Many water treatment companies include in their services free in-home testing of the water, but not all contaminants can be detected this way. For example, organics, which have been associated with serious health problems, must be analyzed in a laboratory with sophisticated equipment. The NSF, whose function is similar to Underwriter's Laboratory, sets performance standards for water treatment devices. Because companies can make unsubstantiated statements regarding product effectiveness, the consumer must evaluate test results of the device to determine if claims are realistic. Less than one percent of tap water is used for drinking and cooking.
Some contaminants are as hazardous when inhaled or absorbed through the skin as when swallowed.
Treatment of all water used in the household may be required. Reverse osmosis and distillation units are connected to a single tap. Activated carbon devices can be installed on a single tap or where water enters the house. The device selected depends on the type of contaminant in question. The consumer must be certain enough treated water will be produced for everyday use.