If your water smells or tastes bad, there is little doubt that you would benefit from a water filter. Municipal water often smells of chlorine, which is used to treat the water. Well water, which is dependent upon many local conditions affecting the water supply, may also smell bad. Water filters often treat these conditions by the use of "Granular Activated Carbon," (GAC) a substance that absorbs contaminants that would otherwise cause offensive tastes and odors.

Filters which use GAC may cause cloudy water for the first couple of weeks after a filter change. This is a harmless condition caused by the release of air from the GAC and can be reduced by running the water for several seconds before each use until the air is flushed out.

If your house relies on a well, your water is more likely to be contaminated by bacteria and parasites. Many bacteria and parasites occur naturally in clear water supplies, while others are the result of water supply contamination by sewage and wastes. Some bacteria and parasites affect the taste and smell of the water, but others do not.

Cysts, such as Cryptosporidium and Giardia, are particularly hearty parasites and have been known to contaminate even chlorinated municipal water supplies. They can cause illness and are a serious hazard to the young, elderly, or those with immune deficiencies. Water filters are available with various filter cartridges which are effective against many of these contaminants.