Methemoglobinemia (blue baby disease) in drinking water Which contaminant is the primary cause of infant methemoglobinemia associated with potable water?

Introduction / Context:
Blue baby syndrome (methemoglobinemia) is a potentially life-threatening condition in infants caused by impaired oxygen transport. Drinking water quality standards restrict certain ions to prevent this outcome.

Given Data / Assumptions:

• Exposure pathway is ingestion of contaminated water or formula prepared with that water.
• Infants are especially susceptible due to gastrointestinal conditions that promote reduction reactions.

Concept / Approach:
Nitrate (NO3−) in water can be reduced to nitrite (NO2−) in the infant gut. Nitrite oxidizes Fe²⁺ in hemoglobin to Fe³⁺, forming methemoglobin which cannot bind oxygen. The initiating contaminant of concern in source water is nitrate; hence regulatory limits (e.g., 10 mg/L as N).

Step-by-Step Solution:

Verification / Alternative check:
Epidemiological cases link private wells with elevated nitrates to infant illness; standards are set accordingly.

Why Other Options Are Wrong:

• Chlorides (a) are mainly aesthetic/corrosion related.
• Nitrites (b) are toxic, but the regulated source contaminant leading to nitrite in infants is nitrate.
• Sulfides (d) cause odor; not linked to methemoglobinemia.
• Iron (e) is an aesthetic parameter.

Common Pitfalls:
Confusing nitrate with nitrite in the water standard context; the driver contaminant to control in supply is nitrate.

Final Answer:
Nitrates