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Non-Hodgkin's lymphoma and nitrate in drinking water
  1. MARY H WARD,
  2. STEVEN D MARK,
  3. KENNETH P CANTOR
  1. Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, USA
  2. University of Nebraska Medical Center, Department of Pathology and Microbiology, Omaha, USA
  3. National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, USA
  1. Dr Ward, Division of Cancer Epidemiology and Genetics, National Cancer Institute, 6120 Executive Blvd, EPS 8104; Bethesda, MD 20892–7240 USA (wardm{at}exchange.nih.gov)
  1. DENNIS D WEISENBURGER
  1. Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, USA
  2. University of Nebraska Medical Center, Department of Pathology and Microbiology, Omaha, USA
  3. National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, USA
  1. Dr Ward, Division of Cancer Epidemiology and Genetics, National Cancer Institute, 6120 Executive Blvd, EPS 8104; Bethesda, MD 20892–7240 USA (wardm{at}exchange.nih.gov)
  1. ADOLFO CORREA-VILLASENOR
  1. Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, USA
  2. University of Nebraska Medical Center, Department of Pathology and Microbiology, Omaha, USA
  3. National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, USA
  1. Dr Ward, Division of Cancer Epidemiology and Genetics, National Cancer Institute, 6120 Executive Blvd, EPS 8104; Bethesda, MD 20892–7240 USA (wardm{at}exchange.nih.gov)

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Editor,— We read with interest the article by Law et al.1 This ecological study examined the incidence of non-Hodgkin's lymphoma (NHL) in Yorkshire and North Humberside in 1984–1993 in relation to nitrate concentrations in drinking water. Nitrate exposure was estimated for 148 water supply zones by calculating a mean of monthly means based on six years of monitoring data (1990–1995) that largely post-dated the time period of NHL incidence. The results showed a relation between NHL incidence during 1984–1989 and nitrate concentrations in the early 1990s but not for incidence during 1990–1993.

The authors compared the distribution of their recent nitrate average with the distribution of long term average nitrate values from our population-based case-control study of NHL in the state of Nebraska, USA.2 They also described the long term average nitrate metric we calculated as “an imprecise estimate of nitrate exposure assessed from single annual measurements for a city or town”. The characterisation of our historical nitrate estimates as “imprecise” is inappropriate; rather, our nitrate metric had most elements required for an accurate estimate of historical nitrate exposure. An accurate estimate of historical nitrate requires individual level information on water source for many decades preceding cancer incidence, water consumption amount, and historical data on nitrate levels in water supplies. We had all this information, as well as individual information on potential confounders such as pesticide use and dietary intake of nitrosation inhibitors.

We collected complete residential and water source histories so that we could determine who drank community water and so that we could …

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