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 a s a predictor of long-term average nitrate exposure calculated from histori c data. Exposure estimates were calculated for 214 study participants who u sed public water supplies between 1947 and 1980 in Minnesota. Long-term ave rage nitrate was calculated by linking residential histories to historical nitrate data. For recent exposures, we averaged nitrate measurements in 198 0, or the next closest year with measurements. The Spearman correlation coe fficient for the relationship between the two measures was 0.54 (95% confid ence interval [CI] =0.44-0.63). Agreement was highest among those residing 34 or more years in their town as of 1980 (r(a) =0.70; 95% CI =0.55-0.80). These findings suggest that taking into account the study participants' dur ation of residence may enhance the validity of using a recent measure as an indicator of past exposures.