Methaemoglobinemia has long been considered to be the end-point of concern for humans from exposure to nitrate in drinking water. Scientific studies show cases of methaemoglobinemia occurring in bottle-fed infants, which are the vulnerable population for these effects. Recent evidence from animal and human studies suggests that effects on thyroid gland function are also an end-point of concern. Studies have seen an effect in school-age children, but no study has looked at this health effect in infants, who would also be expected to be the most vulnerable population for this health effect. In addition, current science suggests an association between cancer and exposure to nitrates in drinking water when conditions result in nitrosation within the human body.
Although no single study can be used to establish a guideline for nitrate in drinking water, available studies in humans show no adverse health effect (either methaemoglobinemia or thyroid effects) below 45 mg/L. At this level of exposure from drinking water, the estimated increased cancer risk would be in the range considered by Health Canada to be essentially negligible.
