Fresh foods like fruits, vegetables and animal products contain low levels of formaldehyde due to its use as a pesticide and fumigant. Many preserved and fermented foods also contain low levels of formaldehyde for the same reasons as fresh foods and also because of the chemical's use as a preservative and a natural byproduct of bacterial and fungal fermentation.
Formaldehyde is most widely known as an embalming agent that prevents dead organisms from decaying and decomposing. However, the chemical also occurs widely in nature, and it is even produced inside the human body as a natural part of the digestive process. When enzymes break down larger organic molecules like sugars, proteins and alcohols, they form many decomposition products, including formaldehyde.
Large amounts of formaldehyde can cause tissue damage and may contribute to incidences of throat and stomach cancer. While formaldehyde causes numerous negative health effects in large quantities, the levels of formaldehyde found in most foods or produced internally are far too small to cause concern. Just as the body possesses enzymes that break down more complex organic chemicals into formaldehyde, it also contains other enzymes that break formaldehyde and other byproducts down into even simpler compounds. It is only when the levels of formaldehyde in a person's system exceed the amounts enzymes can effectively control that the chemical becomes a potential health threat.