Nitrites and nitrates, the salts of nitrous acid and of nitric acid, are normally found in water, especially the nitrates. The presence of these bodies in minute quantities is therefore not objectionable. When, however, there is any considerable increase in the quantity of nitrites especially, pollution of an organic nature may be suspected. Either in the decay of this organic matter nitrous acid may be produced, or the organic matter present may work upon the nitrates normally present and reduce them to a lower state of oxidation, namely, nitrites. The nitrites are extremely transient in character. They are likely to oxidize rapidly to nitrates when the water is brought in contact with free oxygen.

If a water contains more than 0.003 part of nitrites per million, it is sufficient indication of contamination to warrant an investigation. The normal quantity of nitrites and nitrates occurring in the water should be taken as a basis of comparison in each locality. Any sudden increase in the amounts of these bodies is almost a certain indication of contamination.