Nitrate and Nitrite
Nitrogen makes up 78% of the Earth’s atmosphere. Available nitrogen is a major limiting factor in plant growth. Nitrogen is naturally broken down in a series of steps called the nitrogen cycle.
From the above it can be seen that atmospheric nitrogen eventually becomes part of the bio-chain. Nitrate is an important compound in the nitrogen cycle in that it is used directly by plants for their growth, which explains its use in fertilisers and there has been a 20-fold increase in the use of nitrogen fertilisers in the UK in the last 40 years. Nitrates are extremely water-soluble and are soon washed off the land by rainwater into rivers and eventually end up in our drinking water supplies. Nitrates are not removed by the water treatment process or the sewage works without the use of a specialised plant.
Nitrate in water can give rise to methaemoglobinaemia, which is sometimes called “blue baby” syndrome. Babies have favourable conditions in their gastrointestinal tracts for the existence of nitrate-reducing bacteria, so-called because they reduce nitrate to nitrite. These nitrites convert the iron in a baby’s haemoglobin to an oxidised form that is unable to carry oxygen. This effect can cause a baby to appear to suffocate. These symptoms only tend to affect babies who are less than six months old. A fairly low limit for nitrate is therefore required.
Another risk thought to be associated with nitrate is cancer. Again, nitrate is reduced to nitrite in the digestive system. Nitrite can react with amino acids in the body to form nitrosamines which have been shown to be strong initiators of cancerous growths. It is interesting to note that nitrites are widely used as food preservatives in the food industry. A fairly low level for nitrite in water is set.