A comparison of recent and long-term average measurements of nitrate in drinking water.

We evaluated the usefulness of a recent measure of drinking water nitrate as a predictor of long-term average nitrate exposure calculated from historic data. Exposure estimates were calculated for 214 study participants who used public water supplies between 1947 and 1980 in Minnesota. Long-term average nitrate was calculated by linking residential histories to historical nitrate data. For recent exposures, we averaged nitrate measurements in 1980, or the next closest year with measurements. The Spearman correlation coefficient for the relationship between the two measures was 0.54 (95% confidence interval [CI]=0.44-0.63). Agreement was highest among those residing 34 or more years in their town as of 1980 (r(s)=0.70; 95% CI=0.55-0.80). These findings suggest that taking into account the study participants' duration of residence may enhance the validity of using a recent measure as an indicator of past exposures.