Human Health Effects of Exposure to Nitrate, Nitrite, and Nitrogen Dioxide

Human exposure to nitrogen (N) pollution via nitrate in food and water and nitrogen oxides in the ambient air is reviewed. Increased risks of myocardial infarction, respiratory problems, and asthma have been linked to higher exposures to nitrogen oxides in ambient air. Excess nitrate/nitrite exposure in food and water may be harmful to human health by: (1) its contribution to the endogenous formation of N-nitroso compounds, demonstrated carcinogens and teratogens in animal models; (2) its potential role in methemoglobinemia; and (3) in high doses, its ability to competitively inhibit iodine uptake and induce changes in the thyroid. High nitrate levels in drinking water have been linked to methemoglobinemia in infants (“blue baby syndrome”) and children. In 2006, an International Agency for Research on Cancer expert panel concluded that ingested nitrate/nitrite under conditions that result in endogenous nitrosation is probably carcinogenic to humans; subsequent studies have continued to support this conclusion. Maternal exposures to higher nitrate levels in drinking water have been associated with a variety of adverse pregnancy outcomes. In contrast, maternal dietary intake of nitrate/nitrite has not been associated with birth defects in offspring unless coupled with prenatal nitrosatable drug exposure. Further research is indicated on the beneficial and harmful effects of nitrate on human health. Given what is currently known about the harmful effects of nitrogen oxides and ingested nitrate/nitrite, it appears prudent to support measures to reduce nitrogen emissions in the atmosphere and to continue to maintain current limits and guidelines on nitrate concentration in drinking water.