Health risk assessment for exposure to nitrate and nitrite in drinking water in Iran: a systematic review and meta-analysis

The aim of this study was to investigate the health risk assessment of nitrate and nitrite in drinking water resources in Iran by conducting a systematic review and meta-analysis. Electronic databases were searched before August 2017, and methodological quality was assessed by a modified Downs and Black checklist. Of 1,539 searched references, 49 studies were selected and included in the meta-analysis study. We used a random effects model to estimate the effect size with 95% confidence interval (CI) to summarize the results. The hazard index values for health risk assessment were calculated for accrued data. The pooled mean concentration of nitrate and nitrite in drinking water resources was 24.9 mg/L (95% CI 16.79-32.39) and 0.05 mg/L (95% CI 0.03-0.07), respectively. Using the random effects model, the pooled nitrite hazard index was found to be 0.40 (95% CI 0.30-0.48). The hazard index of 12.5% of drinking water resource was above 1. In central and northwestern parts of Iran, the concentration of nitrate and nitrite exceeded the drinking water standards (50 and 3 mg/L for nitrate and nitrite according to WHO guidelines and Iran standards) and could be associated with health risks in these regions. The main reasons for the elevating nitrate and nitrite concentration in Iran could be due to the usage animal and chemical fertilizers as well as lack of wastewater collection systems, which requires the necessary authorities to establish new laws.