Nitrate in community water supplies and incidence of non-Hodgkin's lymphoma in Sardinia, Italy

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Abstract
We selected the 153 communes with at least one nitrate measurement in 1993 or earlier (median: 5 measurements; range: 1–51) after excluding the six major urban areas. The population of these 153 communes ranged from 140 to 61 636 (median: 2459) and included a population of 703 000 (41% of the Sardinian population). There were a total of 3909 nitrate measurements. The nitrate ion was measured by spectrophotometry with the sodium salicilate method. For our primary analysis, we used the average nitrate concentration in 1993 as the exposure metric. Fifteen communes (23% of the study population), also had nitrate data in 1971–84. For these we also evaluated the median concentration in 1971–84. NHL incidence in 1974–1993 was determined by commune from the registries of all pathology and haematology departments in Sardinia. Coverage of these registries was estimated as close to 100%.5 We calculated person years for each gender group and 10 year age group in each commune, using the 1971, 1981, and 1991 census enumerations, extended five years backward and four years onward. We aggregated cases and person years over all communes within eight categories of 1993 nitrate level. Incidence rate ratios (IRR) and 95% confidence intervals for NHL were computed for each nitrate category with reference to the lowest using Poisson regression, and adjusting by gender, age, and population size.