Calculating Fluoridated Populations

Surveillance and assessing public health interventions are essential elements of promoting health. A state’s fluoridation status is an indicator of progress; consequently, it is important to document how this status is calculated.

Reporting of Fluoridated Populations by CDC

The first fluoridation census was compiled in 1956 by the U.S. Department of Health, Education, and Welfare. Starting in 1963, the National Fluoridation Census (Census) summarized the fluoridation status for each state and county and listed the fluoridated water systems. States submitted data for the Census, including the fluoridation status of water systems and communities served by those systems; the population receiving fluoridated water; the date on which fluoridation started; and the additive used to adjust the fluoride content. The Census required many resources and was disseminated on an irregular basis; some states lacked the resources to provide accurate information.

CDC has been responsible for tracking the populations in each state served by fluoridated water systems since 1978. CDC compiled the Census up to and including the 1992 Census periodically. Starting with the 2000 National Fluoridation Report (Report), the information has been based on the water system data in the CDC Water Fluoridation Reporting System (WFRS), and the Report has been published on the CDC Web pages instead of in hard-copy format. The report differs from the earlier Census because it compiles the state population that can access water with beneficial levels of fluoride, whereas the Census was a comprehensive list of individual water systems. Since WFRS manages the data on water fluoridation based on individual water systems, the results on state totals are comparable between the earlier Census and the more recent Reports. CDC’s My Water’s Fluoride  continues to provide information on the fluoridation status of individual communities, and CDC’s Oral Health Maps  continues to provide information on fluoridation status on a county level within a state.

Estimation of Populations by State Programs

Estimating the precise service population for water systems is difficult. Challenges include estimating the population per service connection, determining whether the connection is at a primary or secondary residence, and harmonizing different estimation methodologies used by different states. When using a service population estimate, it is important to understand how that estimate was derived.

Each state drinking water program derives its own methodology for estimating water system service populations. Water systems typically track the number of connections or accounts for billing purposes, and not the number of people served by that account. Therefore, most service populations are estimates derived from the number of connections, normally based on an assumed number of people per connection. However, this can introduce errors, because even though a state average might be 2.4 people per household based on Census figures, a state drinking water program might use an average of 2.6 people per billing account to address the accounts serving multiple dwelling units on a single meter, such as the case with apartments. Although this may provide a satisfactory service population estimate for the state as whole, it may result in discrepancies for individual water systems, since the applied factor is an assumed value and may be inconsistent between communities.

Double-counting of individuals is a potential risk. Water systems may report total people served, which could include commuters that would also be counted as a resident in an adjacent water system, or people with primary and secondary residences, such as college students or recreational homes. It is important to identify which location is an individual’s primary residence.

EPA Data Resources on Populations

The Environmental Protection Agency (EPA) uses the Safe Drinking Water Information System (SDWIS) to track approximately 160,000 public water systems. Many public water systems serve schools, gas stations, parks, and other service districts that may have only a few or no residents. Of the public water systems tracked by EPA, approximately 54,000 are considered Community Water Systems, because they serve a threshold number of residents in addition to other uses. CDC bases the fluoridation status of a state on the Community Water Systems.

State drinking water programs normally update populations served by water systems during the Sanitary Survey process, which is conducted every 3 years for most surface water systems and every 4 years for most groundwater systems. The Sanitary Survey process is a detailed review of a water system and its condition. States may update the population estimate for an individual water system more often if new information is brought to their attention, but would be unlikely to update the estimates for all water systems in any single year. The EPA Web site presents more information on SDWIS and documentation on its data fields, definitions, and uses.

EPA SDWIS Data Fields

State SDWIS administrators generally update SDWIS files by December, and these revisions are compiled by EPA by February of the following year. CDC and EPA have agreed to allow comparison of data concerning water fluoridation and population between SDWIS and WFRS. The two databases can be compared, because both are written using the Public Water System (PWS) ID number as the common reference. CDC conducts an annual comparison to identify discrepancies between the databases, which can then be used to improve the records. CDC compares the year-end data in SDWIS to the data entered in WFRS the following April or May, since many states do not complete their fluoridation records update until March 1. Updating the information in both WFRS and SDWIS is a state program responsibility.

SDWIS is capable of identifying up to four service populations for each water system. Of these, the one that is most useful for the population estimate in the water fluoridation report is the resident population. The resident population in SDWIS-Fed is identified as RetPopSrvd, and is normally in the field known as TINSWYS column in SDWIS-State entitled D_POPULATION_COUNT/AVG_DAILY_CNT. The resident population should represent the service population expressed as the location of residence. Systematically using the resident population should minimize the number of people that might be double-counted, because each person is only counted once for location of residence, presuming that the state drinking water program has carefully addressed this issue.

Additional Considerations on Population Estimates

A state program sometimes contacts an individual water system to find out whether that water system has made a concerted effort to derive a more precise service population. Sometimes a system will compile a more precise number for a municipal bond report or other accounting purpose.

In every state, a small portion of the population consumes water from unregulated private wells. As a result, the reported population consuming water from community water systems typically is less than the U.S. Census population for that state. CDC reports percentage fluoridated populations relative to the populations served by community water systems in that state.

Many state drinking water programs or state oral health programs compare the compiled state totals entered for individual water systems and against the state census totals to determine whether the compiled totals appear reasonable. They will then adjust the populations to be entered into WFRS by a factor so that the compiled totals accurately reflect the actual state population.

For a few states, the total reported service population on community water systems exceeds the actual census population for that state. If the compiled total in WFRS is greater than the estimated census population for that state, then the service populations on public water systems will need to be adjusted for use in establishing the total population in a state with access to fluoridated water. Because the state will have estimated the service populations in a consistent manner, the proportion of fluoridated population to the total population as reported by WFRS will be accurate. This percentage can be applied to census total for the state to derive the fluoridated population for the Water Fluoridation Report.

Reference Census Populations

The census figures used in calculating fluoridated status of a state are those released by the U.S. Census Bureau. Decennial figures are normally released 4–5 years after they are collected, and then are revised and extrapolated for subsequent years. CDC uses the most recent census figures available for a state, and revises the previously reported fluoridated population if subsequent updates have revised totals for a state.

Date last modified: December 4, 2007
Content source: Division of Oral Health, National Center for Chronic Disease Prevention and Health Promotion