Question

Which of the following when present in excess in drinking water causes the disease methemoglobinemia ?

• A. SO\(_4^{2-}\)
• B. NO\(_3^-\)
• C. F\(^-\)
• D. Pb

Hint:

The disease methemoglobinemia (often called "Blue Baby Syndrome") is directly caused by the excessive presence of nitrate ions (NO\(_3^-\)) in drinking water, especially when consumed by infants. These nitrates are converted to nitrites, which interfere with the oxygen-carrying capacity of hemoglobin in the blood.

Answer 1

The Correct Option is B

Step 1: Understand the question and the disease "methemoglobinemia".
The question asks to identify the substance that, when present in excess in drinking water, causes methemoglobinemia. Methemoglobinemia is a blood disorder in which an abnormal amount of methemoglobin (a form of hemoglobin) is produced. Methemoglobin cannot release oxygen to body tissues, leading to symptoms like shortness of breath, cyanosis (bluish skin), and in severe cases, central nervous system depression and death. This condition is particularly dangerous for infants, where it is commonly known as "blue baby syndrome".
Step 2: Analyze each option in the context of water pollution and health effects.
SO\(_4^{2-}\) (Sulfate ions): Excess sulfate in drinking water can have a laxative effect, particularly on unaccustomed individuals, and can contribute to scaling in pipes. However, it is not primarily known to cause methemoglobinemia.
NO\(_3^-\) (Nitrate ions): Excess nitrate in drinking water is a well-documented cause of methemoglobinemia, especially in infants. When ingested, nitrates are converted to nitrites by bacteria in the digestive system. Nitrites then oxidize the iron in hemoglobin from its ferrous (Fe\(^{2+}\)) to its ferric (Fe\(^{3+}\)) state, forming methemoglobin. This reduces the blood's oxygen-carrying capacity.
F\(^-\) (Fluoride ions): Excess fluoride in drinking water can lead to dental fluorosis (discoloration and pitting of tooth enamel) and, at very high concentrations, skeletal fluorosis (bone and joint problems). While harmful in excess, it is not linked to methemoglobinemia.
Pb (Lead): Lead contamination in drinking water is a serious public health concern, causing various health problems, particularly in children. These include developmental delays, learning difficulties, nervous system damage, kidney damage, and high blood pressure. Lead exposure can also affect the blood by causing anemia, but it does not directly cause methemoglobinemia.
Step 3: Conclude the substance that causes methemoglobinemia.
Based on the analysis, excess nitrate ions (NO\(_3^-\)) in drinking water are a known cause of methemoglobinemia, particularly in infants.
The final answer is \( \boxed{\text{NO}_3^-} \).