the association of drinking water hardness and cardiovascular diseases ( cvds ) has been studied since more than five decades ago .
a study in england and wales showed that cardiovascular death - rates had a favorable effect in towns with harder water and had an adverse effect in towns with softer water .
another studies showed that high levels of drinking water hardness can be protective against cvd .
however , a review of ecological studies showed that the results are inconsistent between studies .
a meta - analysis revealed a negative association between concentration of magnesium in water and cvd mortality .
moreover , another review showed protective role of magnesium concentration in water against cvd in some case - control studies and one cohort study , but the analytical studies showed little evidence about the association of calcium and magnesium levels in drinking water and cvd risk .
water hardness may be also associated with cvd risk factors , for instance positive correlations of water magnesium and calcium with blood pressure is documented . in the netherlands ,
another study found no overall association between calcium , magnesium or total hardness and ischemic heart disease or stroke mortality .
water hardness may be also associated with cvd risk factors , for instance positive correlations of water magnesium and calcium with blood pressure is documented .
case - control and cohort studies are more useful than ecological epidemiologicalstudies for investigating cause- and - effect relationships .
the guidelines for drinking - water quality of the world health organization in 2011 reported that one of the case - control studies addressed the association between calcium and acute myocardial infarction and three reported the association between calcium and death from cvds .
the aim of this study is to investigate the association of calcium and magnesium concentration of drinking water with cvds in urban and rural areas of a city in iran .
this case - control study was conducted in 2012 in khansar county in isfahan province , iran .
its area is 900 km and it includes 18 towns , 3 villages and one central city .
we used the official data of the provincial health center regarding the chemical analysis data of urban and rural areas including the hardness , calcium and magnesium content of drinking water .
data of patients hospitalized for cvd in the only specialty hospital of the city was gathered . by using the population of these areas , we calculated the relative frequency of cvd in urban and rural areas .
we compared the frequency and relative frequency of cvds in the population studied according to the concentrations of ca and mg hardness in drinking water in 2010 and 2011 .
we compared the frequency and relative frequency of cvds in the population studied according to the concentrations of ca and mg hardness in drinking water in 2010 and 2011 .
the findings of the study in 2010 and 2011 are reported in table 1 . in 2010 ,
the increase in the calcium hardness above 72 mg / l , the prevalence of cvds in 1000 population decreased [ figure 1a ] ; in 2011 this decrease in cvds was observed for calcium hardness of more than 75 mg / l [ figure 1b ] . in 2010 , the level of mg hardness in water ranged from 23 to 57 mg / l . by increasing mg hardness level above 31 mg / l in 2010 [ figure 2a ] and above 26 mg / l in 2011 [ figure 2b ] , the number of cvd in 1000 people decrease .
characteristics of variables studied in 2010 and 2011 cardiovascular patients in 1000 people according to the calcium hardness in water in ( a ) 2010 and ( b ) 2011 cardiovascular patients in 1000 people according to the magnesium hardness in water in ( a ) 2010 and ( b ) 2011
our study suggests favorable protective effects of water hardness , mainly water magnesium content , on cvds .
several epidemiological studies have confirmed a negative association between water hardness and cvd among adults .
however , some other studies did not confirm such relationship controversial results exist on the association of water hardness with cvd and their risk factors among the adult population .
the health effects of hard water are mainly considered to be because of the effects of its dissolved salts , primarily ca and mg .
some studies documented that the softer the water , the higher the cvd death - rates and suggested that water ca may have such protective role .
most previous studies showed a significant inverse association between mortality from cvd and water levels of mg , but not ca levels .
a review of the current literature reported that 10 of the 19 geographical studies published since 1979 , found a significant association , but two geographical studies from sweden , reported no association .
moreover , one of the six case - control studies that considered ca , found significant protective effect of high ca levels ( > 70 mg / l ) in drinking water .
mg is a protective element against soft - tissue calcification and has a role in prevention from myocardial infarction .
many , but not all , of the geographical correlation , cohort and case - control studies suggested that a high mg water concentration protects against cvd mortality .
likewise , a recent review confirmed the protective effects of water hardness , notably its ca level against many chronic diseases including cvds . the main limitation of the current study is its cross - sectional nature , thus the associations documented in this study should be considered with caution .
the main limitation of the current study is its cross - sectional nature , thus the associations documented in this study should be considered with caution .
water hardness , as well as calcium and magnesium content of drinking water may have a protective role against cvds .
further experimental studies are necessary to determine the underlying mechanisms and longitudinal studies are required to study the clinical impacts of the current findings .
 background : the aim of this study is to investigate the association of calcium and magnesium concentration of drinking water with cardiovascular disease ( cvds ) in urban and rural areas of a city in iran.methods:this case - control study was conducted in 2012 in khansar county in isfahan province , iran . 
 we used the official data of the provincial health center regarding the chemical analysis data of urban and rural areas including the hardness , calcium and magnesium content of drinking water . 
 data of patients hospitalized for cvd in the only specialty hospital of the city was gathered for the years of 2010 and 2011.results:in 2010 , water calcium content above 72 mg / l was associated with reduced number of cvds in 1000 population ; whereas in 2011 this decrease in cvds was observed for calcium levels of more than 75 mg / l . in 2010 , the level of water mg content ranged from 23 to 57 mg / l . by increasing mg hardness level above 31 mg / l in 2010 and above 26 mg / l in 2011 were associated with decreased number of cvds in 1000 people . 
 decrease.conclusions:our study suggests favorable protective effects of water hardness , mainly water magnesium content , on cvds . 
 water hardness , as well as calcium and magnesium content of drinking water may have a protective role against cvds . 
 further experimental studies are necessary to determine the underlying mechanisms and longitudinal studies are required to study the clinical impacts of the current findings . 
INTRODUCTION
METHODS
Statistical analysis
RESULTS
DISCUSSION
Study limitations and strengths
CONCLUSIONS