Nitrate ions / nitrite ions (<50 mg / l / 0.5 mg / l) - Shows the effect of environmental pollution on the water source or the presence of sewage or fertilizers in the water. High levels of nitrite are toxic to humans and animals, especially infants. It can enter the body as nitrate, a nutrient that is essential for plant growth, and can be converted to nitrite, which disrupts the ability of hemoglobin to supply oxygen to the blood.