What is a known effect of elevated levels of nitrate/nitrite in drinking water?

Elevated levels of nitrate and nitrite in drinking water are particularly concerning due to their association with various health effects, one of which is known as blue baby syndrome, or methemoglobinemia. This condition primarily affects infants under six months of age who consume water with high concentrations of nitrates. Nitrate is converted into nitrite in the body, which can interfere with the blood's ability to carry oxygen. This can lead to a bluish discoloration of the skin, especially around the lips and fingertips, indicating reduced oxygen levels in the blood, hence the term "blue baby syndrome."

The other options, while related to water quality and health, do not specifically correlate with the effects of elevated nitrate and nitrite levels. Bad taste is subjective and can arise from various contaminants, mottling of teeth is associated with excessive fluoride rather than nitrates, and hardness refers to the mineral content in water, which is unrelated to the health impacts of nitrate-nitrite concentrations. Thus, blue baby syndrome is the well-documented health risk directly tied to high levels of nitrates and nitrites in drinking water.