Introduction

One of the most important issues in monitoring water chemical parameters is the relationship between the concentration of these parameters and non-communicable diseases such as cardiovascular diseases (Reimann et al. 2003). Cardiovascular diseases are the main reason for mortality, which annually lead to death of 17 million people around the world. In Iran, we see that 46% of overall deaths and 27.2% of the lost years of life are due to cardiovascular diseases in 18 provinces of the country and a large part of the health system resources is spent on their treatment (Sepanlou et al. 2015). Today, hypertension is one of the most important and expanding worldwide public health problems, especially in developing countries, due to its high prevalence and association with cardiovascular diseases (Rasic-Milutinovic et al. 2012). Until 2005, one billion people with high blood pressure existed in the world, and four million people died as a result of high blood pressure. By 2025, the prevalence of high blood pressure is expected to increase by 60% and reach 1.56 billion people. The data show the emergency of prioritizing for screening, early detection and management of worldwide hypertension through the plans based on society (Rasic-Milutinovic et al. 2012; Chockalingam et al. 2006). In assessing risk factors for non-communicable diseases of the country in 2009, about 16% of 15–64-year-old people had hypertension. Some factors such as obesity, high level of cholesterol, low physical activity, positive experience of cardiovascular and kidney diseases and diabetes are effective in creating hypertension. Other risk factors for hypertension include race, gender, smoking, aging, hereditary background and water chemical quality (Yousefi et al. 2018a; Mohammadi et al. 2017; Yousefi et al. 2017). In recent years, different studies have investigated the effect of elements of calcium, magnesium, potassium and sodium on hypertension (Rasic-Milutinovic et al. 2012). An inverse correlation between hardness of drinking water and cardiovascular diseases has been reported by a number of studies in Japan, USA, England, Finland, Sweden, Canada, South Africa, etc. (Nerbrand et al. 2003). The study of Bostick et al. (1999) showed that increased intake of calcium, magnesium and potassium was associated with a decrease in blood pressure and caused a reduction of 30–35% in ischemic diseases or heart failure. Different epidemiological studies have shown a clear relationship between different drinking waters and cardiovascular diseases, which is 10–30% higher in soft water areas than in hard water areas (Woodman et al. 2002). In some studies, mineral water consumption for 4 weeks reduced systolic blood pressure by about 5 mmHg. In a study on 76 metropolitan Sweden areas, a significant difference was observed between the rate of mortality and soft water consumption compared to low mortality in regions with high hardness water consumption (Luft et al. 1990). The wide prevalence of cardiovascular diseases is different in diverse geographical regions around the world, and epidemiologic studies have been carried out to identify variables which can explain this reality (Jenkins 1988; Sengupta 2013). The effect of water hardness has been widely investigated on prevalence of cardiovascular diseases. Previous studies have approved the relationship between hypertension and diet and water containing calcium and magnesium (Rasic-Milutinovic et al. 2012; Crawford et al. 1971; Gumashta et al. 2012; Tubek 2006; Leurs et al. 2009). For example, in a study conducted as a control study on the rate of hardness taken from drinking water, the results showed that the rate of mortality of acute infarction decreased. Today, the concerns of obesity problem are increasing in epidemic proportions worldwide and also are growing in developing countries. High blood pressure is associated with obesity. Among the indicators of obesity, we can mention body mass index (BMI) and waist-to-hip ratio (WHR) (Yousefi et al. 2018a). Most studies in this regard have been carried out in advanced countries, and there is insufficient information about the less developed countries. Therefore, the main purpose of this study was to investigate the relationship between blood pressure with hardness, BMI and WHR in people living in two regions of the West Azerbaijan Province in northwest Iran.

Materials and methods

Study area

Two study areas were selected in the West Azerbaijan Province in the northwest part of Iran, including Agh Otlogh and Sarisoo with high water hardness and Shiblou Olia and Sofla as well as Gharghologh Olia and Soufla with soft drinking water that have similar socioeconomic status and dietary habits (Fig. 1).

Fig. 1
figure 1

Location of the study areas in the Poldasht County, the West Azerbaijan, Iran

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Study population

The population studied in the villages of Agh Otlogh, Sarisoo (soft drinking water), Shiblou Olia and Sofla as well as Gharghologh Olia and Sofla included 462, 779, 278, 277, 271 and 348 people, respectively (Yousefi et al. 2018a, b). The data on the incidence were derived from the health records of the areas being investigated. Based on the results of the study, 501 of over 30-year-old people with hypertension were studied based on the history of health in the two regions of the West Azerbaijan Province with high and low hardness. The rates of hardness of the existing resources were classified based on the standard state of water resources as

Data collection

The data related to hypertension patients were collected by referring to the registration offices for hypertension patients in the health centers of the region. The data were also collected based on gender and place of residence for individuals of over 30 years old through screening. Then, the hypertension status sample was calculated and classified based on gender, infection percentage and location. If the mean score of maximum blood pressure was 140 mmHg or more and that of minimum blood pressure was 90 mmHg or more in several times and states, it was considered as hypertension. The understudy villages were in a similar situation in terms of race as well as economic and social circumstances, and their drinking water was provided from underground resources. All the samples with hypertension were over 30 years old, who lived in the village since birth time and used their village water resources and also had similar situation in terms of healthcare status. People having chronic kidney disease or diabetes and those taking antihypertensive drugs/oral calcium supplements were excluded.

Determination of hardness in drinking water

The total hardness (TH) of drinking water of the understudy village was determined based on 10-year results of standard experiments of the Water and Sewage Company in the village. (In this study, 132 samples were analyzed by descriptive and analytical statistics.) Ten percent of overall water resources was assessed in terms of chemical parameters by standard methods and calibrated devices; the assessment was repeated twice to insure data accuracy (Rice et al. 2012).

Statistical method

To present data, we used mean, standard deviation, median and range. To compare the results between the two groups, we used independent sample t test and Mann–Whitney test and Chi-square test. To evaluate the simultaneous effect of variables on the hypertension (HTN), we used logistic regression analysis. The precision of the statistics were presented by 95% confidence interval (CI). All the statistical methods were performed by SPSS software (Version 24.0). P values less than 0.05 were considered statistically significant. The association between hypertension death with calcium and magnesium levels in drinking water was measured using odds ratios (OR) and their 95% confidence intervals (95% CIs) from conditional logistic regression models.

Results

Anthropometric features, systolic and diastolic blood pressure and total hardness of the two groups are provided with high and low water hardness in Tables 1 and 2. In the two regions with the average high and low water hardness, the Shiblu and Gharghologh area had a four times higher mean total hardness level in drinking water (968.69 as CaCO3) than the Sarisoo and Agh Otlogh area (180 mg/L as CaCO3). As it can be observed, the mean systolic and diastolic blood pressure was significantly higher in areas with a low level of hardness in drinking water (P < 0.001).

Table 1 Classification hardness in the two study areas
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Table 2 Basic characteristics of the study subjects
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The two groups had a significant difference in terms of age groups, BMI, waist circumference and mean arterial pressure (MAP) (Table 2).

According to the results of the study, the overall prevalence of the HTN in areas with high and low water hardness was 6.11% (3.67–9.54) and 12.63% (8.09–18.79), respectively. The prehypertension prevalence in areas with high and low water hardness was calculated as 23.15% (18.11–29.15) and 46.84% (37.61–57.64), respectively. The prevalence of the HTN and prehypertension was higher in regions with low hardness than those with high hardness, which was statistically significant (P < 0.001) (Table 3).

Table 3 Comparison of the prevalence of the ISH, IDH, SDH, prehypertension and HTN in the two areas with different hardness of drinking water
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Adjusted for other variables in the model, the population in high hardness area has 0.6 times odds to have non-normal hypertension compared to low hardness area (95% CI 1.2–3.0, P = 0.011) Table 4.

Table 4 Abnormal hypertension status (including prehypertension and hypertension) prevalence and simultaneous effect of the related risk factors
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As it can be observed from the Fig. 2, the abnormal hypertension was higher in the low hardness area compared to that in the high hardness area separated for the BMI and abdominal obesity status (Fig. 2).

Fig. 2
figure 2

Comparison of abnormal hypertension in different regions with high and low hardness separated for the BMI and abdominal obesity status

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Discussion

According to the results of the study, the overall prevalence of the HTN in areas with high and low water hardness was 6.11% and 12.63%, respectively. The prehypertension prevalence in areas with high and low water hardness was calculated as 23.15% and 46.84%, respectively. The prevalence of the HTN and prehypertension was higher in regions with low hardness than those with high hardness, which was statistically significant (P < 0.001). Moreover, in order to eliminate the effects of other variables and risk factors, a multivariate logistic regression analysis was used to investigate the relationship between hardness and blood pressure. The results of this analysis showed that the population in areas with high water hardness had 0.6 percent higher odds ratio of having non-normal hypertension compared to areas with low water hardness (95% CI 1.2–3.0, P = 0.011). Overall, in our study, we found a significant association between total hardness concentrations and the HTN and prehypertension prevalence. To compare the findings obtained in the present study with those in the literature, results of some studies were described. For example, in a study on the association of blood pressure and metabolic syndrome, it was reported that areas with hard drinking water and adequate supply of Mg from drinking water may prevent hypertension and help maintaining normal triglyceride levels in healthy adults (Rasic-Milutinovic et al. 2012). Many epidemiological studies in Canada and Italy reported an inverse relation between cardiovascular diseases and water hardness; this can be confirmed with the role of water hardness concentration in the etiology of hypertension. However, these results are not consistent with the results obtained in the current study (Anderson et al. 1975; Leoni et al. 1985). Moreover, the analytical research revealed little evidence that drinking water hardness caused cardiovascular risks (Anderson et al. 1975; Leoni et al. 1985). In another research, the influence of calcium and magnesium in drinking water on cardiovascular risk factors was studied in individuals living in hard and soft water areas. The findings showed that individuals living in soft and hard water areas showed significant correlations between the content of calcium in water and major cardiovascular risk factors (Derry et al. 1990; Galan et al. 2002; Yang and Chiu 1999; Monarca et al. 2006). In a study conducted by Momeni et al. (2014) on the association between drinking water hardness and cardiovascular diseases in the Khansar County in the Isfahan Province, it was found that increasing Mg hardness level above 31 mg/L in 2010 and above 26 mg/L in 2011 was associated with a decreased number of CVDs in 1000 people (Momeni et al. 2014). The study of Bostick et al. (1999) showed that increased intake of calcium, magnesium and potassium was associated with a decrease in blood pressure and caused a reduction of 30–35% in ischemic diseases or heart failure. In a study on the effect of drinking water hardness on decreasing cardiovascular and kidney diseases in the Samirom city using RS and GIS technologies, Bahramian et al. (2013) showed that due to low calcium and magnesium in drinking water compared to optimal limitation of the elements, the percentage of cardiovascular diseases (hypertension) was significant relative to the region population, which is consistent with the results of our study. In contrast, in a case study, hypertension was evaluated in people consuming hard water in Haji Samoa in Pakistan. Based on this study, hypertension was no more common in people taking freshwater than in those consuming hard water; however, the underground well water in the area of Keti Bandar had high levels of hardness (Razzaque et al. 2015). In a study carried out in Iran, the concentration effect of chemical parameters such as hardness, TDS, sodium and chloride on the prevalence of hypertension was studied. However, other risk factors such as hereditary and family background in hypertension, unnecessary consumption of salt, obesity and diabetes can be effective in creating hypertension (Hojatzade 2012).

Conclusion

In this study, the association between consumption of excess hard water, BMI and waist circumference with risk of hypertension was investigated in individuals living in hard and soft water areas. The results proved that the Shiblu and Gharghologh areas had a four times higher mean total hardness level in drinking water (968.69 as CaCO3) than the Sarisoo and Agh Otlogh areas (180 mg/L as CaCO3). Moreover, the result revealed that the prevalence of HTN and prehypertension was significantly higher in regions with low water hardness than those with high water hardness. Logistic regression showed that age, BMI, waist circumference and hardness of drinking water was significantly related to hypertension. Accordingly, hypertension was low in people of regions with high hardness. We believe that the use of water with high hardness up to the recommended standard limitation is effective in decreasing hypertension prevalence and consequently in decreasing cardiovascular diseases. As food is the major source of magnesium and calcium intake, the main limitation of our study was the lack of individual data on dietary and water intake. This, of course, requires constant monitoring and periodic conduction of epidemiological studies that will allow permanent assessment of environmental and dietary influences on a population living in a particular region.