Drinking water health effects: Excess nitrate (NO3-) in water above about 50 ppm is associated with which human health condition, especially in infants?

Introduction / Context:
Nitrogen species in drinking water are tightly regulated. Elevated nitrate levels are a particular concern in agricultural regions due to fertilizer leaching. Infants are vulnerable to nitrate's conversion to nitrite in the gut, which can oxidize haemoglobin to methemoglobin, reducing oxygen transport. This item asks you to identify the classic health outcome linked to nitrate above approximately 50 ppm as NO3-.

Given Data / Assumptions:

• Nitrate concentration in water exceeds about 50 ppm as NO3-.
• Population of concern includes infants under six months.
• Standard physiological mechanism of nitrate–nitrite–methemoglobin formation applies.

Concept / Approach:
In the infant gastrointestinal tract, certain bacteria reduce nitrate to nitrite. Nitrite oxidizes Fe2+ in haemoglobin to Fe3+, forming methemoglobin, which cannot carry oxygen. The resulting hypoxia manifests as cyanosis (blue baby syndrome). This pathway underlies regulatory limits and treatment goals for potable water supplies in agricultural catchments.

Step-by-Step Solution:
Identify the contaminant and its typical health endpoint: nitrate → methemoglobinemia.Recall the susceptible group: infants have higher gut conversion and lower methemoglobin reductase activity.Choose the option explicitly naming methemoglobinemia.

Verification / Alternative check:
Public health advisories and WHO/EPA guidance reference nitrate limits to prevent methemoglobinemia in infants, confirming the association.

Why Other Options Are Wrong:
Gastroenteritis: May co-occur from pathogens but is not the hallmark effect of nitrate.Asphyxiation: Implies airway obstruction or CO intoxication, not the nitrate mechanism.Tooth decay: Unrelated to nitrate exposure.

Common Pitfalls:

• Confusing nitrate with nitrite; nitrite is the proximate oxidant, but nitrate is the regulated precursor.
• Assuming adults have the same susceptibility as infants; risk is lower in adults.

Final Answer:
methemoglobinemia (blue baby syndrome)