Methaemoglobinaemia is caused by the decreased ability of blood to carry vital oxygen around the body. Here's more on the condition.

One of the most common causes is the presence of nitrate in drinking water. It is most commonly found in bottle fed infants.  Controlling nitrate levels in drinking water sources to below around 50mg/litre is an effective preventive measure. Methaemoglobinemia is characterized by reduced ability of the blood to carry oxygen because of reduced levels of normal hemoglobin. It is uncommon. Infants are most often affected, and may seem healthy, but show signs of blueness around the mouth, hands, and feet, hence the common name “blue baby syndrome”. These children may also have trouble breathing as well as vomiting and diarrhoea.

In extreme cases, there is marked lethargy, an increase in the production of saliva, loss of consciousness and seizures. Some cases may be fatal. In the body nitrates are converted to nitrites. The nitrites react with hemoglobin in the red blood cells to form methaemoglobin, affecting the blood's ability to carry enough oxygen to the cells of the body.

Bottle-fed infants less than three months of age are particularly at risk. The hemoglobin of infants is more susceptible and the condition is made worse by gastrointestinal infection. Older people may also be at risk because of decreased gastric acid secretion. Malnutrition and infection seem to increase the risk of methaemoglobinaemia (McDonald and Kay, 1988). The general health of the infant as well as Vitamin C intake may determine whether or not the condition develops (Super et al, 1981). Others at risk for developing methaemoglobinaemia include: adults with a hereditary predisposition, people with peptic ulcers or chronic gastritis, as well as dialysis patients.