Recent studies have presented the relationship of various geological, ground-water chemistry, and other environmental factors to death rates for cardiovascular-renal (CVR) and other chronic diseases in the same geographic areas. Several studies report a negative correlation of hardness in the drinking water with CVR diseases death rates. Other studies do not support the hypothesis of a protective role for hard water. In view of the complexity of the environment, other variables in the physical environment, as well as cultural and socioeconomic factors, also require careful consideration.

In Indiana, a variation in CVR diseases death rates follows the Wisconsinan glacial boundary and areas of Wisconsinan outwash, with higher rates north of the boundary. Between these areas, there is a significant difference in soil pH, soil organic carbon, hardness, and possible sulfate in municipal water supplies. For the 92 counties of Indiana, CVR death rates for white males, ages 35 to 74 (age-adjusted), for 1959–1961 show a significant correlation with soil parameters pH and organic carbon, hardness of the drinking water, population density, occupation, income, dust in the air, and other variables. Multiple correlation of eight variables with CVR diseases death rates produces a value of 0.49. Additional measures of the death rate for varying time periods confirm the general conclusions.

Although this moderately homogeneous group of counties presents problems in analysis, the study suggests that many variables often not taken into account may exert considerable influence. An appreciation of the complexity of environmental factors is needed in order to proceed with appropriate caution in the study of the extent and nature of the relationship of geological variables to chronic diseases rates.