Fluoride is a naturally occurring mineral that can help protect teeth from decay.  All water contains some amount of fluoride, though generally not at a level that is effective at preventing tooth decay.  Some water providers therefore add additional fluoride to the drinking water they provide, a process known as fluoridation.  Roughly 73 percent of Americans served by community water systems receive fluoridated water.  As of 2015, 10 counties in Washington had 66 percent or more of the population receiving fluoridated water; seven counties had between 33 percent and 66 percent of the population receiving fluoridated water; and 22 counties had less than 33 percent of the population receiving fluoridated water.

A public water system is any system providing water for human consumption through pipes or other constructed means of transference, except for systems serving only a single residence or a system with four or fewer connections that serves a single farm.  The water system includes collection, treatment, storage, and distribution facilities, as well as other facilities primarily used in connection with the system.

Public water systems are divided into two categories:  Group A and Group B.  Group A water systems are those that have met one or more of the following criteria:

• The system has more than 15 service connections, regardless of the number of people served.
• The system serves an average of 25 or more people for at least 60 days in a year, regardless of the number of service connections.
• The system serves at least 1,000 people for at least two consecutive days.

Group B water systems are those systems not included in Group A.

The Washington State Board of Health (Board) is required to adopt rules for public water systems.  The Board has done so, including rules for Group A systems providing fluoridation of drinking water.  In order to provide fluoridation, the operator of the water system must receive written approval of its fluoridation treatment facilities from the Department of Health (Department).  The Board has prescribed the optimal fluoride concentration (0.7 milligrams per liter), the operating tolerance for fluoridation (0.5 to 0.9 milligrams per liter), as well as requirements for fluoridation testing that must occur each business day and each month.

The test results must be submitted to the Department each month.  Public water systems are not required to adopt fluoridation.

A public water system that is considering commencing or discontinuing fluoridation of its water supply must notify its customers and the Department of its intentions at least 90 days prior to the vote or meeting at which the decision will be made.  This notification can be made by radio, television, newspaper, mail, electronically, or by any method that effectively notifies customers.

Only once notification has been properly provided can a public water system commence or discontinue fluoridation.  Any public water system that violates the notice requirements must return the fluoridation of its water supply to its previous level until the notification requirements have been satisfied.