Which of the following pollutants is most strongly associated with methemoglobinemia (blue-baby syndrome) in infants?

Question

Which of the following pollutants is most strongly associated with methemoglobinemia (blue-baby syndrome) in infants?

1. A. Sulfur dioxide (SO2)
2. B. Nitrate (NO3^-) in drinking water (Correct answer)
3. C. Lead (Pb) in soil
4. D. Ozone (O3) in air

Correct Answer

Option B — Nitrate (NO3^-) in drinking water

Detailed Solution & Explanation

The correct answer is Nitrate ($\text{NO}_3^-$) in drinking water.

Key Points

• Methemoglobinemia, or blue-baby syndrome, is caused by nitrate in drinking water. In an infant's gut, nitrate is reduced to nitrite, which oxidises the iron in haemoglobin from the ferrous to the ferric state, producing methaemoglobin, which cannot carry oxygen. The result is cyanosis — a bluish tinge to the skin.
• The other pollutants cause different, well-defined harms:
  • $\text{SO}_2$ irritates the airways and contributes to acid rain,
  • Lead is a neurotoxin causing developmental and cognitive damage,
  • Ozone at ground level is a respiratory irritant.

Additional Information

• Infants under six months are uniquely vulnerable because their gastric pH is higher, favouring the nitrate-reducing bacteria, and their foetal haemoglobin is more readily oxidised while the enzyme that reverses the change, methaemoglobin reductase, is not yet fully active.
• The usual source is agricultural runoff of nitrogenous fertiliser and sewage seeping into shallow wells, which is why the problem is characteristically rural and groundwater-based.
• The Indian standard (IS 10500) sets the acceptable limit for nitrate in drinking water at 45 mg/L, with no relaxation permitted — unlike many parameters, there is no higher permissible limit in the absence of an alternative source.
• Boiling does not help and in fact concentrates nitrate; treatment requires ion exchange, reverse osmosis or blending with a low-nitrate source.