Oxalic acid is a dicarboxylic acid with the formula H2C2O4.
This chemical is a reducing agent and its conjugated base, oxalate, is a strong chelating factor for metals. Ingestion of high doses of oxalic acid, as well as prolonged contact with the skin, can be extremely dangerous. It is found naturally in many foods, including dark-green leafy vegetables, but under normal circumstances, consumption never reaches hazardous levels. However, accidental ingestion of bleach, anti-rust products or metal cleaners may cause oxalic acid poisoning.
The dangers caused by oxalic acid derive from its tendency to generate oxalates which quickly 'arrest' other chemicals, including calcium. For the vast majority of healthy people, no problems arise, as average consumption never reaches dangerous levels and normal metabolism can dispose of relatively high concentrations of oxalic acid. However, some people have a genetic condition that prevents their metabolism from efficiently eliminating this chemical. This in itself would not be a problem, however oxalic acid quickly precipitates as oxalate combined with calcium, not only depriving the body from this nutrient, but also increasing the risk of kidney stones, rheumatoid arthritis or gout. About 80% of kidney stones or gout is caused by sodium oxalate crystals. These patients need to be vigilant about their ingestion of oxalic acid, similar to the way diabetics need to control sugar intake.
Although it is known that oxalic acid decreases availability of some minerals, calcium in particular, this is not meaningful under normal circumstances as the impact in the overall calcium absorption is minimal. It's also been shown that increasing calcium and potassium intake when eating foods rich in oxalic acid, such as spinach or sweet potatoes, helps the body not only to absorb calcium better, but also to discard of oxalic acid more efficiently.
Furthermore, for many years it was assumed that high consumption of vitamin C would result in kidney stones, as this vitamin is easily converted to oxalic acid when in excess, but it's been recently demonstrated that there is no relationship between the two factors.
Although unlikely due to ingestion through food, oxalic acid poisoning may occur by accident due to ingestion of products which include this chemical, including bleach, anti-rust products and metal cleaners. Symptoms include burns to the skin and throat, abdominal pain, convulsions and tremors or even collapse. It's is advisable for the patient to drink plenty of fluids and seek medical assistance.