Longitudinal Analysis of Fluoride Levels in Irish Water Supplies: A 52-Year Review.

1 Introduction

Community water fluoridation (CWF) involves the addition of fluoride compounds to Public Water Supplies (PWS) to achieve a given target concentration to improve dental health [1]. Recognised as one of the Ten Great Public Health Achievements of the 20th Century [2], CWF has been implemented globally across 25 countries, serving an estimated 400 million people [3]. The United States maintains the most extensive fluoridation program, with approximately 63% (209 million individuals) receiving fluoridated water [4]. Approximately 3% of the Western European population, in Ireland and selected regions of the UK and Spain, receives fluoridated drinking water [1, 3].

Ireland is the only European country with mandatory legislation requiring the fluoridation of PWS [5]. This legislative framework also mandates daily fluoride monitoring at treatment plants (operational monitoring) and monthly testing of fluoride samples collected from representative consumer sites by Environmental Health Officers from the Health Service Executive (HSE) and sent to Public Analysts Laboratories (compliance monitoring) in accordance with the European Communities (Drinking Water) Regulations 2000 [5, 6]. Currently, approximately 71% of the population in Ireland receives fluoridated drinking water [7].

International drinking water fluoride limits (maximum allowable concentration that must not be exceeded) and standards (enforceable regulatory requirements adopted by competent authorities) vary considerably across jurisdictions. The World Health Organisation (WHO) recommends a maximum of 1.5 mg/L fluoride in drinking water, emphasising that “the volume of water consumed and intake from other sources should be considered when setting national standards” [8]. Under the European Communities (Drinking Water) Regulations 2000, which implemented EU Directive (No. 98/83/EC) during the present study period, the European Union permitted up to 1.0 mg/L of fluoride for fluoridated supplies and 1.5 mg/L for naturally occurring fluoride [9]. The most recent EU (Drinking Water) regulations 2023, which implement the EU drinking water directive, apply a single maximum allowable concentration (parametric value) of 1.5 mg/L of fluoride in drinking water intended for human consumption, with no distinction between fluoridated and non-fluoridated supplies [10]. In Ireland, the Environmental Protection Agency (EPA) enforces a national fluoride standard of 0.8 mg/L for fluoridated water supplies and 1.5 mg/L for supplies with naturally occurring fluoride [11]. This contrasts with the United States, where the EPA’s drinking water standard for fluoride is 4.0 mg/L (primary maximum contaminant level (MCL)), with a secondary guideline of 2.0 mg/L [12].

In Ireland, separate from drinking water quality standards, the Fluoridation of Water Supplies Regulations 2007 establish operational requirements for deliberately fluoridated public water supplies [5]. The current target concentration (an optimal concentration set to achieve a public-health benefit) for CWF is 0.7 mg/L, with an accepted performance range of 0.6–0.8 mg/L for both operational and compliance monitoring samples. The target concentration and accepted performance range were lowered in 2007 from 1.0 mg/L (range 0.8–1.0 mg/L) to reduce dental fluorosis risk while maintaining caries prevention efficacy [7, 11]. The Irish 2007 policy adjustment aligned with international trends toward lower optimal levels. The United States Public Health Service (USPHS) similarly reduced its recommended target concentration of fluoride to a uniform 0.7 mg/L in 2015, reflecting evidence of increased fluoride availability from multiple sources, including toothpastes, supplements, and mouth rinses, combined with a lack of correlation between climate and water consumption [13, 14]. Other countries implementing 0.7 mg/L CWF targets include Canada [15], New Zealand [16] and Brazil [17]. While Singapore has further reduced the recommended target fluoride levels from 0.7 to 0.5 mg/L [18], in Australia, a target range of 0.5–1.0 mg/L is recommended for CWF programmes, depending on the climate [14].

Since the introduction of CWF in the 1960s, fluoride dosing pumps at water treatment plants have been used to add fluoride to treated water. Initially, the process was supervised by humans and mechanically controlled. However, over time, operations shifted toward more automated systems (e.g., flow-proportional dosing) [19, 20]. Three chemicals used in water fluoridation are sodium fluoride, sodium silicofluoride, and hydrofluosilicic acid. In the early years of fluoridation, all three chemicals were used in Ireland. However, after a few years, all water treatment plants began using hydrofluosilicic acid at a strength of 10.9% [5, 19].

Drinking water in Ireland originates from groundwater, surface water (including rivers and lakes), and springs. Public water supplies mostly use surface water sources, while private group and private supplies depend on groundwater and spring water [21]. Water supply system in Ireland comprises multiple sources with varying fluoridation requirements and oversight (Table S1, Data S1). Uisce Éireann (formerly Irish Water) manages 962 PWS serving 83.3% of the Irish population [11]. Group and private supplies, serving the remaining population, are exempt from fluoridation legislation but remain subject to quality monitoring requirements under drinking water regulations [22]. This bifurcated system creates potential disparities in fluoride exposure across different population segments. Uisce Éireann (Irish Water) is responsible for monitoring of fluoride levels in Irish PWS in accordance with Fluoridation of Water Supplies Regulations 2007 (S.I. No. 42 of 2007) and European Communities (Drinking Water) Regulations, 2023 (S.I. No. 99 of 2023) [10, 23]. However, local authorities often monitor group and private water supplies on behalf of their owners and trustees [21]. Previous analyses of the water fluoride levels in Irish PWS have examined monthly distillation records [22, 24, 25]. There have been gradual improvements in both result availability and compliance with national recommended limits since water fluoridation was introduced. From 2013 to 2015, the proportion of samples exceeding the national standard of 0.8 mg/L, as reported by the EPA, ranged between 0.9% and 1.3% [11, 21].

To our knowledge, detailed fluoride monitoring data are not publicly available. This study addresses this gap by analysing water fluoride testing results across all water supply types in Ireland over five decades (1964–2016), providing the first comprehensive evaluation of compliance patterns, temporal trends, and disparities in fluoride access. Specific objectives include: (1) characterising long-term compliance with statutory fluoride limits; (2) identifying temporal and geographic variations in water fluoride concentrations in fluoridated PWS; (3) assessing fluoride access disparities between public, group and private water supplies; and (4) informing policy recommendations for optimising fluoridation programme effectiveness and equity.

2 Methodology

3 Results

3.2 Public Water Supplies

3.2.1 Data Quality

Between 1964 and 2016, 129 831 fluoride test results were collected, of which 15 626 results were missing (12%). The variation of missing results ranged between 1.7% (County Cavan) and 30% (County Laois) (Table S2, Data S1). Data availability improved substantially over the study period. By 1970, 95% of expected monthly fluoride test results were available. However, availability fluctuated in subsequent decades: approximately 85% during the 1970s–1990s, declining to 65% around 1999, before stabilising at 75% by 2016 (Figure 1a). Missing data showed no significant seasonal variation (p > 0.05).

3.2.2 PWS Compliance

Between 1964 and 2006, satisfactory results increased progressively from 17% to 60% (1994–1999), before declining to 48% in 2006. Marginal results peaked at 57% (1976–81) then decreased, while unsatisfactory high results ranged between 1.2% and 5.5%. Following the 2007 target adjustment (0.6–0.8 mg/L), compliance improved across all categories. Satisfactory results increased from 49% to 62%, while marginal and unsatisfactory results decreased across all categories (Figure 2a and Table S3, Data S1). Mean fluoride concentration was 0.74 mg/L between 1964 and 2006, decreasing to 0.62 mg/L between 2007 and 2016, with reduced variability in the later period (SD 0.34 vs. 0.19) (Figure 1b and Table S4, Data S1). Across both time periods, there was relatively little variation in fluoride levels between different PWS categories from the smaller (50–499 people) to the larger (20 000+ people) PWS categories (Figure 3 and Tables S5 and S6, Data S1). In the 2007–2016 period, the larger PWS appear to cluster more consistently around the optimal fluoride concentration of 0.7 mg/L (Figure 3).

4 Discussion

This is the first study to analyse fluoride trends across all Irish water supply types over five decades. Previous Irish studies examined only Public Water Supplies (PWS) over shorter periods [22, 24, 25]. International studies using similar monthly monitoring approaches have been conducted in the US [26, 28, 29], UK [30–32] and Brazil [33–35], though most used existing data while Brazilian studies involved primary data collection.

The observed patterns of data quality reflect the evolution of Irish fluoridation infrastructure and monitoring capacity. Early data gaps resulted from equipment failures, chemical supply shortages, distribution challenges to smaller supplies, and inconsistent sampling protocols [19, 25]. Notable disruptions included a 9-month hydrofluorosilicic acid shortage in 1975 and supply interruptions during 1980–1982 [36].

Irish PWS compliance rates (70% within target range, 2012–2016) compare favourably with international standards: 76% in the USA [26] and 28%–78% in the UK [30]. However, Ireland shows notably lower exceedance rates (0.27% above 0.9 mg/L) compared to 7%–30% (USA) and 3%–24% (UK) above their respective thresholds, suggesting conservative dosing practices. This conservative approach may reflect Ireland’s comprehensive legislative framework requiring mandatory fluoridation with regular monitoring. Methodological differences limit direct comparisons.

In contrast to the US study, which analysed monthly average fluoride readings (MAFRs) provided by state drinking water programmes, and the UK study, which utilised daily or weekly measurements recorded by the water companies, the present study used individual monthly samples for each supply collected by Environmental Health Officers and analysed by public laboratories. This sampling framework has constituted the statutory fluoridation monitoring method in Ireland since the 1960s [22]. Performance monitoring improved markedly in the final decade of the study period, coinciding with two policy changes. The Fluoridation of Water Supplies Regulations 2007 (S.I. No. 42 of 2007) reduced the fluoride target range from 0.8–1.0 to 0.6–0.8 mg/L, requiring more rigorous operational control [5]. Under the Water Services (No. 2) Act 2013 [37], Uisce Éireann (Irish Water) consolidated responsibility for public water supplies in 2014, transferring oversight from local authorities into a single national utility with standardised protocols and centralised management. While the present analysis extends only through 2016, limiting assessment of Uisce Éireann’s full impact, these regulatory and structural changes likely contributed to enhanced monitoring practices.

Proposed characteristics that are important in maintaining the recommended levels of fluoride include an increase in the size of population served, source of public water supply, an increase in tenure of the chief operator of the water supply because of internal consistency, stable staff, and natural fluoride content nearer the legal range [28]. However, the applicability of these factors to the Irish context is uncertain. PWS serving populations of more than 5000 people demonstrated more consistent fluoride control, consistent with international experience [26, 30]. This reflects better staffing, automated monitoring technology, and more rigorous quality protocols in larger systems [30].

Group water supplies with public sources matched PWS performance, as both receive water from Uisce Éireann [11]. However, private water supplies and group supplies with private sources showed very low natural fluoride levels. Before introducing water fluoridation, fluoride levels in more than 660 public water supplies were analysed, and only 5 exceeded 0.3m g/L [22]. More recently, raw water sampling by Irish Water and EPA audits have confirmed that the fluoride levels in naturally occurring water were generally below < 0.3 mg/L [11, 21]. These findings demonstrate that Irish waters have consistently low baseline fluoride levels regardless of source type.

A recent systematic review [38] examined national, regional, and county-level dental health surveys of Irish children from 1950 to 2021 summarising dental caries trends over the last seven decades. The review reported substantial reductions in dental caries prevalence over time, with greater reductions in fluoridated areas than in non-fluoridated ones, suggesting water fluoridation is an effective intervention for preventing dental caries in the Irish population. CWF started in Dublin in 1964, and between 1961 and 2014, the mean dmft/DMFT (decayed, missing and filled teeth) scores among 5 and 12-year-olds living in County Dublin decreased by approximately 88% and 90% respectively [38]. While most of the population receives optimally fluoridated water through PWS, approximately 15% rely on private supplies with suboptimal fluoride levels. Private water supplies are predominantly located in rural and remote areas where connection to public water connections is unavailable or economically unfeasible [14, 22]. These areas face multiple disadvantages that compound oral health risks. Rural populations have limited access to dental services due to geographic barriers and lower dentist-to-population ratios [39]. Rural communities also typically have lower income and education levels, both established risk factors for poor oral health [39]. The concentration of private water supplies in already disadvantaged areas suggests that improving fluoride access alone may be insufficient [40]. Importantly, dental caries levels have declined even in non-fluoridated areas of Ireland, largely due to fluoridated toothpaste use and the “halo effect” from consuming foods and beverages processed with fluoridated water [41]. A comprehensive approach to oral health disparities should address multiple factors: alternative fluoride delivery methods, improved dental service access, oral health promotion, and broader social determinants of health in these underserved communities.

This is the first comprehensive analysis of community water fluoridation trends across all Irish water supply types over five decades (1964–2016). The categorical classification system (satisfactory/marginal/unsatisfactory low/unsatisfactory high) provides more meaningful insights than simple means, which can be misleading due to missing data and high variability [42]. Population-based stratification revealed important patterns in fluoridation coverage and compliance that aggregate data would mask. Additionally, including private and group water supplies demonstrates disparities in fluoride access in populations not covered under the statutory fluoride legislation. However, there were some data and methodological limitations. Key data limitations include 12% missing data with substantial geographic variation (1.7%–30%), exclusion of small PWS (< 500 people) due to limited data availability, and absence of data from approximately 170 000 household wells which remain exempt from regulatory oversight. These constraints may affect trend interpretation and limit generalisability to the full population. Additionally, the unavailable geographic boundary data limited assessment of county-level patterns. An important methodological limitation is that the environmental, political, and operational factors influencing fluoridation implementation and compliance were not investigated.

Future research should focus on longitudinal monitoring to identify compliance challenges, factors driving successful fluoridation maintenance, and operational barriers faced by water treatment facilities. Qualitative studies exploring public awareness and community perceptions of fluoridation would inform implementation strategies. An annually updated national fluoride database is needed, managed by a public health body or an academic institution, similar to that in Australia [43]. Longitudinal supply coverage maps should be accessible to researchers to monitor changes in fluoridation levels. Additionally, fluoridation monitoring data should be made publicly available, enabling communities to advocate for improved performance when local water fails to meet target fluoride levels. A community water fluoridation flagging system, similar to the UK model, would improve identification of fluoridated supply zones [31]. Comparative analyses with international programs could identify best practices for implementation and monitoring.

5 Conclusions

This analysis demonstrated progressive improvement in the CWF programme in Ireland, with compliance rates reaching 70% by 2016. There has been marked improvement in fluoride control in PWS over the decades, with the majority of PWS across all population categories consistently maintaining fluoride concentrations within recommended limits. The natural fluoride levels in group water supplies with a private source and small private supplies were suboptimal. In areas where optimal fluoridation is not achievable, alternative fluoride delivery methods should be considered.

Author Contributions

Conceptualisation: V.S., M.C. and B.O.; Methodology: V.S., M.C., E.O., J.R. and B.O.; Data acquisition and statistical analysis: V.S., E.O., J.R., M.C., M.C., B.O. and O.C.; Interpretation of findings: V.S., M.C., M.C., B.O. and L.W.; Drafting and critically reviewing the article: V.S., M.C., M.O., M.C., L.W., J.M. and B.O.; Project administration: B.O. All authors have read and approved the final article.

Funding

The Health Research Board in Ireland supported this study. Research grant number: APA-2016-1882.

Ethics Statement

The authors have nothing to report.

Consent

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

All data generated and analysed during this study are not publicly available but can be made available on request from the corresponding author.