Introduction: In the last decade, the concerned researchers issued warnings related to high phosphorus daily intake, which significantly exceeds the recommended dietary allowance. Chronic exposure to high daily phosphorus intake is claimed to lead to adverse health effects. One of the so-called hidden sources of phosphorus is also chemically softened drinking water. Aim: The study purpose was to determine the health risk assessment resulting from exposure to chemical softeners (sodium polyphosphate) in the drinking water. Methods: The study is divided into several sections. The analysis was focused on concentrations and frequency of phosphate softeners (chemical analysis of hot water samples with the spectrometric method with ammonium molybdate) and the awareness of consumers about water softening (survey questionnaire). Since concentrations of phosphates at different sampling sites vary substantially, additional analysis was conducted to measure changes in concentration through time. Additionally, the exposure assessment for phosphate in case of consuming chemically softened drinking water was established. The next section of the research presents the analysis of daily menus in six different institutions providing the data on daily phosphorus intake by different age groups. The toxicity test on Vibro fisceri to determine any potential acute toxicity for water microorganisms was conducted. In order to highlight the indirect risks, which are not included in the standard list of toxicity and eco-toxicity tests, the impact of commercial product (sodium polyphosphate) on the growth of Legionella pneumophila was determined. Results and discussion: In nearly half (45 %) of the samples of the drinking water (N = 242), the presence of phosphates was confirmed in concentrations from 0.2 to 24.6 mg PO4/L. The majority of the consumers (70 %) were not aware of the chemical softening of their drinking water. The consumption of chemically softened drinking water presents a minor share of the daily phosphorus intake. However, it should be noted that chemically softened drinking water is one of the hidden sources of phosphorus, which cumulatively adds to its exceeded dietary intake. More important is the phosphorus intake through the consumed food, which significantly exceeds the recommended dietary allowance in nearly all participating groups. The phosphate additives (commercial product for drinking water softening in concentrations 3.6 and 39.5 mg PO4/L) enhance the growth of L. pneumophila by log 1.28 and log 1.19 CFU/mL, respectively. According to the response of the L. pneumophila in vitro, it can be inferred that the added phosphorus promotes the growth of bacteria also in water systems and thus indirectly presents additional threat to the health of consumers. The modified approach to the risk assessment in case of water softening includes (i) exposure assessment, (ii) assessment of safe daily intake, based on toxicity test and also on (iii) indirect impact of the added phosphates on microorganisms, (iv) potential additive effects of various pollutants in water, (v) the impact of phosphates on nutrient absorption in gastro–intestinal tract, and finally (vi) the risk management. The research findings indicate that the health risks deriving from phosphate exposure in drinking water are not negligible.
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