chemical disinfectants with catalytic carbon helps water-using appliances, like your thankless water heater, last longer.