Eighteen communities of Weld County, Colorado, USA were selected to study the possible relationship between high nitrate levels in potable water supplies and the incidence of hypertension. The control group consisted of nine communities that had no nitrates in their potable water supplies and the study group of nine communities that had nitrate levels ranging from 19 to 125 ppm. A survey of Weld County General Hospital records yielded 487 cases of both primary and secondary hypertension in patients from the exposed and control communities for the period 1 January 1969 to 1 January 1975. The mean annual incidence rate for the control group was 7·9/1000 population compared with a rate of 5·9/1000 population for the exposed group. There was an earlier onset of hypertension amongst those residents of the exposed communities; i.e. the exposed group exhibited a peak among the 50 to 59 age group and the control group peaked later in the 70 to 70 age group. © 1978.
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