Which of the following NO3 concentrations would be considered hazardous to an infant?

Nitrate (NO3) concentrations in drinking water are a significant concern for infants, particularly due to the risk of methemoglobinemia, commonly referred to as "blue baby syndrome." This occurs when nitrate interferes with the ability of blood to carry oxygen, which is especially dangerous for infants.

A concentration of 44 mg/L of nitrate is recognized as hazardous for infants. The U.S. Environmental Protection Agency (EPA) has established a maximum contaminant level (MCL) for nitrate in drinking water of 10 mg/L, primarily to protect vulnerable populations such as infants. While levels below this threshold may not pose immediate health risks to adults, infants are more susceptible to elevated nitrate levels due to their immature digestive systems and lower body weight.

At 44 mg/L, the concentration exceeds the recommended safety levels, indicating a significant health risk for infants. Lower concentrations such as 10 mg/L, 25 mg/L, and even 75 mg/L (which would be outright unsafe) might not portray the same level of imminent danger as 44 mg/L in the context of established guidelines. Therefore, the identification of 44 mg/L as hazardous aligns with health and safety standards regarding nitrate levels in drinking water for infants.