Nitrite is a nitrogen compound from fertilizers and waste. It converts to nitrate in the body and can cause oxygen deficiency in infants, known as "blue baby syndrome".

Nitrite enters drinking water primarily through agricultural runoff, sewage discharge, and leaching from septic systems. It's measured in milligrams per liter (mg/L) or parts per million (ppm). In the body, nitrite rapidly converts to nitrate. While adults can typically process nitrate, infants under six months are at risk. Their digestive systems can convert nitrate back to nitrite, potentially leading to methemoglobinemia or "blue baby syndrome," a serious condition affecting oxygen transport in the blood.