Calculating Fluoridation Statistics
Community water systems (CWS) conduct regular sampling and testing of water to monitor operations and to confirm compliance with state and local regulations and industry standards. CDC maintains the Water Fluoridation Reporting System (WFRS), the principal tool CDC uses to help states manage the quality of their water fluoridation programs. State drinking water programs or state oral health programs in participating states submit relevant data to WFRS. These data are the source for CDC’s water fluoridation statistics as well as the annual fluoridation information in the National Oral Health Surveillance System.
Data from about 52,000 water systems are captured in WFRS, and states report to WFRS that about 18,200 of those systems are fluoridated, meaning they either adjust fluoride to the recommended level of 0.7 mg/L, have naturally occurring fluoride at or above the recommended level, or buy optimally fluoridated water from a neighboring water system.
Each year, CDC works with state programs to improve the accuracy of water system statistics by identifying and correcting discrepancies between the Environmental Protection Agency’s Safe Drinking Water Information System (SDWIS) and WFRS databases. Biennially, CDC summarizes the available WFRS data into National Water Fluoridation Statistics that describe the estimated proportion of the US population receiving fluoridated water and provide the estimated percentage of the population on CWS receiving fluoridated water in each state.
The state-reported CWS service populations in WFRS are estimated for each state based on an assumed number of people per connection. Because these estimation methods may under- or over-estimate the populations served by CWS, CDC adjusts state-reported CWS population estimates to more closely reflect the likely true population on CWS in that state.
CDC has updated the methodology for calculating fluoridation statistics, beginning with the 2016 data (see Fluoridation Statistics — Population Methodology Changes). Estimates for CWS populations and fluoridated populations vary based on the adjustment methodology. While the population estimates using the current methodology are not directly comparable to previous years’ estimates, the percentage of state populations on CWS receiving fluoridated water will remain comparable across old and new methodology.
Corrective Methodology for Population Estimates
The current methodology for adjusting CWS population estimates utilizes two established data sources: the US Census Bureau State Population Estimates, which are published annually, and the US Geological Survey (USGS) Public Supply Water Use data, published every 5 years.
The USGS Public Supply Water Use data include an estimate of percentage of population served by public water systems for each state and account for state residents who are served by private water sources (e.g. residential private wells). CDC determines each state’s fluoridated population using the following steps:
- The Census Bureau State Population Estimate is multiplied by the USGS estimate for the Percentage of State Population served by Public Supply to determine the Adjusted State CWS Population.
- The Adjusted State CWS Population is divided by the state-reported Population Served by CWS from WFRS to obtain a “Control Factor.”
- Each water system’s reported population is multiplied by the Control Factor to produce Individually Adjusted CWS Populations.
- Finally, the Individually Adjusted CWS Populations for all fluoridated systems in a state are added together to determine the State Fluoridated Population.
Detailed information on how these data are calculated can be found at Estimating Community Water System Populations.
Before 2016, state-reported data were only adjusted if the state-reported population served by CWS exceeded the US Census Bureau’s State Population Estimates. In this case, the Census Bureau state population estimate was used as a replacement for the CWS population estimate. The state-reported fluoridated population was adjusted downward by multiplying it by the ratio of the Census Bureau state population estimate to the state reported state-reported Population Served by CWS. If the state-reported population did not exceed the Census Bureau estimate, the state-provided data from WFRS was used and no adjustment was made.
A limitation of this methodology is that it did not account for situations where states may have under-reported the CWS population. Additionally, the data produced were known to still be inflated, as no state CWS population is 100% the total population of that state.
Detailed information on how these data were calculated can be found at Estimating Community Water System Populations.
- Census Population Count 2018. Annual Estimates of the Population for the United States, Regions, States, and Puerto Rico: April 1, 2010 to July 1, 2018. Source: U.S. Census Bureau, Population Division. Release Date: December 2018. Available at https://factfinder.census.gov/faces/tableservices/jsf/pages/productview.xhtml?pid=PEP_2018_PEPANNRES&src=pt
- US Geological Survey: Maupin MA, Kenny JF, Hutson SS, Lovelace JK, Barber NL, and Linsey KS. Estimated use of water in the United States in 2010: US Geological Survey Circular 1405; 2014. https://dx.doi.org/10.3133/cir1405